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S95
1974
POPULATION DYNAMICS AND SEASONAL
MOVEMENT PATTERNS
OF DALL SHEEP IN THE ATIGUN CANYON
AREA,
BROOKS RANGE, ALASKA
By
Bob L. Summerfield, B. S.
CAME
POPULATION DYNAMICS AND SEASONAL MOVEMENT PAITERNS
OF DALL SHEEP IN THE ATIGUN CANYON AREA,
BROOKS RANGE , ALASKA
A
THESIS
Presented to the Faculty of the
University of Alaska in Partial Fulfillment
of the Requirements
for the Degree of
MASTER OF SCIENCE
By
Bob L. Summerfield, B. S.
Fairbanks, Alaska
December, 1974
ARLIS
Alaska Resources
Library & lnfè..~ination Services
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POPULATION DYNAMICS AND SEASONAL MOVEMENT PAITERNS
OF DALL SHEEP IN THE ATIGUN CANYON AREA,
BROOKS RANGE, ALASKA
REC(M.ŒNDED:
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ABSTRACT
Population dynamics and seasonal movements of Dall sheep (Ovis
dalli) were studied in the Brooks Range in 1973 and 1974. Background
data were available on the population from 1970 and 1971.
The population size decreased from over 300 in 1970 to about 275
in 1974. Productivity fluctuated from 10 to 59 lambs per 100 ewes, and
lamb survival was high. Adult mortality was low during early age and
increased rapidly after about age eight.
Winter range was a small portion of the total range used in summer.
Summer movements began in June and extended up to 25 miles. High use of
mineral licks occurred during movements between winter and summer ranges,
with an average of 90.0 minutes spent per lick visit. Sheep returned to
winter range in August and September.
The sizes of ram, ewe, and mixed bands were compared, and seasonal
and annual changes in band size were analyzed.
iii
ACKNOWLEDGMENTS
This study was financed by Federal Aid to Wildlife Restoration,
Alaska Project Nos. W-17-5 and W-17-6, Job No. 19.13, through the Alaska
Cooperative Wildlife Research Unit, University of Alaska, Fairbanks.
1 extend my sincere thanks to the following people:
Dr. David R. Klein, Leader, Alaska Cooperative Wildlife Research
Unit, for his advice and encouragement from inception through completion
of the study, for critical reading of the manuscript, and for helpful
days spent in the field.
Mr. David C. Allen, my field assistant, for his dedication and en-
durance under frequently adverse conditions, for his willingness to always
do more than required, and for his companionship during long days away
from home.
Alyeska Pipeline Service Company, for logistic support and frequent
use of their Galbraith Lake facilities.
Mr. John Clark, Mr. Ted Hill, and many others stationed at the Gal-
braith Lake construction camp whose hospitality, encouragement, and
assistance will long be remembered.
The Bureau of Land Management, for assistance in conducting aerial
surveys.
Dr. Keith Van Cleve, Associate Professor of Forestry, for analysis
of mineral lick soil samples.
Dr. Russell D. Guthrie, Professor of Zoology, Dr. Samuel J. Harbo,
Jr., Head of the Department of Wildlife and Fisheries, and Dr. Peter C.
iv
Lent, Assistant Leader, Alaska Cooperative Wildlife Research Unit, for
critical reading of the manuscript and many helpful suggestions for its
improvement.
v
......._,
TABLE OF CONTENTS
INTRODUCTION •
THE STUDY AREA
Location.
Climate •
Geology and Physiography.
Vegetation. . • .
Vertebrate Fauna.
STUDY METHODS ....
POPULATION DYNAMICS.
Size and Structure of the Population.
Productivity and Lamb Survival.
Survival and Mortality of Adult Sheep
Accidents ..•...•
Disease and Parasites.
Hunting .•
Predation.
SEASONAL MOVEMENTS
Winter Range.
Spring Movements.
Mineral Lick Use
Summer Range ..•.
Fall Movements .•.
Seasonal and Annual Variation in Band Size and
composition.
SUMMARY OF CONCLUSIONS AND MANAGEMENT RECOMMENDATIONS.
Conclusions . • • . • • . .
Management Recommendations.
APPENDIX: .
LITERATURE CITED
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LIST OF TABLES
Page
Table 1. June sheep classification counts in Atigun Canyon .... 18
Table 2. Percent of rams represented by each horn curl class
during four years. . . . . . . • . • . . . . • . . . . • 20
Table 3. Numbers of sheep counted on wintering areas 2-4 in
1974. . . . . . • . • . . • • • . • • . • . • • . . . . 23
Table 4. Productivity and lamb survival in Atigun Canyon
during four years . • . • . • . • • • . • . • . . . . . 25
Table 5, Dall sheep remains found on the Atigun study area. . • . 33
Table 6. Construction of a synthetic cohort from rams found
dead in each horn curl class . • . • . . • • . • . . . . 37
Table 7. Harvest of Dall sheep rams from the study area and
vicinity. . .
Table 8. Mineral licks used by Dall sheep on the study area
Table 9. Chemical analysis of soils from a lick and a non-
lick area •.•.•.
Table 10. Duration of mineral lick use by Dall sheep in Atigun
47
63
68
Canyon, June-August, 1973 .••..••••.•••••• 70
Table 11. Percent occurrence of each sheep class in ram, ewe,
and mixed bands. • . . • . • • . • • . . . . . • . • • • 85
vi
LIST OF FIGURES
Figure 1. Map of the Atigun study area. ..
Figure 2. Dall sheep winter range and lambing areas
Figure 3. Age distribution of Dall sheep remains found on
the Atigun study area.
Figure 4. Comparison of the percèntage of rams in each horn
curl class for the living population and a recon-
Page
6
17
• •• 35
structed cohort. • • • . • . • • • • • • • • . . • • • 38
Figure 5. Relationship of horn growth rate to age at death .••• 40
Figure 6. Sheep feeding on low slopes in the spring ••••••• 51
Figure 7. Winter range directly across Galbraith Lake from
the Alyeska pipeline construction camp ••..••.•• 51
Figure 8. Winter range used by rams in a small drainage into
the Atigun River •••••.•••••••••..••• 53
Figure 9. The main lambing area in the northeastern portion
of Atigun Canyon. • • • • • • • • • • • • • • • • • • • 53
Figure 10. Lick A, the most heavily used mineral lick on the
study area ••••••••••.•••••••••••. 54
Figure 11. Lick B, a primary lick near the eastern end of
Atigun Canyon • . • • • • • • • • • • • • • • • • • • • 54
Figure 12. Mineral licks and travel routes utilized by Dall
sheep on the study area • • • • . • • • • . • • • • • • 62
Figure 13. Mean duration of lick use for ewes and rams two
years of age and older •••••••••.••.•••• 72
Figure 14. Relationship of horn curl to age ••••.••••••• 74
Figure 15. Relationship of age to time spent in mineral licks ••. 75
Figure 16. Mean elevation of Dall sheep bands in and near
Atigun Canyon during May-August • • • • • . • . • • • • 80
Figure 17. Mean size of ram, ewe, and ali bands during
May-August. • • . . • • • • • • . • • • • . • • . • • • 88
vii
List of Figures, continued.
Figure 18. Percent occurrence of band sizes for ram, ewe,
and mixed bands during March-September ..
Figure 19. Ram, ewe, and mixed bands as the percent of total
identifiable bands recorded in and near Atigun
viii
Page
• •. 88
Canyon during May-August ...•.....••.•••. 91
INTRODUCTION
Mountain sheep are one of the most highly esteemed inhabitants of
our wildlands, yet they are among the most sensitive of all mammals to
changes in the environment induced by man. Herds of mountain sheep were
once widely dispersed throughout western North America from Alaska to
Mexico. In most localities, however, numbers of sheep have dwindled
steadily since the coming of the white man, and present distribution in-
eludes only a fraction of the historie range. After years of study, the
reasons for this decline --introduction of disease and parasites, re-
striction of habitat and competition from domestic livestock, improper
game laws and illegal hunting --are now at least in part understood.
In the light of this information, most western states have now undertaken
programs to increase the dwindling numbers of mountain sheep on areas
which they have historically occupied. This, however, requires both
time and money and is usually complicated by complex social and economie
conflicts. Alteration of traditional grazing practices or long-estab-
lished game law policies, for instance, has often been difficult or im-
possible to accomplish.
In Alaska, Dall sheep (Ovis dalli) populations have received less
impact from human encroachment than is true ofmountain sheep habitat
farther to the south. Dall sheep numbers in Alaska, therefore, are
probably not much different today from what they were a century ago.
In recent years, however, the increased focus on development of
resources in the far north has been strongly felt in Alaska. The results
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2
have been increased disturbance of Dall sheep and, in sorne cases, the
physical disruption of their habitat. In at least one instance, devel-
opment activities appear to have directly contributed to the abandonment
of portions of traditional sheep range (see Linderman, 1972). Increasing
levels of development have also produced increasing human populations and,
therefore, greater hunting pressure. This bas led to a decline in the
quality of trophy animais collected by hunters and a scarcity of legal
rams in the most accessible areas. The future welfare of Dall sheep is,
therefore, in question unless wise management policies are initiated to
cope with the increasing stresses of development, higher human population
levels, and more intensive hunting. This is a challenge which can be met,
but only through well planned research contributing to our understanding
of mountain sheep ecology and by adjusting our political, economie, and
social attitudes.
One of the areas of greatest potential development in Alaska is the
Brooks Range and northern coastal plain. This is an area richly supplied
r
with natural resources which have scarcely been tapped. The Brooks Range
also contains much excellent Dall sheep habitat, and the potential for
conflict between human development in the area and perpetuation of the
sheep populations there is great. Development of petroleum and mineral
resources, for example, will necessitate greater accessibility. This will
not only increase disturbance from aircraft operation and road construc-
tion, but will also place the region within easier reach of the public,
resulting in still more human disturbance to wildlife and perhaps even-
tually the loss of critical habitat.
This study was undertaken on the northern side of the Brooks Range
adjacent to the route of the Trans-Alaska hot oil pipeline, which will
1
3
run from Prudhoe Bay to Valdez, Alaska. This 48-inch pipeline and the
accompanying gravel road that will ultimately become a part of the state
highway system will run through Dall sheep habitat on both sides of the
Brooks Range. On the north side of the range, a pumping station, a
major construction camp, a staging area and an airstrip will be located
within three miles or less of winter range and lambing areas used by the
study population. Some sheep within this population are also known to
cross the pipeline route during their summer movements.
It seems inevitable that this degree of development will affect the
adjacent sheep population. To properly assess this effect, we must know
something of the natural ecology of the population. The focus of this
study, therefore, has been toward an assessment, before substantial de-
velopment has occurred, of the population dynamics and seasonal movement
patterns of these sheep, which live near the northern extremity of moun-
tain sheep distribution in North America. This information should make
it possible in future years to better evaluate any man-induced changes
that may come about within this mountain sheep population.
THE STUDY AREA
Location
This study was conducted at the juncture of the Endicott and
Philip Smith mountains on the northern face of the Brooks Range between
the latitudes of 68°10' and 68°35'N and the longitudes of 148°45' and
149°25'W. Topographical features comprising a loose boundary around
the study area (see Fig. 1) are the Atigun River and Galbraith Lake on
the west, the foothills to the north, the Sagavanirktok River on the
east, and the continental divide or crest of the Brooks Range on the
south. Along the west boundary, the study area lies adjacent to approx-
imately 25 miles of the route for the Trans-Alaska hot oil pipeline and
associated road system as well as the Galbraith Lake construction camp.
Within these boundaries, the bulk of study effort was concentrated
on the area known as Atigun Canyon, which lies between Galbraith Lake and
the Sagavanirktok River where the Atigun River turns from its northward
course and begins flowing in a northeasterly direction. At the onset of
the study the Trans-Alaska pipeline was scheduled to pass through this
canyon, but it has since been rerouted around the canyon on the north.
Atigun Canyon lies approximately 30 miles north of Dietrich Pass in the
continental divide.
Climate
Weather information is available from the Galbraith Lake construc-
tion.camp on a sporadic basis since October, 1970. The nearest climat-
ological station with an extended period of operation is located at the
4
5
Figure 1. Map of the Atigun study area (map from U. S. Geological
Survey, Philip Smith Mountains Quadrangle, 1956).
Legend:
Study area boundary
..___._ Approximate Trans-Alaska pipeline route
1 = Atigun Canyon
2 = Black Mountain
3 = Galbraith construction camp and airfield
4 = Guard House Rock
5 = Pump Station No. 4
6
village of Anaktuvuk Pass, sorne sixty miles west-southwest of Galbraith
Lake. Specifie and detailed analysis of the climate of the study ar~a
is, therefore, not possible. With the information available, however,
it is possible to discuss the climate of the area in broader terms.
Temperatures in this section of the Brooks Range may vary from
7
below -50° Fahrenheit in winter to near 80° in summer. The lowest tem-
perature recorded at Galbraith Lake during the winter of 1970-1971 was -49°
on January 23 and February 28, and the highest temperature recorded by
myself on the study area during the summer of 1973 was 75° on July 23.
The mean monthly temperature rises above freezing only during the months
of June, July, and August, and freezing may occur during any month of
the year. Temperature inversions are characteristic of arctic Alaska
during both summer and winter (Searby, 1971), a fact which would favor
sheep on mountain slopes during the cold months.
Strong winds frequently combine with low temperatures in this area
to make the climate among the most severe in the state. During the summer
of 1973, winds reached 50 m.p.h. on two occasions and 20-30 m.p.h. sev-
era! times. Wind speeds may also reach 35-50 m.p.h. in association with
winter storms, but fortunately are lighter during periods of extreme cold
(Searby, 1968). Still, moderate winds in conjunction with minimum winter
temperatures may produce a chili factor lowering temperatures to the equiv-
alent of at least -80° .
Precipitation on the study area amounts to only 8-10 inches per
year, most of which falls during the summer months. Annual snowfall at
Anaktuvuk Pass averages 63 inches (Searby, 1968), but is probably a good
deal lower in the Atigun and Sagavanirktok drainages as indicated by clim-
atological data from Galbraith Lake and observations made in March and
8
April of 1971, 1973 and 1974. Light, temporary snows may occur through-
out summer, but accumulation in the valley bottoms usually begins in late
September. Wind transport of snow and drifting are prevalent, filling
small ravines and depressions and exposing ridgetops where sheep are able
to forage. Late snowmelt resulting from relatively cold spring temper-
atures combines with a permafrost layer which halts downward percolation
of water to produce an abundance of soil moisture throughout the growing
season in what might otherwise be considered an arid environment.
A climatic factor which uniquely affects sheep populations above the
Arctic Circle is length of day. There is nearly a three-month period of
continuous daylight during summer and a period of similar length during
winter when darkness predominates. This presents extra opportunities for
observation of sheep from May through July but makes observation from
November through January impractical.
Geology and Physiography
The bedrock forming the backbone of the Brooks Range in the region
of the study area is derived from Paleozoic marine sediment and consists
of limestone, quartzite, shale and schist. Skirting the higher mountains
and forming the north front of the range are younger rocks of sandstone,
conglomerate, and slate dating from the Permian through early Cretaceous
periods (Gryc, 1958; U. S. Dept. of the Interior, 1971). Periodic up-
lifting and eroding away were characteristic of the area from Jurassic
until late Tertiary times, when massive uplifting then gave the range its
present form as a high mountainous region. Renewed uplift; erosion and
glaciation since that time account for its present appearance (Gryc,
19S8).
,..
9
The U-shaped valleys typical of the area were widened, straight-
ened, and scoured by four major glacial advances during the Quaternary
period (Detterman, 1953). These glaciers originated in the valley heads,
moved downstream, and merged at the mountain front. The highest moun-
tains protruded above the ice even during maximum glacial development
(Keller et al., 1961). After recession of the glaciers, moraine and out-
wash deposits covered the valley floors with a veneer of gravel, and this,
along with silt, sand, and localized bedrock, now forms the major stream-
beds (U. S. Dept. of the Interior, 1971).
The elevation of the study area ranges from 2200 feet at the mouth
of Atigun Canyon to 7610 feet, with five mountain peaks exceeding the
7000 foot level. Crest lines connecting these peaks are formed by inter-
secting cirques, aretes, homs and irregular ridges (Keller et al., 1961).
Several remnant glaciers remain in the higher north-facing cirques while
the cirques at lower levels are now ice free.
The north-south oriented valleys of the Atigun and Sagavanirktok
rivers are broad and of relatively low gradient, showing the knob and
kettle topography typical of glaciated areas as well as polygonal ground,
frost boils, mud slumps and other features associated with permafrost
regions. Near Galbraith Lake, however, the Atigun River turns sharply
in a northeasterly direction and drops 400 feet in approximately 8 miles
through Atigun Canyon. Throughout this section the river is normally
too swift and contains too many rapids to either wade or navigate safely,
and in the lower three miles it frequently cuts through nearly vertical
slopes making foot travel adjacent to the river impossible.
10
Vegetation
Vegetation on the study area may be classified into four plant com-
munity types described by Spetzman (1959). These are flood-plain and
cutbank vegetation, cottongrass meadows, dry upland meadows, and outcrop
and talus vegetation. Wet sedge meadows and aquatic vegetation (also
described by Spetzman, 1959) occur in much more limited extent at the
lower elevations and are of minor importance to Dall sheep.
The most conspicuous species of flood-plain and cutbank communities
is feltleaf willow (Salix alaxensis).1 This is the tallest vegetation of
the area, reaching a height of as much as 20 feet. It represents, how-
ever, only the second of four successional stages typical of flood-plain
and cutbank areas. Associated plants occurring in this community are
various species of grasses, sedges, shrubs, herbs, mosses and lichens.
Willows and the other associated species are utilized extensively by
Dall sheep, as well as by moose and caribou, on localized areas at cer-
tain times of the year.
Cottongrass meadows occur abundantly in the valley bottoms and on
mountain slopes up to about 3600 feet. The cottongrass Eriophorum ~
inatum spissum is the dominant species, forming tussocks 6-12 inches high
and equally wide. The tussocks are separated by mossy channels a few
inches wide which during summer usually contain standing water. Other
species scattered through the relatively closed stands include severa!
grasses, sedges, small shrubs and herbs. Sheep frequently graze this
community type near the margins of more rugged slopes during both summer
and winter.
1Plant names are from Spetzman (1959).
11
Dry upland meadows are found on well-drained coarse mineral soil
from about 2500 to 4000 feet in elevation. Vegetation is usually only a
few inches high and somewhat sparse, and it may vary considerably in
species composition from one site to another. Dryas octopetala and
lichens, however, normally are common to all sites. The associated
species of grasses, sedges, shrubs, herbs and mosses and their eleva-
tional position on mountain slopes make this community type an important
one to Dall sheep.
Plant communities among rock outcrops and on talus slopes are per-
haps the most important vegetation in Dall sheep habitat, since these
provide available forage near escape terrain on winter range and are
abundant throughout the higher summer range. Vegetation is found in
these sites up to 6000 feet in elevation, although it may become quite
sparsè above 4000 feet. Even at lower levels plants are scattered on
shallow soil between areas of exposed rock. Species composition may vary
considerably, depending on rock type, but generally consists of a mix-
ture of scattered saxifrages, ferns, grasses, dwarf herbs and mat shrubs.
Vertebrate Fauna
The study area is inhabited by a variety of vertebrate wildlife
species including fish, birds and mammals. A portion of these are res-
ident to the area throughout the year, and others only pass through
during the course of their seasonal movements.
Sorne of the mammalian species affect Dall sheep directly as pred-
ators, and this relationship will be discussed in a later section.
Many of the nonpredators also play a role in sheep ecology, however, as
competitors for food.
Most of the bird species occurring on the study area visit for only
12
a short period during the summer months. As Murie (1944) noted, however,
sorne of these, such as the snow hunting (Plectrophenax nivalis), may corn-
pete with Dall sheep for food. A total of 72 bird species were identi-
fied in 1969-1970 in the Atigun and Sagavanirktok valleys by Sage (ILS.
Dept. of the Interior, 1971), and 52 identified species plus several
unidentified species were observed by D. Allen and myself in 1973.
Listed in the Appendix are the scientific and coDDilon names of mam-
mals identified on the study area and those few birds which remain there
the year round.
-----" __ " ________ _
STUDY METHODS
The initial studies of Dall sheep in the Atigun Canyon area were
conducted by Ron Andersen during the summer of 1970 under the project
title "Effects of Human Disturbance on Dall Sheep." This project was
continued by Ray Priee and assistant James Male through the 1971 field
season. During both years, valuable baseline information was collected
on population parameters and movement patterns in addition to observations
of human disturbance. Their work allowed me to anticipate what to expect
when I began my study and has made analysis of population ecology over
a four year period possible.
Field observations for the present study were conducted during
March 28-April 2, 1973; May 23-September S, 1973; April 9-12, 1974;
and June 3-13, 1974. A total of 127 days were spent observing sheep on
the study area. During the May 23-September S, 1973 period, field work was
conducted with the aid of assistant Dave Allen from a base camp established
near the center of Atigun Canyon. Backpacking trips of up to four days
were frequently made from this camp. The remaining periods of investig-
ation were conducted solely by backpacking.
Three basic techniques were used in making observations. Recon-
naissance flights were conducted with fixed-wing aircraft at irregular
intervals to determine population size, distribution and movements.
Ground surveys were made during ail periods of investigation and were
conducted on an approximately weekly basis during the May 23-September S,
13
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1973 period for more frequent and detailed information on population
characteristics and movement patterns. During these surveys the size,
composition, activity and elevation of ali sheep bands observed were
recorded on prepared forms, and the location of ali bands was keyed to
topographie maps. Sheep were classified according to age as lambs,
yearlings, and adults. Only adults were classified according to sex as
no lambs and only a few yearlings could be sexed at a distance. Adult
rams were further classified by horn size to the nearest one-quarter
curl using the technique described by Murphy (1974:18).
Daily observation of the population was used to gain an understanding
of various other aspects of sheep behavior and ecology which were useful
in interpreting population ecology and movement information. A 15-60X
spotting scope and 7x35 binoculars were used as an aid in ali ground
surveys and daily observations.
i
POPULATION DYNAMICS
There are four major sheep wintering areas within the study area
(see Fig. 2). Sorne of these areas are used primarily by ewes and young
sheep, others primarily by rams. The sheep within each of these areas
are isolated from other sheep groups for several months of the year.
Mixing of these groups occurs, however, during the rut when rams visit
the ewe wintering areas and during the summer when sheep congregate at
mineral licks and then disperse over summer range. Since all sheep found
on the study area could potentially interbreed and come into contact on
summer range, they should all be considered as one population. It should
also be noted, however, that sorne sheep move out of and into the study
area during summer and probably during the rut, and therefore, the study
population may be considered a subunit of a much larger population.
Size and Structure of the Population
The analysis of the size and structure of the study population is
facilitated by considering separately each wintering area, in view of the
fact that these sheep are concentrated on winter range for a large por-
tion of the year. Area 1, Atigun Canyon, (Fig. 2) is the major winter
range unit located within the study area bath in terms of size and the
number of sheep present. Table 1 lists the results of population com-
position counts made in this area during June of 1970-1974. It is
apparent that Atigun Canyon is used primarily by ewes and young sheep,
with legally huntable rams (3/4-curl or larger) averaging only 5.2% of
15
16
Figure 2. Dall sheep winter range and lambing areas (map from
U. S. Geological Survey, Philip Smith Mountains
Quadrangle, 1956).
Legend:
approximate winter range boundary
-----areas receiving occasional winter use
• lambing area
r
Table 1. June sheep classification counts in Atigun Canyon.
1970 1971 1973 1974
(18-20 June) (8-9 June) (13 June) (8-9 June)
No. % No. % No. % No. %
Lambs 66 26.4 21 9.5 8 4.8 52 28.6
Yearlings 19* 7.6 42 18.9 18 10.8 8 4.4
Ewes 121* 48.4 107 48.2 81 48.5 88 48.3
1/4-curl rams 19 7.6 16 7.2 28 16.7 8 4.4
1/2-curl rams 16 6.4 11 5.0 24 14.4 17 9.3
3/4-curl rams 6 3.4 11 5,0 5 3.0 7 3.9
Full curl rams 3 1.2 3 1.3 3 1.8 2 1.1
Total rams 44 17.6 41 18.5 60 35.9 34 18.7
Unidentified 0 0 11 4.9 0 0 0 0
Total 250 100.0 222 100.0 167 100.0 182 100.0
*Estimated figure: count included 91 ewes, 14 yearlings and 35 unclassified ewes and yearlings,
hence (14/105)X35•5 sheep were added to the yearling count and (91/105)X35=30 sheep were added to
the ewe count.
,_.
00
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19
the total. Also apparent is a continuous decline in the adult popula-
tion, exclusive of lambs and yearlings, since 1970. The number of adults
counted was 165 in 1970, 159 in 1971, 141 in 1973, and 122 in 1974. For
this four-year period, an average decrease of 10.8 adult sheep occurred
each year. Considering this decline, the percentage of ewes in the pop-
ulation has remained remarkably stable since 1970, varying around 48%.
Age structure changes among the rams in Atigun Canyon have also
occurred during this period. Table 2 shows each ram class as the percent
of total rams seen on Area 1 each year. A slight shift from the younger
age classes (1/4-and 1/2-curl) to the older age classes (3/4 and full
curl) occurred between the June, 1970 and June, 1971 composition sur-
veys, a large increase in the younger age classes occurred between the
June, 1971 and June, 1973 surveys, and another shift towards the older
age classes occurred between the June, 1973 and June, 1974 surveys. Such
age structure changes are probably due to large variations in initial
cohort size from year to year and the effect of their carryover into the
older age classes. The production in 1970 illustrates such effects. A
large lamb crop was produced in 1970 with a lamb:ewe ratio of 54:100.
By 1973 these sheep were three years old and most of the rams were small
1/2-curls, producing the increase in the younger age classes in 1973.
The shift back towards the older age classes in 1974 was due to smaller
lamb crops in 1971 and 1972 and probably also to the movement of sorne
young rams out of the area to join ram bands at other locations.
Age structure changes are more difficult to detect among ewes than
among rams. However, the sex ratio in the genus Ovis is near unity at
birth (Geist, 197lb), and the large fluctuations in the size of the lamb
crop which have been observed probably also have resulted in changes in
Table 2. Percent of rams represented by each horn curl class during
four years.
Ram Year
Classification 1970 1971 1973 1974
1/4-curl 43% 39% 47% 23%
1/2-curl 36 27 40 50
3/4-curl 14 27 8 21
full curl 7 7 5 6
Total 100 100 100 100
20
21
in the ewe structure. Age structure changes may also be influenced by
differentiai survivorship and mortality rates among cohorts w~ich, as
noted by Murphy (1974), may occur at any age throughout life as a result
of annually variable weather conditions, changes in food availability,
disease, predation, intense social interaction, or any other factors
which affect the energy intake and expenditure of cohorts.
A further trend evident from the annual composition counts in
Atigun Canyon is a changing sex ratio. During 1970, 1971, and 1974, the
ratio of adult males to adult females (two years and older) was 36, 38,
and 39 rams per 100 ewes respectively. In 1973, however, the proportion
of rams to ewes was about doubled (74:100). The large number of rams
that year, as mentioned before, resulted from the large lamb crop in
1970. However, one would have expected an equally large increase in the
number of ewes in 1973, which would have stabilized the ram:ewe ratio
somewhat. Since there was actually a decrease in the proportion of ewes
seen in 1973, a lower birth rate and/or subsequent survival rate for
ewes than for rams is indicated between 1970 and 1973. The Atigun sheep
population reached its greatest size and density in 1970 and thereafter
began to decline. Female lambs characteristically show lower survival
rates in declining populations than do rams (Geist, 197lb). Adverse
forage conditions, which may be associated with declining populations,
may also result in poorer intrauterine growth for females than for males
(Robinson et al., 1961), and presumably could affect sex ratios at birth.
Sex ratios on ewe wintering areas are not only a product of dif-
ferent natality and mortality rates for ewes and rams, but are also
influenced by the social structure of mountain sheep. The large increase
in the proportion of rams to ewes from 1973 to 1974 is a case in point.
Geist (197lb) has shown that young rams remain attached to ewe bands
until 2-4 years of age at which time they join ram bands and establish
permanent home range patterns. Many of the young rams which wintered
22
in Atigun Canyon in 1973 did not return in 1974 but probably remained
with the ram bands on Areas 2 and 3 (see Fig. 2). Population counts
taken in Area 2 in 1973 and 1974 show evidence of this. Twenty-one rams
were counted there in June, 1973, but four or five mature rams were
killed from this area in August by hunters working out of a nearby
guide's camp (D. Keyes, Bureau of Land Management, pers. comm.). In
June, 1974, however, 21 rams were counted there again and the average
age (as indicated by horn curl) had decreased somewhat.
The above discussion of spring sex ratios in Atigun Canyon was not
intended to present a picture of the actual sex ratio of the study pop-
ulation but was given only to illustrate the changes in sex and age
composition which have occurred on this one wintering area since 1970.
Geist (197lb:288) noted that "sex ratios can be considered valid only
if taken within two weeks prior to the ewes entering estrus," and, I
might add, if taken over a sufficiently large area to include both ewe
and ram home ranges. The entire study area is probably large enough to
give an accurate indication of the true sex ratio from composition counts
made in the spring as both ram and ewe ranges are included. This infor-
mation is available from April and early-June surveys of wintering areas
2-4, the results of which are shown in Table 3. Area 2 is an area used
almost exclusively by rams. Only one and two ewes were counted there in
1973 and 1974 respectively, while the total estimated number of sheep
using this range is 25. Areas 3 and 4 are also used mainly by rams but
contain a few ewes as weil. A total of 30 sheep is estimated to winter
Table 3. Numbers of sheep counted on wintering areas 2-4 in 1974.
Area and Date
Classification 2: (June 7) 3: (June 5) 4: (April 10)
Lambs 0 0 0
Yearlings 0 0 1
Ewes 2 3 2
1/4-curl rams 1 1 0
1/2-curl rams 10 9 0
3/4-curl rams 4 5 1
Full curl and larger rams 6 1 0
Total rams 21 16 1
Total 23 19 4
Total
0
1
7
2
19
10
7
38
46
N ....
24
on these areas.
Over the entire study area, an actual 228 sheep were counted in the
spring of 1974. Of the 167 sheep which were two years of age or older,
72 were rarns and 95 were ewes, giving a sex ratio of 76 rarns per 100
ewes. This figure is intermediate in comparison to sex ratios given in
the literature for nonexploited and lightly exploited mountain sheep
populations.
An estimate of the total study population size can be calculated
from the June, 1974 composition counts in wintering Areas 1-4. A total
of 182 sheep were counted on Area 1, which is probably close to the
actual number present. Combined with the 55 total sheep estimated on
Areas 2-4, a minimum of 237 sheep were present. By early June, however,
sorne sheep may have already begun to wander from the· wintering areas.
It is also likely that sorne sheep spent the winter of 1973-1974 at
locations other than wintering areas 1-4 since snow accumulation was
unusually light (see discussion of winter range in Seasonal Movements
section). With these considerations, a fair estimate of the total size
of the wintering population on the entire study area in 1974 would seem
to be approximately 275 animais.
Productivity and Lamb Survival
Table 4 summarizes lamb production and survival in Atigun Canyon
for the four years in which classification counts were made. It is
evident that productivity fluctuates between wide extremes and, there-
fore, results in large differences in initial cohort size. These fluc-
tuations might occur as a result of annually variable pregnancy rates,
re~orption of fetuses, stillbirths, or neonatal mortality. In the latter
case it must be remembered that June classification counts represent
,.......---,----~--"-~
Table 4, Productivity and lamb survival in Atigun Canyon during four years.
Year
1970 1971 1973 1974
No. lambs 66 21 8 52
% survival ~ 18~8 19 42 No. yearlings
No. ewes 121 107 81 88
Lam:ewe ratio 54:100 20:100 10:100 59:100
Yearling: ewe ratio 16:100 39:100 22:100 9:100
Average
37
22
99
37:100
22:100
N
"'
·~·-
only apparent natality since many lambs probably die within a few days
after birth and are never counted.
26
The number of ewes which become pregnant in a mountain sheep pop-
ulation depends on at least two things, range quality and the age struc-
ture of the population. The age at which ewes reach sepual maturity is
closely related to their nutritional state. Under conditions of average
range quality, most females probably breed first at 2-1/2 years of age,
but on exceptionally good range sexual maturity may be achieved at
1-1/2 years (Geist, 197lb; Streeter, 1970). Hence, on high quality
ranges the number of potentially reproductive females may be higher than
on ranges of lower quality, even if the actual number of ewes is the
same. Also, it is known from domestic sheep that ewes achieve their
highest reproductive output during middle age. The highest conception
rates, highest number of lambs born per ewe, and highest survival of
lambs occurs among ewes 5-7 years of age (Turner and Dolling, 1965).
On the other hand, reproductive performance in female ibex (Capra ibex),
a close relative of mountain sheep, has been shawn to decline after about
10 years of age, while young ibex and domestic sheep ewes lose or desert
a higher percentage of their lambs than older females (Nievergelt, 1966;
Alexander, 1960). Populations of middle-aged sheep, therefore, probably
demonstrate the greatest production, while reproductive output in pop-
ulations with large young-and old-aged segments would likely be some-
what lower.
In applying this information to the study population, one might
speculate that the large lamb crop of 1970 was a product of a healthy
mi"ddle-aged ewe population. In 1973, however, these ewes had passed
their reproductive prime and the large crop of lambs they produced in
27
1970 had not yet reached their highest reproductive potential. Con-
sequently, production in 1973 was much reduced. An excellent lamb çrop
was produced in 1974, but it is doubtful if one year's increase in the
age of the ewes born in 1970 could have made such a large improvement
in the reproductive performance of the population. It seems likely,
therefore, that other factors have also contributed to the wide flue-
tuations in productivity.
Geist (197lb) compared the number of bighorn ewes seen from a fire
lookout in Banff National Park during eleven years to the lamb:ewe
ratios in the summers following and concluded that productivity is
inversely related to population density (r=-0.75, t=3.4, p<:O.Ol). As
further evidence of this, Murphy (1974) showed that the lamb:ewe ratio
in the area of Mt. McKinley National Park where population density was
highest was significantly lower (X 2=6.43, p<:O.Ol) than for the park
as a whole. Finally, Woodgerd (1964) demonstrated that among the Wild-
horse Island population of bighorns, productivity dropped as the pop-
ulation expanded and density increased to a stable level. Composition
counts from Atigun Canyon substantiate the inverse relationship of
population density and productivity. The peak population level, 250
sheep in 1970, was followed by a low lamb:ewe ratio (20:100) in 1971
while the smallest population count, 167 sheep in 1973, was followed by
a high lamb:ewe ratio (59:100) in 1974.
Weather during both gestation and the lambing season also has
profound influences on lambing success. Murphy (1974) showed a sig-
nificant negative correlation (y=65.9-0.3lx, r=-0.81, F(l,lO)=l7.6,
p<O.OI) between snowfall during winter and the lamb:ewe ratio in the
summer following for Dall sheep in Mt. McKinley National Park. Good
28
nutrition is important to "every phase of the reproductive process from
conception to parturition and post-partum vitality," (Pitzman, 1970:37).
However, during winters of deep snow sheep are restricted to small por-
tions of their habitat, which increases population density and reduces
the amount of forage available per sheep. Pawing for food through deep
snow also increases the energy expenditure for sheep. Normal winter
weight losses of 15% and 13% have been reported for Dall sheep from the
Alaska Range and the Kenai Peninsula in Alaska respectively (Heimer,
1973). Among reindeer, weight losses of 17-24% early in gestation have
been shown to result in the frequent resorption of fetuses (Preobrazh-
enskii, 1961). Assuming a similar condition exists for Dall sheep, it
seems almost certain that during severe winters many ewes are unable to
provide enough energy for optimum fetal growth. Even if a fetus is
successfully formed, it is known from experience with domestic sheep
that lambs born after severe winters are smaller and less viable than
those produced in mild years (Robinson et al., 1961). Small, weak lambs
born to nutritionally stressed ewes are especia11y susceptible to
immediate postnatal death caused by desertion, failure to initiate nursing,
or hypothermia. If they survive, they are subject to several physio1-
ogical and behavioral disorders which may be retained throughout 1ife
(see Geist, 1971b:286-287).
The winters of 1969-1970 and 1973-1974, which according toU. S.
Weather Bureau statistics appear to have been relatively mild throughout
northern Alaska, were followed by large lamb crops on the study area
(54 and 59 lambs: 100 ewes respectively). The poor lamb crop of 1971
(20 lambs:lOO ewes) followed a severe winter; however, the virtual
failure of the lamb crop in 1973 followed a winter which was not
l
29
especially severe. The most reasonable explanation for low production
in 1973 is the cold, wet weather which occurred during the lambing sea-
son and probably resulted in a high rate of neonatal mortality by hypo-
thermia.
Predation may influence lamb production in either a positive or a
negative manner, depending on the relative numbers of predators and prey
and the size of the prey population in relation to the quality of its
habitat. Moderate levels of predation may reduce competition for avail-
able food in high density sheep populations, thereby placing ewes on a
higher nutritional plane and increasing productivity. A high ratio of
predators to prey, however, could concentrate sheep within the more
rugged portions of their winter range where vegetation is often less
abundant. This would not only lower the nutritional plane of ewes and
place them under.physiological stress, but if harassment were frequent
enough might cause psychological stress as well. Experimentally-induced
stress led to neurosis in domestic sheep (Liddell, 1954) and produced
behavioral abnormalities in the offspring of pregnant female rats
(Thompson, 1957). Nutritional and psychological stress ind~ced by
predators might, therefore, contribute to the loss of embryos or the
production of smaller, less viable, or behaviorally affected lambs.
Predation pressure may vary annually as the numbers of predators and
prey change, as alternate prey species become available and, because
predation is greater during winters of heavy snowfall (Murphy, 1974), as
the weather fluctuates.
Evidence from Dall sheep studies in Alaska indicates that lambs
experience little mortality for the remainder of the summer after they
reach about a week of age (Murie, 1944; Murphy, 1974).. During their
30
first winter, however, they frequently suffer high mortality, usually
at a greater rate than other age classes (Murie, 1944; Murphy, 1974;
Nichols, 1973). Lambs born in Atigun Canyon probably follow a similar
pattern, although mortality data for this age class are sparse. Remains
of only six lambs were found on the study area in 1970 and 1971, and no
lamb carcasses were found in 1973 or 1974. The small number of lamb
carcasses found is not surprising in view of their high perishability
and the rugged areas in which lambing occurs.
Low summer mortality among lambs in this area is suggested by the
1973 lamb crop. Eight lambs were counted on the lambing areas in Jqne
and at !east seven of these were present when the sheep returned to
Atigun Canyon in late August.
Information on survival through the first winter is available for
the 1970" and 1973 cohorts (see Table 4). Of 66 lambs counted in 1970,
42, or 64%, survived to be counted as yearlings in 1971. Of the eight
lambs counted in 1973, all survived until the 1974 count. Survival
rates of 64% and 100% are probably not typical, however, but were more
likely the result of favorable circumstances in those years. Conditions
which produced a large lamb crop in 1970 may also have favored good
survival of those lambs. For instance, Murphy (1974) found a strong
negative correlation between snowfall in the year before birth and sur-
vival to yearling age among sheep in Mt. McKinley National Park during
recent years. Wolf densities were also apparently lower on the study
area in 1970 than more recently.
The eight lambs which were counted in June, 1973 probably repre-
sènted the largest and strongest of their cohort since high neonatal
!osses seem to have occurred that year. Snowfall was exceptionally light
.._
31
during the winter of 1973-1974, and because the sheep population was at
its lowest level since at least 1970, more food was apparently avaif-
able per sheep. In addition, wolf densities apparently dropped some-
what from a high the previous summer and caribou were present in the
area most of the winter and were likely the focus of wolf predation.
The combination of all these factors suggests highly favorable survival
conditions for the 1973 lamb crop.
Survival and Mortality of Adult Sheep
From this and other studies it is apparent that patterns of survi-
vorship and mortality among adult mountain sheep almost always follow a
similar form. Sheep which live past yearling age show high survival
during the early years of life and then accelerated mortality beginning
about midway through the maximum life expectancy. This pattern may vary
slightly between populations or between cohorts within the same popula-
tion, but the general characteristics remain the same for both rams and
ewes in all populations.
Geist (197lb:294) has noted three means for estimating the age-
specifie survival and mortality of adult mountain sheep. These are:
(1) aging skeletal remains found in the field by the horn annuli tech-
nique (Geist, 1966); (2) comparing the size of successive age (or horn
size) classes observed in the living population; and (3) observing the
return of individually identifiable sheep to a given seasonal home range
in successive years. Since no sheep in the study population were marked
and few could be recognized individually, the first two methods must be
used in this analysis.
A total of 86 sheep remains were found on the study area between
32
1970 and 1974. A breakdowp of the number found for each sex and horn
curl class is shown in Table S. As the majority of remains were col-
lected in 1973, it is believed that most of these sheep died during or
after 1970. The homs on sorne skulls were quite decayed, however,
indicating that a few remains may have accumulated as early as 1965.
Unfortunately, mortality data compiled from remains has been shown to be
subject to severa! biases. These include variation in the perishability
of carcasses among the various age and sex classes and the tendency of
finding relatively more of certain sex and age classes depending on
whether ram or ewe wintering areas are more closely searched for remains
(Geist, 197lb). Neither of these biases appears serious for the ram
remains data collected on the Atigun study area, as will be shown later.
Age structure changes within a population also may lead to unreal-
istic conclusions concerning the age-specifie mortality patterns of that
population. Murphy (1974) hypothesized that large fluctuations in
initial cohort size and variable conditions (e.g., weather) which in-
fluence subsequent survival lead to large differences in age-specifie
mortality patterns among cohorts. These differences may be so great
that remains which accumulate over a period of severa! years will not
reflect the true mortality patterns of the population. Such appears to
have been the case in Mt. McKinley National Park when Murie (1944) col-
lected his sample of over 800 skulls. There have also been age structure
changes and large variations in initial cohort size in the Atigun
population. The remains found' during this study, however, indicate
mortality patterns which are not inconsistent with those that would be
ex~ected from the age structure of the living population (see p. 36
and Fig. 4).
r
.........
Table S. Dall sheep remains found on the Atigun study area.
Year
Classification 1970 1971 1973 1974 Total
Lamb 1 5 0 0 6
Yearling 0 0 0 0 0
Ewe 4 0 8 0 12
l/4.:curl ram 0 0 0 0 0
1/2-curl ram 0 1 4 1 6
3/4-curl ram 0 5 4 1 10
Full curl ram 0 2 8 3 13
Full curl plus ram 0 0 2 1 3
Large ram, class unknown 4 2 12 1 19
Adult, class unknown 0 1 2 1 4
Unidentifiable 0 10 3 0 13
Total 9 26 43 8 86
"' "'
~
34
Of the ram remains found, 24 had homs in good enough condition to
allow confident aging by horn annuli while 32 could be classed according
to size to the nearest one-quarter curl. Fig. 3 shows the age distrib-
ution of the 24 aged ram remains. The mean age at death was 9.00 years
with a standard deviation of 2.67 years and a standard error of the mean
of 0.54 years. The minimum life expectancy of adult rams may be defined
as the age at which 95% of those surviving to yearling age are still alive
while the maximum life expectancy is the age at which 95% are dead
(Geist, 197lb). For this population, the calculated minimum and maximum
life expectancies are 3.5 years and 14.5 years respectively (for details
of calculations see Geist, 197lb:294). These figures are close to the
ages of the youngest and oldest remains found, which were four and thir-
teen years of age respectively.
For purposes of analysis of the data from the ram remains, the 24
known-aged rams can be considered as members of the same cohort. Average
mortality for any age interval may then be calculated as the sum of the
number found dead each year in that interval over the sum of the number
alive in each previous year (Geist, 197lb:295). This method gives an
average mortality between the ages of two and eight years of 5.2% and
between the ages of nine and thirteen years of 38.1%, clearly illustrating
the two-phase survivorship pattern noted earlier. For all rams greater
than two years of age, the average annual mortality rate is 12.9\.
A second synthetic cohort may be constructed from the 32 ram remains
classed to the nearest quarter curl. Such a cohort, shown in Table 6,
contains 41\ 1/4-curl rams, 34\ 1/2-curl rams, 21% 3/4-curl rams, and 4\
ful1 curl rams. These percentages were compared to the percentage com-
position of living rams recorded in classified counts of the study area
...
'E • u
20
15
.. tiO
5
0
-
-
-
2 3
"""" -~ -
~ -r-
l""" -
. .
4 5 6 7 8 9 10 Il 12 13
Age at De ath (Yeora)
Figure 3. Age distribution of Dall sheep ram remains found on the
Atigun study area •
35
36
during 1970-1974 and the two were found to be very similar (Table 6 and
Fig. 4). The 1/2-and full curl classes are slightly over-represented
in the remains data while the 1/4-and 3/4-curl classes show slight
under-representation. These differences in percentage composition may
result from one or more of the biases suggested by Geist (197lb) and
Murphy (1974), but since the discrepancies are of small magnitude, these
biases apparently are not large in the Atigun carcass sample.
The causal faètdrs behind the two phase mortality pattern exhibited
by rams greater than two years of age have been described by Geist
(197lb :295):
The ages of low mortality in rams coïncide with their low
dominance status and near exclusion from breeding by larger-
horned, older rams. Conversely, when rams reach near ultimate
body and horn size, and become dominant breeding rams during
rut, their mortality increases.
Large, dominant rams, notes Geist, return from the rut in an ex-
hausted condition because they '~irtually do not feed while guarding
ewes and.they.fight extensively and do mut:h running and chasing when
following the estrous ewe and discouraging competitors." Having lost most
of their fat deposits, these dominant. rams are vulnerable to mortality
during the severe winter months following the rut.
' Rams which are on high quality range and thus grow fast and achieve
dominance status relatively early in life will be subjected to the
physiological stresses imposed by the rut at an earlier age than slow-
growing, late maturing rams on poor range. One would expect, then, that
on the average, rams reaching sexual maturity on good range would have
a shorter life span than those on range of lower quality. This principle
has peen demonstrated for both mountain sheep and European ibex by corn-
paring longevity in introduced, expanding populations (where forage
r-
Table 6. Construction of a synthetic cohort from rams found dead in each horn curl class.
Horn Curl Class
1/4 1/2 3/4 Full Full+ Total
No. remains found 0 6 10 13 3 32
Reconstructed cohort
No. 32 26 16 3 0 77
% 41 34 21 4 0 100
% observed in population 39 38 16 6 1 100
"' "
____ A
-c • u
~ n.
50
1/4
••--• Observed Population
•--• Reconstructed Cohort
1/2 3/4 4/4 4/4+
Horn Curl C lass
Figure 4. The percentage of rams in each horn curl class in the
living population compared to a reconstructed cohort.
38
39
conditions are good and individual growth rates are expected to be high)
to longevity in stable or declining populations on poorer range (whe~e
individual growth rates should be somewhat lower; see Geist, 197lb:296).
There is also considerable difference in growth rates among rams from the
same population, however, and evidence from the Atigun study population
indicates that the inverse relationship of growth rate and longevity is
equally true for these rams.
Individual horn segments were measured for twelve of the ram remains
found on the study area and the total length of horn segments 3-5 (pro-
duced between the ages of 1-1/2 and 4-1/2 years) was used as an indica-
tion of growth rate. There are several reasons for choosing these seg-
ments. The first and second segments (lamb and yearling) are almost
always partially worn away and thus would not give an accurate indic-
ation of growth. The third through fifth segments are usually the lar-
gest and most easily discernible and would, therefore, give the most
precise index of growth. Since these are the years of fastest growth,
any subtle differences in growth rates would show up here best. After
age four, the fastest growing rams may begin taking part in the rut,
which could affect their horn growth and complicate any comparison based
on later segments.
By regression analysis a negative correlation was found between the
total length of horn segments 3-5 in millimeters and age at death in
years (y=535.9-11.3x, r=-0.693). This relationship is illustrated in
Figure S. Within the same population, therefore, there exist faster
growing individuals of relatively short life span and slower growing
individuals of relatively long life span. Since fast growers are known
to predominate in expanding (i.e., high quality) populations and slow
40
~
E s
10
1 550 ,.,
• y=535.9-11.3x .. c r= -0.693 • • E
11:11 500 • • U)
c
~
0 :z: 450 ....
0
z: ..
11:11
c 400 • .J
0 ..
0
~ '350
4 6 8 10 12 14
Age at Death (Years)
Figure S. Relationship of horn growth rate ta age at death.
41
growers are known to predominate in stable or declining (i.e., lower
quality) populations, the existence of both types within the same pop-
ulation may be due simply to individual nutritional variation or possibly
occurs as a genetic adaptation evolved to cope with a periodically
changing environment (e.g., fluctuations in weather, density, predation,
disease) which sometimes produces sudden changes in population quality.
Mortality and survival data for ewes in the Atigun population are
sparse, however, better information is available from other mountain
sheep populations. Large carcass samples of Dall sheep ewes collected
in Mt. McKinley National Park (Murie, 1944) and Nelson's bighorn (Ovis
canadensis nelsoni) collected on the Desert National Wildlife Range
(Hansen, 1967) indicate a two phase survivorship pattern for ewes similar
to that exhibited by rams. As with rams, mortality rates of ewes seem
to be tied closely to the energy expended on reproduction.
Young ewes probably can maximize lifetime reproductive output by
channelling relatively more energy into maintenance and growth than into
early reproduction (Murphy, 1974). As a consequence, survivorship of
ewes in the young age classes is enhanced. It has been inferred from
knowledge of domestic sheep and European ibex that reproductive output
for mountain sheep ewes probably peaks at about six years of age (Geist,
197lb). Middle-aged ewes should, therefore, channel all available energy
into reproduction if maximum lifetime reproductive output is to be
achieved (Murphy, 1974). As age continues to increase, however, recovery
from the physiological stresses of reproduction becomes more difficult
each year. Consequently, one would expect increasing mortality rates
and/or decreasing productivity in the older age classes. The available
evidence suggests that both occur after about age 8-10.
42
Mortality data from both the Nelson's bighorn and Dall sheep pop-
ulations noted previously indicates that age-specifie mortality rates
begin to increase when ewes first reproduce, increase at a greater rate
through middle age, and achieve their highest rate of increase bètween
9 and 10 years of age (for life tables of these two populations see
Bradley and Baker, 1967). The reproductive output of female ibex was
shown by Nievergelt (1966) to decline after about 10 years of age. If a
similar decline takes place for mountain sheep, then it seems certain that
the mortality rate increases and reproductive performance wanes as ewes
enter the older age classes.
Survivorship may actually be increased as a consequence of decreased
reproductive success if barren females or those which have lost their
young have a better chance of survival than those which are pregnant or
lactating (Geist, 197lb). On the other hand, the available evidence for
caribou and muskox (~moschatus) indicates that pregnancy may have
certain hormonal and behavioral effects which increase chances for winter
survival (P. Lent, Alaska Cooperative Wildlife Research Unit, pers.
comm.).
While the physiological stresses incurred as a result of reproductive
effort probably contribute to increased mortality rates among adult moun-
tain sheep of both sexes, these stresses are usually not the ultimate
causes of death. Severa! direct causes of mortality and their importance
to the study population will be discussed in the following paragraphs.
Accidents
Accidents occur in ali animal populations and populations of mountain
sheep are no exception. Although the percentage of the population affected
L
43
is usually thought to be low, nearly every published report concerned
with the life history and population dynamics of mountain sheep mentions
the occurrence of accidentai injury or death.
While accidents reported from different localities have been attrib-
uted to such events as poisoning, puncture wounds, drowning, hanging,
collision with vehicles, avalanches, and freezing in overflow ice OMUrie,
1944; Smith, 1954; Spalding, 1966; Geist, 197lb), by far the greatest
number of accidents appears to involve falls. Lambs are particularly
vulnerable to falls during the first month of life because, although
they may be more agile than older sheep, they lack experience in traversing
the precipitous terrain. Among adult sheep, most falls probably occur
during the rut when aggressive behavior and chasing are common. Also,
males travel extensively between bands at this time when ice and snow
reduce footing to its poorest.
Evaluation of the importance of accidents as a mortality factor from
remains found in the field is difficult since accidentai death is easily
confused with death from other sources. Sorne falls, for instance, may be
caused by weakness or dizziness resulting from pathological conditions
(Smith, 1954). Sick sheep also tend to bed at the base of a cliff where
the cause of death may be interpreted as a fall (Welles and Welles, 1961).
Other sheep which do die from accidents may later be fed upon by predators
and the results interpreted as death from predation.
Only three instances of accidentai injury or death have been observed
in the Atigun area, all apparently involving falls. Andersen (1971) dis-
covered a dead ewe on July 3, 1970 lying on a steep grassy slope with its
head turned back in an abnormal position. He concluded it had probably
fallen while running and had broken its neck. Andersen also saw a lamb
become injured in 1970 while fleeing from a helicopter (pers. comm. to
D. Klein, Alaska Cooperative Wildlife Research Unit). On June 27, 1973
D. Allen observed a lamb with a bad limp in one rear leg which was ap-
parently sustained in a fall. It was seen only once thereafter and
probably either died or recovered from its injury and became indistin-
guishable from other lambs.
Disease and Parasites
44
Although a great deal of information has been compiled on the dis-
eases and parasites of mountain sheep, most of it pertains to Ovis ~
densis. The pathology of Ovis dalli has, by comparison, received very
little study. Severa! diseases have been detected in Aiaskan Dall sheep
(Ericson and Neiland, 1973), but no reports of diseased sheep have come
from the Brooks Range. Rausch (1951) found no sign of disease or hel-
minth parasites among six sheep examined from Anaktuvuk Pass region and
no instances of gross pathological conditions were observed during the
present study. The Atigun population thus appears to be in generally
good health with the exception of a few minor anomalies.
The most common anomaly noted was broken homs. This was observed
among severa! ewes and rams but most homs break above the terminus of
the horn core and do not cause further complications. One ewe was also
seen with a horn which grew almost lateral! y from the top of her head.
The left jaw of a mature ewe was found with the third premolar
missing. The tooth may never have been present or was lost at an early
age as the cavity in which it should have fitted was not fully developed
and the first two premolars were slanted backwards leaving no gap between
them and the first molar. The teeth and jaw otherwise appeared in good
.....
45
condition. A similar condition, but involving the second premolar, has
been reported in a significant proportion of Ovis canadensis nelson~ and
is more prevalent in ewes than in rams (Bradley and Allred, 1966).
Hemming (1969) also found the second premolar missing in 5 of 63 Dall
sheep examined. It should be noted here that examination of many sheep
jaws from the Atigun area has revealed no trace of bene necrosis which is
common among sheep in certain areas of Alaska.
One of the eight ewes which was followed by a lamb in 1973 still
retained most of her winter coat after all ether sheep had nearly com-
pleted molting. Her delay in shedding could possibly have been due to
pathological conditions.
Hunting
The number of sheep hunters in Alaska and the harvest of Dall sheep
rams have risen annually for several years. This increase in hunting
pressure has prompted many hunters to search for less accessible sheep
ranges where greater numbers of harvestable rams remain. The Brooks Range
has traditiona1ly been one of these areas of 1ow accessibi1ity, and during
past years the sheep hunting season has begun ear1ier there and bag limits
have at certain times been greater than in the rest of the state. Prob-
ably as a result of this, hunting pressure has increased at a greater rate
in the Brooks Range recent1y than over the state as a whole.
The Atigun study area lies in Game Management Unit 26 where the har-
vest of 3/4-cur1 or Iarger rams has risen from 47 in 1970 to 149 in 1973,
a 217% increase. The exact kil1 of rams from the study area cannet be
determined, but can be estimated from harvest tickets returned by hunters
to the Alaska Department of Fish and Game. The reported numbers of sheep
killed during 1970-1973 in subunits 2606 (Atigun River drainage) and
2607 (Sagavanirktok, Ribdon, and Lupine River drainages) are shown in
Table 7. From these statistics, an annual harvest since 1970 of S-8
sheep from the study area seems probable.
46
With the beginning of major construction activities for the Trans-
Alaska pipeline in sight, the Commissioner of Fish and Game, at the re-
quest of the Alaska State Legislature, issued an emergency order on
February 21, 1974 closing all big game hunting for five miles on each
side of the pipeline corridor north of the Yukon River. This closure is
aimed at avoiding undue impact on big game species during pipeline con-
struction and will continue indefinitely. Approximately one-half of the
Atigun study area has been affected, hence the sheep harvest from this
area will probably be sharply reduced. Most hunters have in the past
entered the study area from the Atigun valley where access by float
plane is good. Hunting will still be permitted in the eastern portion
of this area, but access in the upper Sagavanirktok drainage is limited
to two lakes of marginal size for float planes.
Predation
Potential Dall sheep predators in the Brooks Range include the
golden eagle, raven, wolverine, grizzly bear, red fox and wolf. Of these,
the eagle, raven and fox, because of their small size, would be largely
restricted to attacking young lambs. The lambing season and several
months following, however, is a time of food abundance for these three
predators, with ground squirrels, other rodents, and carrion making up
the bulk of their diet. Because of the relative difficulty of finding
and. killing lambs as compared to finding other food items, predation by
these species must be very insignificant. There have been documented
.loo..
Table 7. Harvest of Dall sheep rams from the study area and vicinity.
Year
1970
1971
1972
1973
1Refers to the Atigun drainage.
2606 1
8
5
7
5
Subunit
2607 2
4
5
1
6
2Refers to the Sagavanirktok, Ribdon, and Lupine drainages •
47
48
cases of ali three species attacking ungulates, but in no instance have
they become an important source of mortality.
The wolverine is a scavenger-predator weil known for its ferocity.
While there are no documented accounts of wolverine attacking mountain
sheep, Guiguet (1951) observed an unsuccessful attack on an adult moun-
tain goat. Considering the low population density characteristic of the
species, however, it seems unlikely that wolverine could be an important
predator of Dall sheep. Only one wolverine sighting occurred during the
present study.
Grizzly bears are the largest carnivores in the area, and if they
could catch sheep they could easily kill them. Healthy sheep of all ages,
however, can probably easily avoid bears. The behavior of both species
reflects this and they usually act with indifference towards each ether
(Sheldon, 1930; Pitzman, 1970). At !east nine, grizzlies frequented the
study area during the present study but no grizzly-sheep interactions
were noted.
The wolf, in contrast to ether predators, is capable of having major
influence on sheep populations. The effectiveness of wolves as sheep
predators is weil documented. In Arctic Alaska they usually hunt in
packs averaging about 4-6 individuals (Stephenson, 1974). but single
wolves have been observed to kil! seemingly healthy shee~ in their normal
habitat with apparent ease (Heimer, 1972).
Between 1970 and 1973, wolf numbers on the Atigun study area appear
to have increased dramatically, probably as a result of the prohibition
of aerial wolf hunting in 1970. Wolves were seen only once each by R.
Andersen in 1970 and R. Priee in 1971, but were observed frequently in
1973. During the summer of 1973, two wolf dens in the vicinity of the
l
49
study area were occupied. One was near the west bank of the Atigun
River two miles south of Galbraith Lake and contained five pups. The
other, on the east bank of the Sagavanirktok River four miles downstream
from Atigun Canyon, contained the litters of two females with a total of
15 pups (T. Hill, Galbraith Lake camp, pers. comm.). In addition to
these, six other wolves which were identifiable by their coloration were
observed several times. The local wolf population at this time, there-
fore, numbered at least 31 (20 pups + 3 adult females + 2 adult males +
6 other wolves).
The diet of wolves seems to be governed largely by the availability
of their various prey species. In the Atigun area during winter, sheep
are apparently taken often, moose occasionally, and caribou when avail-
able. During summer, ground squirrels and other small mammals are abun-
dant and easier to catch than larger species and thus make up a major
portion of the diet. Wolves were frequently sighted capturing ground
squirrels during the summer of 1973, but no attempts at killing large prey
were seen. This same pattern of seasonality in the diet was observed by
Murie (1944) in Mt. McKinley National Park.
The wolves studied by Kelsall (1968) in northern Canada traveled
with the caribou herds in winter, probably because this was their only
major food source. On the Atigun study area, caribou are present sorne
winters and absent others. Wolves, however, remain in the area each win-
ter, although their numbers may not be as high in winter as in summer.
Their year-round existence there probably depends on the availability of
alternate prey species when caribou are absent. Sheep and a few moose are
always present on the study area and constitute acceptable alternate
prey species.
50
Since moose densities north of the Brooks Range are comparatively
low (about 10 winter on the study area), fewer are killed by wolves than
in areas where moose densities are higher. The fresh remains of a cow
and calf were found in Atigun Canyon during April, 1973 and the very old
remains of two others were found later that summer. Sheep remains, while
not ali wolf kills, were ~ch aore abundant; a total of 86 were found
between 1970 and 1974.
The ultiaate cause of death of an animal is almost impossible to
establish with certainty when only its skull and homs, a few bones, or
hair are found. It may have been killed by predators or it may equally
well have died from other causes before being eaten by predators. Sev-
era! points suggest, however, that wolves kill a high percentage of the
sheep which die on the study area. These include the apparent low level
of disease in the population, the generally good condition of the denti-
tion in skulls found, the high level of the wolf population, and the
scarcity of other major prey species during most winters.
A high level of wolf predation on sheep in the Atigun area is fur-
ther suggested by the location at which the sheep remains were found.
Most remains were discovered around the periphery of winter range. More
precisely, most sheep die or are killed near mountain bases on relatively
gentle slopes away from escape terrain. Escape terrain is an essential
element of sheep habitat, but sheep frequently move some distance from
the cliffs and rock outcroppings during winter as grazing may be better
around the margins of the steeper slopes (see Fig. 6). lt is here that
sheep are most easily caught when surprised by wolves. If wolf predation
were not a major cause of mortality among these sheep, most remains would
probably be found in more rugged terrain.
51
The wolf population seemed markedly lower during field trips to the
study area in April and June, 1974 than during the previous sUDDer. It
is not known, however, whether this was a reflection of aortality or of
movement out of the area. Several instances of rabies were discovered
among the local fox population by construction personnel during the spring
and some wolf mortality from the disease aay also have occurred. In-
creased construction activities associated with the Trans-Alaska pipeline,
however. may equally well have caused sœe of the wolves to aove to less
disturbed areas. In any case. predation on sheep seeaed less during the
winter of 1973-1974 than during the previous winter. Only three fresh
sheep remains were found during the two weeks spent on the study area in
1974 while eight were found during a coaparable period in 1973.
Figure 6. Sheep feeding on low slopes in the spring. Most wolf predation
occurs in such locations.
Figure 7. Winter range directly across Galbraith Lake from the Alyeska
pipeline construct ion camp. Note the scarcity of snow except
in draws and ~epressions (April 2, 1973).
52
r Figure 8 .
Figure 9.
l'l'inter range us.ed by rruns. in a small drainage into the
Atigun River (Area 2) six miles south of Galbraith Lake.
(April 11, 1974)
The main lrunbing area in the northeastern portion of
Atigun Canyon.
.s;;
Figure 10. Lick A, the eroding bluff just a.bove the stream bed, is
the most heavily used mineral lick on the study area.
54
Figure 11. Lick B, also a stream-eroded bluff, is a primary lick ne ar
the eastern end of Atigun Canyon.
SEASONAL MOVEMENTS
Mountain sheep in northern climates generally do not remain in one
location throughout the year but seasonally move between various segments
of their habitat, thus optimizing their chances for survival and repro-
duction. Geist (197Ib) has shown that in some localities, particularly
the bighorn and Stone sheep ranges of Canada, rams may have as many as
seven seasonal home ranges and ewes may have up to four, while a minority
of sheep have only two home ranges, summer and winter. In the less di-
verse sheep habitat of the Brooks Range with its long winters and short
summers, the home range pattern of Dall sheep approaches the simplest of
these forms. Both ewes and rams have summer and winter home ranges, and
rams additionally visit the ewe wintering areas during the rut. Both
sexes concentrate at mineral licks during spring and summer while moving
between seasonal range units, but since the mineral licks on the study
area lie between winter and summer ranges, they have not been considered
as separate range units.
Winter Range
Although snowfall on the north side of the Brooks Range is normally
light, it is great enough to concentrate sheep within a small portion of
their total range for severa! months of the year. The se areas of winter
concentration have certain characteristics in common which permit sheep
to survive the cold, dark winters.
55
56
First, winter range must occur in areas of light or no snow accum-
ulation so that sheep can obtain forage with a minimum expenditure of
energy. Evidence indicates that vegetation covered by a thin layer of
soft snow may be preferred to exposed vegetation since the former is
protected from desiccation and nutrient leaching by the snowcover (Nichols,
1973) . The combination of snow depth and snow hardness impose certain
limits, however, beyond which pawing for vegetation becomes energetically
uneconomical. Secondly, assuming snow depth is not limiting, winter
range must show an adequate growth of vegetation during spring and summer
to meet the forage requirements of sheep from at least September through
May. And thirdly, terrain within the winter range must be sufficiently
rugged to permit escape from predators.
Several factors may contribute to the existence of snow free areas
in the Brooks Range during winter. Direction of exposure is important
since insolation on south-facing slopes delays snow accumulation in fall
and hastens melting and regrowth of vegetation in spring. Furthermore,
sunlight on southern slopes in early spring significantly increases tem-
peratures near ground level, benefiting sheep at a time when survival
may be most difficult, i.e., when they have lost their fat reserves and
when range conditions are at their worst.
A second important factor in limiting snow accumulation is steep-
ness of terrain, Snowfall is less per unit of ground surface on steep
slopes and also may cling to these slopes less readily. This factor may
be quite significant in the rugged cliff terrain in which sheep are often
found and may increase the amount of available escape terrain on winter
range.
A final and probably the most important factor contributing to the
l
57
absence of snow on winter range is exposure to wind. The continuously
cold temperatures of the area result in snow which is extremely light· and
easily blown about. Ridgetops and sideslopes which are repeatedly blown
free of snow may provide good grazing for sheep throughout the winter
months. Where drifting snow tends to collect, however, wind contributes
to snow compaction, making these areas of little use to sheep.
Fortunately, the factors contributing to an absence of snow often
exist simultaneously at the same locations. That is, south-facing slopes
on the study area generally receive the most wind in winter, and the
steeper of these receive greater insolation due to a more nearly perpen-
dicular angle of incidence at that time of year. There is, therefore,
a considerable amount of exposed grourid with drifted snow prevalent in
draws, depressions and canyon bottoms (see Figs. 7 and 8). Exposed
ground, however, does not imply an abundance of suitable winter range
because adequate vegetation is frequently lacking. This is especially
true on talus and seree slopes and on ridgetops above 5,000 feet where
vegetation is extremely sparse. Escape terrain is also generally abun-
dant throughout the mountains, but the cliffs and rock outcroppings which
provide the best escape cover usually contain less vegetation than the
more gentle slopes.
Figure 2 depicts the locations within the study area where concen-
trations of sheep were observed in winter. These areas also represent
the major locations meeting the qualifications of limited snow accumul-
ation, adequate vegetation, and good escape terrain. Area 1, Atigun
Canyon, is the major wintering area on the study area and is used
primarily by ewes and young sheep. This range presently supports a
population of about 175 sheep, most of which winter on the north side of
58
the canyon. Area 2 is a wintering area used almost exclusively by rams
and contains about 25 animais. Areas 3 and 4 are used primarily by rams
but also contain ewes. The sexes generally remain separated, however,
except during the rut. An estimated 30 sheep currently winter here.
In addition to these areas, similar populations of sheep exist out-
side the study area across the Atigun and Sagavanirktok valleys. There
are also patches of habitat scattered throughout the study area which are
not choice winter range but which receive occasional winter use by small
numbers of sheep, especially during winters of light snowfall. Three such
areas where sheep have been observed in winter are indicated in Figure 2
by dashed lines. However, there are undoubtedly many other areas which
receive occasional winter use.
Spring Movements
The coming of spring brings dramatic changes to the Brooks Range.
During May, the daylight period increases by about eight minutes each
day, temperatures rise above freezing for the first time in severa!
months, and the snowcover begins to recede. At this time the sheep move
down from the windblown ridges to feed on vegetation which is made avail-
able by melting snow. Although plants do not produce new growth for a
few more weeks, the vegetation which has remained frozen under the snow
all winter is of higher quality than that which has been exposed to the
desiccating and nutrient-leaching effects of dry winds, greater temper-
ature fluctuations, and intense light conditions. Near the end of May
the vegetation on the low south-facing hills above the Atigun River
beg~ns to show new green growth and offers choice grazing. Within a few
days, however, vegetation at the higher elevations on the north side of
the canyon also resumes growth and sheep move back up the slopes where
escape terrain is more abundant and where the ewes can find isolation
for lambing.
59
Published lambing dates for Dall sheep fall between May 5 and June
28 (Hemming, 1969) with the greatest frequency of births usually occurring
in the last week of May. On May 20-22, 1970, R. Andersen counted 19
lambs in a ground survey of Atigun Canyon while R. Priee counted 11 lambs
in a similar su~ey June 2-3, 1971. The first lambs seen in 1973 were
two 1 found on May 28 which were judged to be less than 24 hours and 48
hours old. However, only eight lambs were seen during the entire summer.
In 1974, lambing seems to have occurred earlier than usual with one lamb
reported as early as May 7 (E. Ludlow, Bureau of Land Management, pers.
comm. to D. Klein, Alaska Cooperative Wildlife Research Unit). When 1
conducted a survey of the area on June 8 the lambs appeared quite large
and many had moved well away from the lambing areas.
The lambing areas in Atigun Canyon are shown in Figure 2. The
boundaries of these areas were established by plotting all lamb sightings
through the first week of June in the years 1970-1974 and outlining those
areas which showed definite concentrations. Some movement of lambs away
from lambing areas occurs before June 7, but outlining only the areas
where sightings were concentrated probably minimizes the error created by
these movements. It is apparent that there are two major lambing areas,
both within winter range on the north side of the canyon. One of these
is the southwest slope of Black Mountain directly across Galbraith Lake
from the pipeline construction camp. This area consists of severa!
shallow draws with numerous rock outcroppings which provide good escape
cover. Sheep can be found there throughout each winter, but lambing
apparently occurs on this area only during good lambing years. New lambs
60
were observed there during 1970 and 1974 when large lamb crops were pro-
duced, but no lambs were seen there in 1971 or 1973 which were poor
lambing years.
The second lambing area is located in more rugged terrain in the
lower half of the canyon (Fig. 9). New lambs were observed there every
year of the study and in greater numbers than on Black Mountain; thus
this seems to be the most important lambing site on the study area.
The area around Guard House Rock (see Fig. 1) has been reported by
construction personnel as a good lambing area (Andersen, 1971), but no
new lambs were recorded there in 1970, 1971, or during the present study.
While a few lambs may be born at locations throughout winter range, the
majority of lambing occurs on the two areas just described.
Mineral Lick Use
Concurrent with the lambing season, sheep begin to show increasing
interest in the severa! natural mineral licks which are located on the
study area. The location and a description of these licks are given in
Figure 12 and Table 8 respectively. Two basic categories of mineral
licks exist in this area. Those which receive the greatest volume of
use and which seem to be preferred because the sheep travel relatively
long distances to reach them have been designated "primary" licks (A and
B in Fig. 12; see also Figs. 10 and 11). Licks which receive only oc-
casional light use or licks which are used as substitutes when the pri-
mary licks cannat be reached have been designated "secondary" licks
(C-H in Fig. 12).
Mineral licks are apparently used by sheep to satisfy bath nutri-
tional and social needs. It has been generally assumed that licks
61
Figure 12. Mineral licks and travel routes utilized by Dall sheep
on the study area (map from U. S. Geological Survey,
Philip Smith Mountains Quadrangle, 1956).
Legend:
A, B
C,D,E,F,G,H
primary mineral lick location*
secondary mineral lick location*
sheep,trail
other major travel route
*Letters refer to lick descriptions in Table 8 and
the text.
62
,..----------------~~----------~~--~--------
Table 8. Mineral licks used by Dall sheep on the study area.
Lick Designation Location
Primary Licks
A
B
Secondary Licks
c
D
E
S. side of Atigun Canyon 2-1/2
mi. from W. end; adjacent to
tributary stream at 2700' elev.
S. side of Atigun Canyon 1-1/2
mi. from E. end; adjacent to
tributary stream at 2400' elev.
W. side of Sagavanirktok valley
6 mi. S. of Atigun Canyon; ad-
jacent to tributary stream at
2700' elev.
N. side of Atigun Canyon oppo-
site lick A; elev. 2600'.
N. side of Atigun Canyon 1/2
mi. E. of lick D; elev. 2600'.
Description
Moist cutbank of the
black clay and shale
SOxlSOm. White crusting
of minerais when dry.
Moist cutbank of black
clay and shale 75x200m.
White crusting of min-
erais when dry.
Moist cutbank of black
clay and shale 75x50m.
Black clay and shale
on cutbank of river
SOxlOOm.
Black clay and shale
on cutbank of river
SOxlOOm.
Extent of use
Most important lick on
study area. Heaviest use
in late June with up to
70 sheep observed at one
time.
Nearly as important as
A. Heaviest use during
late June.
Visited only once during
summer. Use appears less
than A or B. Probably
used as sheep move toward
summer range.
Used during mid-June when
river is too high for
sheep to reach lick A.
Used during mid-June when
river is too high for
sheep to reach lick A.
"' "'
Table 8, continued.
Lick Designation Location
F N. side of Atigun Canyon 1/4
mi. E. of lick E; elev. 2500'.
G S. side of Atigun Canyon 1/4
miE. of lick A; elev. 3400'.
H Headwaters of tributary 6 mi.
S. of Galbraith Lake; elev.
4800'.
Description
Black clay and shale
on riverbank lOxlOm.
Black clay and shale
on sideslope lOxlOm.
Cutbank of black clay
and shale on small
stream, 30xl00m.
Extent of use
Of minor importance.
Used only once in late
May by one 1/2-curl ram.
Of minor importance.
Used only once in mid-
August by two 1/2-curl
rams.
Visited only once on
ground, but high use ob-
served from air. Sur-
rounded by well-worn
trails. Possibly a major
summer range lick.
"' ~
......
----------------~~~--------~------~-----~
65
provide a source of minerais that are not available in sufficient quan-
tities in forage plants at a time when nutritional demands of the sheep
are highest. Socially, licks faci1itate the mixing of ram and ewe bands
at a time when the sexes generally remain segregated. Geist (197lb) has
shown that young rams leave the ewe bands at 2-4 years of age, join ram
bands, and establish permanent home range patterns. Mixing of ewe and
ram bands at mineral licks may play a vital role in the initiation of
young rams sti11 attached to ewe bands into the social structure of ram
bands. Heimer (1973) sees this function of mineral licks as preventing
genetic drift among isolated groups of rams and ewes through the chance
exposure of young rams to any one of severa1 ram groups.
The timing of mineral lick use varies somewhat from year to year but
generally begins in 1ate May and reaches a peak sometime in June. On
May 23, 1973, there were fresh tracks in lick A and a ram was seen eating
soi1 at lick F on May 30. Both primary licks were receiving moderate use
by June 10 in both 1973 and 1974. In 1973, use of the primary licks was
limited to sheep on the south side of Atigun Canyon during most of June
as the Atigun River was unusual1y high and rapid from snowmelt and spring
rains. On numerous occasions, sheep on the north side of the canyon came
down to the river as if checking to see if they could cross but then re-
turned to the higher s1opes. This was somewhat unexpected since bighorn
sheep are known to regularly cross such large and turbulent streams as
the Salmon River in Idaho (Smith, 1954) and the Athabasca River (Cowan,
1940) and Churn Creek (Sugden, 1961) in Canada. While confined to the
north side of the canyon, sheep regularly made use of secondary licks
D andE. During the last week of June, however, the river dropped
slightly and sheep began crossing in great numbers. Even at this time
66
swimming the river appeared difficult. Sheep always seemed hesitant to
enter the water, standing nervously for long periods at the riverbank
or walking up or down stream severa! hundred yards looking for a better
place to cross. Once they decided to enter the water, however, they
leaped in with a splash and swam across with a bounding motion. The
swift current carried them downstream nearly twice as far as the river's
width. Although the river crossing appeared difficult, many sheep did
not remain on the south side of the river after using the licks, but
returned to the north side for a day or two before permanently crossing
the river again. The two favored crossing sites were near both of the
primary mineral licks; however, sheep occasionally cross the river at
other points between these two locations.
The peak in lick use during 1973 occurred near the end of June
shortly after sheep began crossing the river. The situation in 1974,
however, was somewhat different. At that time the river was much lower
than during the previous spring and could easily be waded. Sheep were
observed crossing as early as June 9 and were crossing in large numbers
by June 12. Use of lick A was moderate to heavy by June 13 when obser-
vations ceased and it appeared that the peak in lick use would occur
within a few days. A similar variation in the timing of the peak in
lick use has been found at the Dry Creek lick in the Alaska Range. During
three years the day of maximum use occurred on June 6, June 19, and June
27 (Heimer, 1973). The apparent reason for this variation is '7he
manner in which winter snows disappear, the conditions of warming in
spring, and immediate weather conditions" (Heimer, 1973 :39).
Ali mineral licks on the study area appear to be of the same soil
type. The soil in these areas is characteristically black in color and
-
67
has a clay texture with an abundance of soft black shale throughout.,
Many lick areas, especially those most heavily used by sheep, are moist
from ground water seepage. However, dry areas are also frequently
licked by sheep. Such dry areas often show a light encrusting of white
mineral salts left by the evaporation of water.
To determine the particular mineral or minerais sought by sheep at
these licks, a chemical analysis of water soluble salts was made on a
soil sample collected from a heavily used spot in lick A. For compar-
ative purposes, a soil sample was also analysed from wintering area 2
(see Fig. 2). The soil in this area is distinctively red in color, so
much that the sheep and even the ptarmigan seen there in'spring appear
pink rather than white in color. No licks have been found within this
area; however, '~ink" sheep were seen on severa! occasions at lick A.
lt is likely therefore, that sheep wintering on area 2 come to lick A
in the spring to obtain minerais they could not get in as great an ahun-
dance on their winter range.
Table 9 shows the results of the analysis of these two soil samples.
The non-lick soil was acidic (pH 5.8) while the lick soil was slightly
basic (pH 7.5). This same relationship (pH higher in lick than in non-
lick soils) was observed for 18 lick and non-lick soils in western
Montana (Stockstad et al., 1953). Of the eight elements for Which
analysis was made, five showed greater concentrations in the non-lick
soil. The three remaining elements, magnesium, zinc, and potassium,
were only slightly higher in the lick soil. It is unclear from these
results what substance is being sought by sheep at this lick. Sodium,
for which analysis could not be made on these samples, was found to be
the primary element sought by bighorn sheep at mineral licks, mineral
Table 9.
Sample
Location
lick A
Non-lick
Chemical analysis of soils from a lick and a non-lick area.
Element 1
pH Ca Mg Mn Zn K Fe p N
7.5 0.012 0.158 0.000 0.004 0.480 0.100 0.068 0.198
5.8 0.020 0.136 0.006 0.002 0.460 0.125 0.125 0.350
1values represent the average of two trials and are recorded as the percent of water soluble salts
extracted at a pH of 7.0.
o-
00
~
-· --~--~---·--~--~~----~. -------~---~--·-······-----------,
69
"cafeterias" and in soil impregnation tests (Stockstad et !.!_., 1953;
Smith, 1954). The soil in the Atigun licks is not salty to the taste
and, therefore, sodium chloride is probably not present in abundance.
Other sodium compounds, however, may be the substances sought. Cowan
and Brink (1949) also suggested the importance of trace elements in
mineral licks. Copper, iodine, cobalt and selenium have been found
necessary for normal reproduction in domestic ewes (Millen, 1962;
Streeter, 1970), but no analysis was made for these elements in the
Atigun soil samples. Clearly, no definite conclusions can be drawn from
the limited analysis of only the two soil samples collected. Others
who have made much more extensive,analysis of mineral licks, however,
also have reached no definite conclusions. ·~o arrive at conclusive
results, methods such as preference tests will probably be more produc-
tive than chemical analysis of the lick soils" (Cowan and Brink, 1949).
The mineral "cafeteria" and soil impregnation experiments of Stockstad
et al. (1953) and Smith (1954) have shown this to be so.
Sheep normally remain in the vicinity of the primary licks for a
few days before moving on to summer range. Their time is divided during
this period between visiting the licks and grazing and resting in the
bills nearby. When moving to the licks, sheep appear quite anxious and
determined, frequently running for the last severa! hundred yards. Warm
sunny weather seems to stimulate .an increase in lick use (Heimer, 1973
and pers. obs.), but sheep were observed on occasion licking vigorously
in a steady downpour of rain.
Table 10 shows the average time spent per lick visit by each sheep
class. In a large proportion of lick visits (27 of 62 observations),
sheep spend an extended period in the licks, then graze or rest above
Tablé 10. Duration of mineral lick use by Dall sheep in Atigun Canyon, June-August, 1973.
Mean No. Minutes per Visit
Classification lst Period 2nd Period Total Visit
lambs 12.2 (n=9) 20.0 (n=l) 14.4 (n=9)
yearlings 72.2 (nell) 20.0 (n=l) 74.0 (n=ll)
ewes 79.6 (n=30) 22.1 (n=l7) 92.1 (n=30)
1/4-curl rams 75.3 (n.,ll) 17.5 (n=4) 81.6 (n=ll)
1/2-cur1 rams 90.2 (n=4) 20.0 (n=2) 100.2 (n=4)
3/4-curl rams 93,0 (n=2) ---93.0 (n=2)
full curl and larger rams 106.2 (n=4) 31.0 (n=3) 129.5 (n=4)
Average (exc1uding lambs) 80.1 (n=62) 22.1 (n=27) 90.0 (n=62)
.....
0
~-. ..J ---.
71
them for 15-30 minutes, and finally return to the licks for a short~r
session of licking. In such cases, the first licking session was termed
the "first period" and the second session the "second period." The total
of both periods constituted one lick visit. The time recorded was the
total time spent in the lick minus any noticeably long intervals of
standing or lying without licking. Such behavior is relatively rare
since sheep have been found to spend 93-95\ of the time they are in licks
actually eating soil (Heimer, 1973). Only lambs spend a significantly
high amount of time in licks without licking, but they do lick for short
periods of time.
No significant difference (p< 0.05) was found in the time spent
licking by adult ewes and adult rams (two years and older) in either the
first (t=l.38) or second (t=O.l8) periods (see Fig. 13). Adult ewes
spent an average of 79.6 minutes (n=30) in the first period and 22.1
minutes (n=l7) in the second period for an average visit of 92.1 minutes.
Adult rams spent an average of 85.7 minutes (n=21) in the first period
and 22.6 minutes (n=9) in the second period for an average visit of 95.4
minutes. The similarity of ram and ewe lick times in 1974, however, may
have been an unusual occurrence resulting from an unusually low lamb:ewe
ratio (10: lOO) that year. Heimer (1973) stated, "lt appears that there
is no difference in lick use between ewes which are not lactating and
rams," but that "ewes that are nursing lambs appear to spend more time
licking."
One facet of lick use which has not been previously reported is
the relationship of age to time spent in mineral licks. While my data
on this subject are sparse, they suggest that lick time increases with
increasing age, at least for rams. Although it is difficult to precisely
100-
so-Ewes
60-
fil
0 > 40-~
Il ~ c Cl)
a.. ..
Cl) -~ 20-c
2
0
1'-
ii c
20-....__
40-
Ra ms
~
C\1
Il c
en
Il c
1..-.-
lst Period
2nd Period
Figure 13. Mean duration of mineral lick use for ewes and rams two
years of age or older.
72
l
73
age living rams in the field, an indication of age is given by horn
curl in degrees or quarters of a circle. Figure 14 illustrates the
relationship between age and horn curl for the horns of dead sheep found
on the study area and for a few living sheep which could be approached
closely enough to age with the aid of a spotting scope. Since horn curl
increases throughout life, it has been used as an index of age in
Figure 15 in order to relate age to lick time. The data available clearly
indicate that lick time increases with increasing horn curl, and thus
presumably age, for all ram classes. Such a relationship may hold true
for ewes also, but would be impossible to determine without known-age
animals. The significance of this relationship is difficult to evaluate
because no information is available on the frequency of lick use by the
various age classes. Assuming that licks are visited with the same fre-
quency by young and old rams, however, it is possible that older rams
with more massive bodies require lick visits of longer duration than
younger, smaller rams in order to offset mineral deficiencies built up
during winter.
Following the June peak in lick use, the number of sheep seen at
mineral licks in Atigun Canyon drops rapidly. After the first week of
July, 1973, practically no mineral lick use was observed for several
weeks; however, the number of sheep remaining in Atigun Canyon at this
time is also very low. An occasional sheep band drifts in from summer
range to use the primary licks in mid-summer, but it is presumed that
most lick use at this time occurs at small licks scattered throughout
summer range. Licks C and H (Fig. 12 and Table 8) may be examples of
such summer range licks. Lick use may also decrease during mid-summer
because mineral deficiencies existing after several months spent on
...... ., ...
c
tl >-
/ ., ., .,
n=8
1/4 1/2
1
1
1
1
1
1
1
1
1
1
1
1
/n=~O ., .,
" .,
" ., n=9
-Meon
---Ronge
3/4 4/4
1
1
Horn Curl (Quarters of a Circle)
1n=S
1
4/4+
Figure 14. Relationship of horn curl to age for rams from the study
population.
74
uo~JD:»U!ISDI:> paJDiay-aDv
• ~ ' ~ ~ ~ ~ N
1 1 0
1 1 0 0 ~ 0 0 c: c: c: c: ... ... ~ ... ... Cl
J:
09 • 0
~
II=U : ,
• .,
08 0 o.
c:
3aU n
~
-1
001 3 •
y:U ~
~ c: .. •
021 !!.
SL
76
winter range have been satisfied by that time.
Summer Range
Seasonal shifts from winter to summer range have been found to vary
for different mountain sheep populations from no movement at all (Sugden,
1961) to as muchas 40 miles (Smith, 1954). In most cases, the distance
traveled appears to depend on the distance between suitable winter range
at lower elevations and alpine habitat utilized for summer range at
higher elevations. When the terrain is such that the two habitat types
are contiguous, seasonal movements generally are short. But when the
two are separated by greater distances, the annual migration may encom-
pass many miles.
In many mountain sheep populations studied to date, especially big-
horn populations, the habitat consists of relatively small sections of
winter and summer range separated by broad valleys and forested areas
unsuited for mountain sheep use. These sections of habitat are generally
connected by an elaborate pattern of traditional migration routes over
which sheep move from one portion of range to another.
Sheep habitat is more continuous in the Brooks Range, however,
being broken only by treeless valleys of relatively narrow width. Winter
range on the study area is actually just a restricted portion of the total
range used in summer. The movement of sheep in this area, therefore,
need not be thought of as a migration in the usual sense, but rather is
best described as a drift from or expansion of winter range as snow melts
and vegetation begins to grow on the higher slopes. Because the area
occupied by sheep in summer is much larger than the winter range, how-
ever; sorne sheep may move as much as 20-25 miles between seasonal range
units.
1
77
For practical purposes summer range may be defined as the area
south of Atigun Canyon to the continental divide since only a very few
sheep (8 in 1971 and 1973) remain in Atigun Canyon all summer. At any
given time there is a much greater likelihood of finding sheep on specifie
portions of this area than on other portions, but sheep may visit any
portion of the range at one time or another during the summer and the
entire area must, therefore, be considered summer range.
Between Atigun Canyon and the continental divide, the movement of
most sheep is restricted by the Atigun River on the west and the Saga-
vanirktok River on the east. These are not impenetrable barriers, how-
ever, since sorne sheep do cross these valleys. On June 16, 1973, D.
Allen watched a band of six sheep cross the Atigun River south of Galbraith
Lake at a point where the valley is approximately three miles wide. Sur-
veyors at the Galbraith Lake camp also told me of finding several sheep
remains in the center of the Atigun valley, indicating that sheep cross
it not infrequently during the summer and perhaps during the rut. During
July, sheep were also seen at the extreme crest of the Brooks Range.
Mixing of sheep populations and joint use of summer range with sheep
west of the Atigun River, east of the Sagavanirktok River and south of
the continental divide, therefore, almost certainly occurs.
Emigration or immigration could easily take place under these cir-
cumstances, but since there have been no large unexplained fluctuations
in the population level since at least 1970, movement of sheep into or
out .of the population has probably been negligible. Mass emigration and
immigration of sheep in other areas have been known to occur on rare
occasions when the habitat is subjected to catastrophic conditions (see
Geist, 197lb:97), but traditional patterns of movement are normally
1
!
78
adhered to and fidelity to seasonal home ranges is high (Geist, 197lb;
Heimer, 1973). A possible exception is the Dall sheep population of
Mt. McKinley National Park which may rely more on a strategy of oppor-
tunism than on tradition in the use of habitat (Murphy, 1974).
Southward movement from Atigun Canyon onto summer range begins
immediately after the late-June period of intensive lick use. Since the
majority of sheep wintering in the canyon visit one or both of the pri-
mary mineral licks located there in late spring, movement onto summer
range seems to radiate from the lick sites by weil established routes.
From lick A (see Fig. 12) sheep normally move either through a notch in
the cliffs behind the lick and then southward along the east branch of
the small stream which flows by the lick or else they move westward from
the lick and around Guard House Rock. In both cases weil worn trails
have been established in the talus slopes and vegetation. Sorne sheep
were also seen moving eastward from this lick and then up the stream
which passes lick B.
From lick B sheep generally move southeastward to the rounded
mountain nearest the lick, then through a low pass and up the Sagavan-
irktok valley and its tributaries. Sorne sheep also move up the stream
flowing past this lick and across two miles of flat terrain until they
reach larger mountains.
Figure 12 shows the major movement patterns of sheep to and away
from mineral licks as weil as other established trails and travel routes.
The movement of sheep to summer range and their dispersal within
it appears rapid in the sense that they frequently cover severa! miles
in.a day. But it is in no way comparable to the seasonal movement of,
for instance, caribou. The seasonal movements of Dall sheep normally do
not appear direct or hurried since they feed and rest frequently. Be-
cause they often retrace their steps while feeding, their route is not
necessarily linear.
79
The distribution of sheep on summer range is more closely governed
by elevation than by horizontal distances. Their movements may be de-
scribed'as an upward drift in elevation which begins at the main mineral
licks and generally follows the slopes of small side valleys and tribu-
tary streams. By late summer many sheep are found near the level of the
remnant glaciers remaining on the study area. Figure 16 illustrates the
elevational shifts made by Dall sheep bands during the summer months.
The minimum average elevation, 3200 feet, occurs in late June during the
period of heaviest lick use. The maximum average elevation, 3900 feet,
occurs during mid-July. It should be noted, however, that these figures
represent only average elevations, and sheep were seen at elevations
ranging from about 2500 feet to 5000 feet during the entire summer.
The causes of seasonal migration among mountain sheep have been
the subject of discussion by many who have studied these animais. Blood
(1963), Murie (1944), Smith (1954), and others have indicated that
sprouting of highly nutritious vegetation near receding snowlines is one
stimulus to spring movements. More recently Hebert (1972) has shown the
relationship between the decreasing phosphorus content of vegetation as
it ripens in summer, the increased phosphorus requirements of ewes during
lactation, and the high phosphorus content of summer range vegetation
which commences growth later in the summer than vegetation on the winter
range. Blood (1963) further mentioned increasing temperatures on winter
range in spring as a possible stimulus for migration. lnsect harassment
as a factor motivating movement has been documented for other ungulate
l
80
40
39
......
38 ...: u. ....
0
"' 37
0
0 36
r::
0 -35
c
>
4) 34
LIJ
c 33 Cll
tl
::E 32
31
A 8
;;:; 0 0 0 0 ;;:; 0 <)1 10 0 C\1 10 0 C\1 1 ..!.. ..!.. 1 1 ..!.. ~ 1 C\1 ï C\1 1 C\1 ....... ' ' ....... ~ ~ ~ ' ....... .......
10 <D <D <D co co co
Date (Month/Oays)
Figure 16. Mean elevation of sheep bands in and near Atigun Canyon
during May through August; based on 615 band observations
in 1971 and 1973. A = period of maximum lick use, B =
period of maximum dispersion over summer range.
81
species and is mentioned by both Blood (1963) and Murie (1944) as a pos-
sibility for inducing seasonal movements in mountain sheep in sorne sit-
uations. Neither author, however, felt the level of insects on his study
area was sufficient to contribute to sheep movements.
In regard to fall movements, most authors have felt that the first
snowfalls on summer range precipitate movement back to lower elevations.
Frequently, however, sheep begin to leave the summer range before snow
drives them down. Murie (1944) mentioned the deterioration of vegetation
on summer range by ripening and drying as a likely reason for the timing
of these movements.
The variety of explanations posed for seasonal migration and the
difference of opinion held by researchers seems to indicate that the
seasonable movements of mountain sheep may not be induced by any single
stimulus. Rather, there seems to be a combination of interacting factors
which induce migration. Environmental factors have usually been given
the greatest attention in research because of the relative simplicity of
correlating them with sheep behavior. But there seem also to be factors
inherent to the sheep themselves which determine the timing of movements.
Geist (197lb:95-96) has reached a similar conclusion:
[Seasonal movements] are not simply a response to snow,
forage availability, or insect pests, but are more complex
phenomena. • •• It is more probable that sheep are inter-
nally motivated to migrate, but are synchronized by external
environmental factors.
The factors which internally motivate sheep to migrate are far from
being fully understood. It is likely that there is a genetic influence
on migrational tendency which developed when sheep first began to colonize
northern environments and has been perpetuated until today in all species
living in cold climates. It is also probable that a conditioning response
82
in young sheep occurs through their association with older conspecifics
which results in migration when a correct set of environmental eues
appears. Photoperiod seems to have a major influence in determining
movements (Geist, 197lb), but all external stimuli in a sheep's environ-
ment are probably important in triggering its motivation to migrate.
The environmental eues influencing sheep in Atigun Canyon are not
unlike those affecting other sheep populations. Sheep moving from the
canyon to summer range do appear to be seeking better forage and cooler
temperatures. Contrary to what has been found true for other areas,
however, insects, especially ~squitos, seem to be important to sheep on
the study area as a stimulus for movement. Sheep seen in the canyon after
mid-June often appeared to be greatly bothered by insects while those on
the higher ridges where winds were stronger and temperatures cooler were
much less harassed.
Snowstorms at the higher elevations occur throughout summer on the
study area. The beginning of .avement back to winter range in 1973 did,
however, correspond roughly with an increase in the frequency and sever-
ity of these storms.
Fall Movements
A comparison of mountain sheep populations from different latitudes
shows a definite trend in the timing of the fall migration. Bighorns in
Idaho and Montana were first seen on winter range during mid-October to
mid-November (Smith, 1954; Couey, 1950). In southern British Columbia,
bighorns returned to winter range in mid-to late October (Sugden, 1961).
Bighorns in Banff National Park and Stone sheep in northern British Col-
umbia first appeared on their winter range during late September (Geist,
83
l97lb). In Mt. McKinley National Park, most Dall sheep return to winter
range in September, although movement may begin in August and is not
completely over until early October (Murie, 1944). These records show
clearly that the return of mountain sheep to winter range occurs earlier
as one progresses northward. lt is not surprising, therefore, that the
sheep of Atigun Canyon, which are the .ost northern population for which
information is presently available, return to their winter range at an
earlier date than noted for other sheep populations.
Freezing temperatures and persistent snowfall on summer range ends
the growing cycle of vegetation and begins a decline in mosquito popula-
tions. This may occur on the study area as soon as early August in some
years. With these occurrences, sheep begin to aove down from the higher
elevations and soon reappear in the canyon. As the number of sheep in
the canyon increases, activity at the mineral licks increases also, al-
though the level of use does not compare with the late-June period. For
a few days after drifting into the canyon, sheep bands alternately visit
the mineral licks and feed and rest in the mountains along the canyon's
southern rim. They then cross the Atigun River back to the north side of
the canyon.
The first sheep seen crossing back to winter range in 1973 were a
band of three ewes on August 18. Thereafter, small bands were frequently
seen crossing. The same locations were favored for crossing as during
June, and most sheep still appeared reluctant to swim the river, which
reaained quite high all summer. The fall crossing, as the spring crossing,
is not a one-way movement. Several sheep crossed back to use the licks
or feed on the hills above them for a few days before permanently returning
to the north side.
84
During late August and early September, the number of sheep visible
north of the river increases daily. By September 5, 1973 there were
approximately 80 sheep on the north side of the canyon. No observations
were made after that date, but it seems probable that nearly all sheep
would have returned by the end of September.
Because Atigun Canyon is primarily a wintering area for ewes and
young sheep, mature rams move into the canyon from the i~olated wintering
areas to the south prior to the November-December rut. The fact that they
do is evidenced by the number of large rams which die there during winter.
More skulls may be picked up in the spring each year than would be ex-
pected from the number of rams observed in the canyon in September or
March.
Seasonal and Annual Variation in Band Size and Composition
In classifying sheep bands, ram bands were considered to contain
only rams, ewe bands always contained ewes but could also contain any
combination of lambs, yearlings, and 1/4-curl rams, and mixed bands were
similar to ewe bands except they contained at least one ram of one-half
or greater horn curl. One-quarter curl rams were included in ewe bands
since most of these young rams were two years of age and thus had not
reached maturity. In the social hierarchy, therefore, they held approx-
imately the same position as mature ewes. Most of the 1/2-curl rams,
on the other hand, are sexually mature (although they may be excluded
from breeding by older males) and were found in greatest frequency within
ram bands. Table 11 indicates that only 14% of all 1/4-curl rams recorded
occurred in ram bands whereas 62% of all 1/2-curl rams were observed in
ram bands. By definition, therefore, 1/4-curl rams were included in ewe
Table 11. Percent occurrence of each sheep class in ram, ewe, and mixed bands.
Sheep Classification
Percent Ocur-1/4-curl 1/2-curl
renee in: ewes lambs yearlings rams rams
ewe bands 77 77 75 56
mixed bands 23 23 24 30 38
ram bands --1 14 62
Note: Data based on 1141 sheep observations during May-August, 1973.
3/4-curl
rams
11
89
full curl
rams
20
80
00
tn
86
bands, and bands containing both ewes and 1/2-curl or larger rams were
counted as mixed bands.
Mountain sheep bands are not fixed social units, but are dynamic
aggregations of individuals. Bands frequently split or join other bands,
and individual sheep may enter or leave a band at any time. The mother-
young relationship between a ewe and her lamb and ofter her yearling is
the only association among mountain sheep which can be expected to re-
main stable over a long time interval. Frequent changes in average band
size and composition would, therefore, be expected to occur. Assuming
that band size and composition exist in a particular combination to best
meet the ecological requirements of sheep at a given time, then differ-
ences in average band size and composition may be expected at different
seasons and under different environmental conditions, such as when sheep
are limited to winter range, concentrated at mineral licks, or dispersed
over summer range.
During the period May through August, the distribution of sheep on
the study area progresses from winter range to the mineral licks to
summer range and back to winter range. Analysis of band size and com-
position characteristics during this period reveals severa! distinct
trends.
The most immediately recognizable of these is the difference in rel-
ative size of bands of different composition. It can be seen in Figure
17 that ram bands are the smallest band classification, averaging only
3.1 individuals during May through August. They are also relatively
more uniform in size throughout summer than other band classifications.
Ewe bands, averaging 7.3 sheep during the same period, are of a
larger average size than ram bands and show greater variability in size
87
Figure 17. Mean size of ram, ewe, and a11 bands (inc1uding mixed and
unc1assified) during May through August; based on 858
band observations in 1970, 1971 and 1973.
Figure 18. Percent occurrence of various band sizes for ram, ewe, and
mixed bands during March through September, based on 561
band observations in 1970, 1971 and 1973.
88
10
~ • • ./:.
Ul 8 ....
0
ci
~ 6 • N u;
'0 4 c
tl
CD
c
tl 2 •
----, ,.-..._,"""'' ' / ' '-... / ---Rom
'-/
2
0
ji; 0 0 ~ 0 0 '? 0 0 .., N ~ N 1 "j' .! 1 "j' :r .! 1 Cl) s (\j (\j
.:::::. .:::::. ~ ~ ~ ~ ..... ~ ..... 10 CD Cl) Cl)
Dote (Month /Doys)
Figure 17.
60
---Rom
-•-Ewe ---Mixed
c 40 • .. .. • Q.
20
2
1
CD
Bond
Figure 18.
89
during the summer. In early summer ewe bands average nearly three times
as large as ram bands, but when the sheep are dispersed throughout sum-
mer range they are less than twice as large as ram bands.
~lixed bands were observed too infrequently during certain portions
of the summer to be included in Figure 17, but for the entire period of
May through August they averaged 13.2 sheep in size. This is nearly
twice as large as the average size of ewe bands and over four times as
large as the average size of ram bands during the same period.
A second feature apparent in Figure 17 is that all band classific-
ations reach two peaks in size during the summer. This phenomenon is
quite noticeable in the field and coincides closely with the two crossings
of Atigun Canyon by the sheep. The first peak occurs during the first
half of June when ewes and their lambs move down from the higher ridges
on the north side of the canyon and gather into nursery bands on the
lower slopes and hills prior to crossing the river.
The second peak occurs at the beginning of August a few days prior
to the northward movement across the canyon. At this time sheep begin
moving back into the canyon from summer range and gather into larger
bands near the mineral licks. This peak in band size is somewhat less
dramatic than the earlier one, being smaller in size and progressing over
a longer time interval.
Figure 18 shows ram, ewe, and mixed bands of various sizes as the
percent of all bands for each respective classification. A high percen-
tage (34%) of the ram bands recorded were solitary rams, partially
accounting for the low average size of ram bands. Most ram bands (50%),
however, were groups of only 2-5. A total of 98% of the ram bands
recorded contained 10 or fewer sheep, and the largest ram band observed
90
contained only 16.
Ewes occur singly only about one-fourth as often as rams (8%), but
occur in groups of 2-5 animais 45% of the time, showing little differ-
ence in frequency from ram bands of that size. The largest ewe band
encountered contained 53 animais and, accordingly, was the largest sheep
band recorded in the four years of study.
The 6-10 sheep band is the most frequently occurring size for mixed
bands, comprising 28% of the total. The percent of mixed bands containing
more than 15 sheep drops rapidly but mixed bands of as many as 51 sheep
were observed.
Shawn in Figure 19 is the relative abundance of identifiable ram,
ewe and mixed bands observed during consecutive periods of approximately
10 days each from May through August. The observed proportion of each
band classification varied from one period to another because of sheep
movements within the study area and because certain parts of the study
area were more intensively surveyed during sorne periods than during
others. Actual changes in the proportions no doubt also occurred in re-
lation to changing band sizes and composition. For the entire summer
period, however, ewe bands averaged 64%, ram bands averaged 24%, and
mixed bands averaged 12% of all sheep bands recorded.
The presence of mixed bands during this season should especially
be noted. This contrasts with the observations of Blood (1963) and
others who observed mixing of ram and ewe bands of mountain sheep only
during the winter months. Summer mixing of rams and ewes in Dall sheep
does occur to a limited extent, although mixed bands usually appear to
be ,temporary associations brought together by circumstance, such as
concurrent use of mineral licks or certain travel routes.
91
100
80
~ 60
ID
u ...
ID a.
40
20
0
1
10
1
CD .....
1()
1 .
0
0 N
1
1 -..... ' CD CD
.
0 0 -0 10 0 N 10 0 N 10
1 1 1 1 .!
N 1 -N 1 -N ' -..... ..... ..... ..... ..... .....
CD ,.. ,.. ,.. CD CD CD
Date (Month/Ooys)
Figure 19. Ram, ewe, and mixed bands as the percent of total identifi-
able sheep bands recorded in and near Atigun Canyon during
May through August; based on 538 band observations in 1970,
1971, and 1973.
92
Annual variation in the size of sheep bands on the study area has
been observed, but the reasons are not totally apparent. During June,
July, and August, the average size of all bands observed was 9.5 in
1970, 9.2 in 1971 and 6.0 in 1973. Since these figures represented the
records of three different observers, sorne of the difference may be due
to the thoroughness of searching for sheep bands. Missing a few single
sheep which count as a band of one could raise the average considerably.
Two other possibilities for the large decrease in the 1973 average
involve the population composition in that year. A higher proportion of
rams was counted in 1973 than in 1970 or 1971 (60% of the population as
opposed to 44% and 41% respectively). Since ram bands are normally
smaller than other bands, the higher proportion of rams observed in 1973
would lower the average band size. Also, the average size of ewe bands
in 1973 may have been decreased by the extremely low lamb crop that year.
This would in turn depress the average size of all bands for 1973.
A final possibility exists that band size may be related in an
unknown manner to population density. A decreasing population level
since 1970 has seen a corresponding decrease in average band size. If a
proportional relationship exists between band size and population den-
sity, then fewer sheep could result in smaller rather than fewer bands.
SUMMARY OF CONCLUSIONS AND MANAGEMENT RECOMMENDATIONS
Conclusions
During winter, the Atigun study population separates into four main
wintering groups. The number of adults in the largest of these, the
Atigun Canyon group, has decreased annually since 1970. The apparent
reasons for this decline were poor lamb production during two years and
an increasing level of wolf predation. Within this group, the age struc-
ture of rams and the sex ratio have also fluctuated during this period.
The size of one of the remaining wintering groups remained stable be-
tween 1973 and 1974, although the average age decreased. The overall
sex ratio for all groups was 76 rams per lOO ewes and the total popula-
tion size was estimated to be 275 animais in 1974.
Productivity in the Atigun Canyon group fluctuated between 10 and
59 lambs per lOO ewes. These fluctuations were probably the result of
several factors, including a changing age structure among ewes, fluctu-
ations in population density, severity of winter and spring weather,
and predation.
After their first few days of life, the survival of lambs during
summer is apparently high in this area. Survival during the first winter
was also high for two cohorts (64% and lOO%) . Data from other sheep
populations in Alaska, however, suggests that average survival to year-
ling age is probably much lower than was observed in these two years on
the study area.
93
94
Patterns of adult mortality were estimated by aging ram remains
found in the field and by comparing the horn curl composition of a syn-
thetic cohort constructed from these remains to the horn curl composition
of the living population. The mean age at death of rams was 9.00 years
and calculated minimum and maximum life expectancies were 3.5 and 14.5
years respectively. The composition of the synthetic cohort constructed
from remains was 41% 1/4-curls, 34% 1/2-curls, 21% 3/4-curls, and 4%
full curls. This compared with a composition in the living population
of 39% 1/4-curls, 38% 1/2-curls, 16% 3/4-curls, 6% full curls, and 1%
greater than full curl.
The ram remains showed a two-phase survivorship pattern with 5.3%
average mortality between two and eight years of age and 38.1% average
mortality between nine and 13 years of age. Increasing mortality in the
older age classes is related to achievement of dominance status and
participation in the rut since rutting rams !ose their fat reserves and
become vulnerable to winter mortality. Rams which grow rapidly achieve
dominance early in life and can expect shorter life spans than slower
growing rams. A negative correlation was shown between length of horn
segments 3-5, an index of growth, and age at death for rams from the
study area.
Survivorship patterns of ewes are known from other mountain sheep
populations to be similar to those followed by rams. Mortality rates
among adult ewes begin to increase at the age of first reproduction,
increase at a greater rate through middle age, the years of highest
reproduction, and increase fastest during old age when senescence de-
creases reproductive performance.
The importance of accidents, disease and parasites, hunting, and
95
predation as mortality factors in the study population were discussed.
Of these, predation by wolves seems to be the most significant. Wolf
numbers on the study area increased from 1970 to 1973 and decreased in
1974. Predation on sheep also peaked in 1973. The availability of
other prey species in winter, particularly caribou, seems to markedly
affect the exploitation of sheep by wolves in this area.
During winter, sheep are concentrated within a small portion of
their total range where snow accumulation is light, vegetation is abun-
dant, and escape terrain is adequate. Sorne movement away from these
areas begins in May when melting snow exposes fresh vegetation. Sheep
generally remain on the wintering areas through the lambing season, how-
ever. Lambing in this area occurs during late May and early June. Two
lambing areas exist in Atigun Canyon, but lambs were observed on one of
these only during years of high production.
Concurrent with the lambing season, sheep begin using mineral licks.
There are at least two primary and several secondary mineral licks within
the study area. The use of licks peaks in June, drops markedly during
July, and increases slightly again in August. Sheep apparently seek
mineral substances from these licks to supplement their forage diet, but
the specifie substance sought was not determined. Licks may also serve
a social function by aiding the attachment of young rams to ram bands.
Many sheep divide their lick visits into two segments separated by
several minutes of grazing or resting. The average time spent licking
per lick visit for all sheep excluding lambs was 90.0 minutes (n=62) and
no difference intime was detected for adult rams and ewes. The length
of time spent per lick visit appears to increase with increasing age for
rams. "'·
96
Following the June peak in lick use, sheep rnove to surnrner range.
This rnovernent occurs more as an expansion of winter range than as an
actual migration because winter and surnrner ranges are contiguous and
sirnilar in appearance except that the latter lies at higher elevations
and is covered by deeper snow in winter. Surnrner range encornpasses most
of the study area, hence sorne sheep rnove as far as 20-25 miles. Their
distribution during surnrner is governed largely by elevation, reaching
an average of 3900 feet in rnid-July.
Sheep begin rnovernent back to winter range during August and many
return to Atigun Canyon by the first of Septernber. Most sheep are
probably on winter range by the end of Septernber; however, rarns visit
the ewe wintering areas in Novernber for the rut.
Sheep bands vary in size and composition throughout the surnrner.
Rarn bands are the srnallest band classification, averaging 3.1 sheep,
ewe bands are larger, averaging 7.3 sheep, and rnixed bands are the
largest, averaging 13.2 sheep during May through August. Sheep bands in
Atigun Canyon reach their greatest average size during June prior to
rnovernent to surnrner range and reach a second peak in August upon returning
to the canyon. Thirty-four percent of the rarn bands and eight percent
of the ewe bands recorded were single sheep while 50% of the rarn bands
and 45% of the ewe bands recorded were groups of 2-5 sheep. Larger
bands containing up to 53 sheep were observed. Of all sheep bands ob-
served during May through August, 64% were ewe bands, 24% were rarn bands,
and 12% were rnixed bands.
Annual variations were also observed in average band size. These
may be the result of variations in intensity of observation between
years, or of fluctuations in population composition, density, or produc-
tivity.
97
Management Recommendations
The most serious consideration to be faced in the management of the
Atigun sheep population is the vastly increasing influence of man on the
environment. At present, this influence is strongest in relation to
construction of the Trans-Alaska oil pipeline. But after the completion
of this project, man's impact will continue to be felt. Pipeline main-
tenance and operations crews will be housed in permanent residences near
the pumping station located on the study area, and the permanent airfield
at Galbraith Lake and the road system now under construction will permit
much greater public access to the area than ever before possible. Pet-
roleum and mineral exploration will probably continue at increasing
rates in the Brooks Range, and increasing numbers of tourists and hunters
may be expected. It seems inevitable that greater pressures will in the
future be placed on all natural resources of the area, including wild-
life. If adequate precautions are not maintained, unfavorable effects on
the Dall sheep population may be anticipated, including among other pos-
sibilities the destruction of critical habitat and disturbance to sheep
from construction activities, aircraft, and hunters.
The consequences of disturbance to wildlife may be quite serious,
including the loss of body weight and increased susceptibility to disease,
resorption of embryos or abortion, decrease in birth weight, trampling
and desertion of young, increased predation, neurosis and behavioral
disorders in adults and young, and voluntary withdrawal from the best
habitat (Geist, 197la). Humans need not present a disturbing influence
to wildlife, however, as proven by the coexistence of large numbers of
humans and wildlife in many national parks. To avoid human disturbance
to a wildlife species, we must "act in such a manner that we become
98
acceptable, and remain a harmless part of that species' environment"
(Geist, 197la:414). Too frequently in the past, though, man's inclin-
ation toward the environment has been to dominate it and change it to
suit himself; wildlife species, in contrast, have evolved to suit
existing environments and usually cannot tolerate drastic changes.
The decline in numbers of the bighorn sheep in North America, re-
sulting largely from the loss of traditional range areas through devel-
opment activities and overgrazing by domestic livestock, has shown that
mountain sheep are especially sensitive to the loss of critical habitat.
Dall sheep habitat in the Brooks Range might equally well fall victim to
increased development activities and growing human populations if
adequate protection is not maintained. Future construction activities
in this area should be restricted when critical habitat areas, including
winter range, lambing areas, and mineral licks, are in use and if at
all possible should be planned to completely avoid the vicinity of these
areas. Little apparent effect has been noted thus far from construction
activities in the Atigun area; however, the long term effects of such
disturbance are completely unknown.
Aircraft present a further potential source of disturbance to sheep
in the Atigun area and Andersen (1971) and Priee (1972) cited numerous
instances of aircraft disturbance during their studies there. It has
been my impression that sheep habituate readily to the presence of air-
craft as long as regular flight patterns are maintained and sheep are
not approached too closely. It is the occasional pilot who cornes in low
over a band of sheep to photograph them or just to get a better look who
causes the greatest disturbance. A strong emphasis must be placed on
educating people as to the possible consequences of approaching wildlife
99
too closely, especially during lambing and calving seasons and in winter,
and rules should be implemented to restrict aircraft to definite flight
zones. Sorne progress has already been made in this direction among
construction personnel, but instances of aircraft disturbance still
occasionally occur and a greater public awareness of the consequences is
needed.
With vastly improved access to the Atigun area once pipeline con-
struction is completed, strict limitations on Dall sheep hunting seem
imperative if the population is to be maintained near its present size
and composition. In nearly all instances in Alaska wheie roads have
been built through or near Dall sheep habitat, it has proven necessary
to stop or closely restrict the sheep kill. This has been accomplished
by a number of means, including restrictions on the method of transpor-
tation to the hunting area, the number, sex, and horn curl class of sheep
to be harvested, and the length and timing of the hunting season.
Under existing regulations, big game hunting is closed for five miles on
each side of the Trans-Alaska pipeline corridor. This system seems
presently justified in that the long-term disturbance effects of construc-
tion are unknown and any hunting during construction may later prove to
have been unwarranted. It is my recommendation that this closure remain
in effect until completion of pipeline construction, which is expected
in 1977.
Once pipeline construction is completed, a decision must be made
whether to retain the present closure or to adopt sorne other means of
regulation. This population has in the past sustained a light harvest
of mature rams, thus a low level of hunting might still be permissible.
However, the possible synergistic effects of hunting and human activity
lOO
in the area could lead to abandonment of the range even though hunting
pressure was light. If hunting is perrnitted, improved access to the area
will require rnuch stricter regulation of the kill than was necessary in
the past. The actual means by which the harvest is restricted seems not
so important as the assurance that the intensity of hunting and the kill
are no greater than could be sustained by the population.
A further consideration to be taken into account when deterrnining
hunting regulations for this area is the effect of predation. Wolves
have in the past been an important mortality factor to this sheep popu-
lation. However, the increased hurnan influence in the area may result
in lowered wolf populations. If this occu!s, closely regulated hunting
may prove to be a beneficiai tool for keeping the size and composition
of the sheep population at optimum levels.
The effects of construction and development activities in the Atigun
area and the influence of man on the predator-prey relationships there
are things which can only be deterrnined with time. For this reason, I
feel it is impractical at the present time to recornrnend specifie regula-
tions governing the hunting of this population when pipeline construction
is completed in 1977. The focus of this study has been toward assessing
the natural population dynarnics and seasonal movements of the population
before substantial development and disturbance occurred. It is imperative
that this study be continued after completion of pipeline construction
in order to assess the true impact of human influence on the study pop-
ulation. At that time, the best management policies for maintaining this
population indefinitely into the future could better be determined.
Caution must be used in follow-up studies, however, as this and other
studies have shown that Dall sheep populations fluctuate naturally in
101
time with regard to size and sex and age composition. Future analysis
of this population should take these natural fluctuations into account
before attributing them solely to human influence.
APPENDIX. Scientific and common names of mammals and resident birds
identified on the study area.1
Mammals
Alces alces moose ------
gray wolf
*Clethrionomys rutilus red-backed vole
Dicrostonyx groenlandicus collared lemming
Erethizon dorsatum porcupine
Gulo luscus wolverine
LellllliUS trimucronatus brown lemming
Marmota caligata hoary marmot
*Microtus spp. vole
Mustela erminea ermine
Ovis dalli Dall sheep
Rangifer tarandus caribou
*Sorex spp. shrew
Spermophilus undulatus Arctic ground squirrel
Ursus arctos grizzly bear
red fox
1scientific names are from Hall and Kelson (1959) for mammals and
from Irving (1960) for birds.
*These species were not actually observed during the study but were
noted by Rausch (1951) as common near Anaktuvuk Pass. Hence, they were
probably present on the study area.
102
103
Appendix, continued.
Birds
Aquila chrysaetos golden eagle
Corvus corax ra ven
Falco rusticolus gyrfalcon
Lagopus lagopus willow ptarmigan
Lagopus mutus rock ptarmigan
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