The Thinhorn Sheep is a medium-sized ungulate with a stocky body, slender legs, short ears and short tail. The body colour ranges from the all-white Dall’s Sheep to the dark blackish-brown forms of Stone’s Sheep, with a general cline of all-white individuals in the northwest to darker ones further south and east in the species’ range. Coat colour is the primary character used to distinguish the two subspecies.
Both sexes have horns, which are generally light brown or amber, with rings and ridges over their surface. Adult males have much larger horns than females, and they are roughly triangular in cross-section with an obvious keel on the upper, outer edge. Like those of most male wild sheep, the horn sheaths are sharply tapered and grow in a spiral out from the head so that in mature individuals they complete a full circle alongside the face. Most adult males – even the old ones – have a wide horn span and unbroomed (unbroken) horn tips. The female’s horns are short, gently curved up and back from the head, and elliptical in cross-section.
Identification and Subspecies Information
Subspecies: Dall's Sheep (Ovis dalli dalli)
Dall’s Sheep is characterized by its overall white or creamy white coat and tail. This all-white pelage distinguishes them from Stone’s Sheep with its grey to black body, white rump patch and black tail. But some populations (mainly in the Yukon) have intermediate forms between Dall’s and Stone’s Sheep. Some individuals may be all white with only a black tail, others can show light grey patches, often on the middle of the back.
total length: male: 1,552 mm (1,346-1,740) n = 11; female: 1,372 mm, n = 1 tail vertebrae: male: 94 mm (70-121) n = 9; female: 85 mm (70-90) n = 10 hind foot: male: 402 mm (370-432) n = 10; female: 338 mm (350-390) n = 10 ear: male: 89 mm (85-92) n = 6; female: 87 mm (80-90) n = 9 shoulder: male: 988 mm (927-1090) n = 9 chest: male: 1,159 mm (1,100-1,240) n = 5; female: 1,085 mm (1,050-1,120) n = 2 weight: male: 84.1 kg (70-103) n = 5; female: 48.8 kg (46-51) n = 8 skull length: male: 249.0 mm (235-256) n = 16; female: 228.0 mm (220-238) n = 4 skull width: male: 122 mm (108-116) n = 7; female: 115 mm (112-117) n = 3
In British Columbia, Dall’s Sheep is restricted to the southern extension of the St Elias Range in the Haines Triangle west of Bennett Lake in the extreme northwestern portion of the province. Outside B.C., the subspecies inhabits the Mackenzie Mountains along the border between the Yukon and the Northwest Territories, and various mountain ranges in the Yukon and Alaska.
It is not easy to see Dall’s Sheep in the province, and visitors prepared to travel to the extreme northwestern part of B.C. may have better luck going a little further into the Yukon and visiting Sheep Mountain in southern Kluane National Park Reserve, southwest of Haines Junction.
The 500 Dall's Sheep estimated in British Columbia in 1977 represent only a fraction of the total world population of around 100,000. Because of their low numbers in B.C., Dall's Sheep is included on the province's blue list of species at risk.
Subspecies: Stone's Sheep (Ovis dalli stonei)
Stone’s Sheep varies in coat colour from light grey to a grizzled grey-brown to almost black. This subspecies of Thinhorn Sheep tends to be darker in the southern and eastern parts of its distribution, and lighter in the north and northwest. But a single population may contain almost the entire range of coat variation, although one colour type generally predominates. The colour of the neck and face also varies from light grey to darker grey in the dark-colour forms. Stone’s Sheep also has white inside the ears and grey on the outside, a white belly, white on the backs of the legs, and a large white rump patch with a black tail. The dark tail stripe extending forward from the tail may or may not be continuous across the rump patch, and older males may have a dark band running across the underbelly. The horns of both sexes are similar to those of Dall’s Sheep.
total length: male: 1,595 mm (1,321-1,803) n = 5; female: 1,308 mm (1,295-1,321) n = 2 tail vertebrae: male: 102 mm (76-114) n = 5; female: 89 mm (76-102) n = 2 hind foot: male: 433 mm (419-457) n = 5; female: 380 mm (350-400) n = 16 ear: male: 95 mm n = 1 shoulder: male: 1,106 mm n = 1; female: 883 mm (700-950) n = 16 weight: male: 88.6 kg (77-100) n = 2; female: 51.3 kg (45-61) n = 7 skull length: male: 253.3 mm (250-257) n = 5 skull width: male: 118.8 mm (117-121) n = 5
Stone’s Sheep distribution in northern B.C. runs northwest to southeast from the east side of Bennett Lake on the B.C.-Yukon border, along the east side of the northern Coast Mountains, into the northern end of the Skeena Mountains, through the Cassiar and Omineca Mountains, and in the northern Rockies to about 80 km northwest of Chetwynd. There is also an unconfirmed report of an isolated population further southwest in the Omineca Range on the east side of Takla Lake. The easternmost population is along the west side of the Chief River, west of the Nelson River. The North American distribution of Stone’s Sheep also extends north into an area of integration with Dall’s Sheep in south-central Yukon (Cassiar and Pelly mountains, MacArthur Range, and White Mountains).
Between 1990 and 1993, 28 Stone’s Sheep were transplanted from the north side of the Peace Arm of the Williston Reservoir to the Upper Moberly River Drainage, but so far the new population has had difficulty becoming established. In another introduction made between 1994 and 1995, 24 Stone’s Sheep from the east side of Atlin Lake (Surprise Lake area) were released on the west side of Atlin Lake (Table Mountain). This transplant appears to have been successful and numbers have since increased with as many as 50 animals counted in 1997. The most recent transplant was of 8 sheep from Toad River that were moved to a site 25 km to the east.
Places with reasonably good prospects of observing Stone’s Sheep are: the Spatsizi Plateau Wilderness Park; south of Boya Lake on Highway 37 near Good Hope Lake; along the Alaska Highway in the Muncho Lake Park area; and between Stone Mountain Provincial Park and Muncho Lake near where the Liard River crosses the highway.
Most of the world's population of Stone's Sheep lives in northern B.C., where about 12,000 were estimated in 1997, and another 3,000 in the Yukon. Until 1997, this species was on the province's Blue List of species at risk, but in 1998, it was downlisted to the Yellow List.
The only other B.C. ungulate with the all-white coloration of Dall’s Sheep is the Mountain Goat. Both sexes of Dall’s Sheep are readily distinguished from Mountain Goat by their lack of a beard and more slender legs, and in winter, by their shorter body hair. Male Dall’s Sheep have much larger and massive horns than Mountain Goats; they grow in a spiral and range in colour from light amber to dark brown, rather than black like a Mountain Goat. Female Dall’s Sheep horns, are similar in size to those of Mountain Goats, but have blunt (not sharp) tips, are elliptical (not round) in cross-section, have a ridged (not smooth) surface, and are amber or dark brown (not black). Despite their colour variations, Stone’s Sheep never seem to show the dull, medium-brown coat colour typical of either Rocky Mountain or California Bighorn Sheep. Also, compared to Bighorn Sheep, Dall’s and Stone’s Sheep horns are more amber coloured, and in males have a more prominent outer keel and are less prone to brooming. See also the Identification section for Bighorn Sheep.
Mating usually begins in mid to late November and runs to late December in B.C. Gestation is about 175 days, and females invariably bear a single young during May or June. The few recorded birth weights range from 3 to 4 kg. Like Bighorn Sheep, female Thinhorns give birth away from their group and when they rejoin it, their young use the follower strategy. Sexual maturity is attained at 18 to 30 months of age, depending upon population conditions, but males fully participate in mating only when much older.
Thinhorn Sheep eat grass and low shrubs.
Age determination and life expectancy
The age of a Thinhorn Sheep can be determined by tooth succession, cementum annuli counts or the distinct rings on their horns, with the same limitations as described for Bighorn Sheep. The average longevity of Thinhorn Sheep is probably around 10 years for both sexes, but older animals are not uncommon.
Predators and other mortality factors
The main predators of Thinhorn Sheep are Wolves, Grizzly Bears, and sometimes Wolverines and Lynxes. Golden Eagles may also prey on young sheep in the first few weeks of life. Like Bighorns, Thinhorn Sheep frequently carry one or two types of protostrongylid lungworm. Fortunately, Thinhorn populations do not appear to suffer the die-offs experienced by Bighorn Sheep herds, although little monitoring has been done and Thinhorn Sheep rarely reach the high densities that some Bighorn Sheep populations do. The tooth problems described in Bighorns may be more common in Thinhorn Sheep.
Social organization, grouping and behaviour
Most aspects of the Thinhorn Sheep basic biology, ecology and social behaviour are similar to those described for Bighorn Sheep. Some data suggest that Stone’s Sheep may live in smaller groups than Bighorn Sheep.
Thinhorn Sheep inhabit open mountain slopes up to the alpine zone.
Populations with intergrades between Dall’s and Stone’s Sheep occur in northwestern British Columbia and also in the Yukon. In the southern Yukon, I observed individuals in the same group that had all-white coats, white coats and a black tail, and others like the light grey form of Stone’s Sheep. Such non-white forms were originally referred to as Fannin’s Sheep, but are now simply regarded as intergrades between Dall’s and Stone’s Sheep. A third subspecies of Thinhorn Sheep recognized by Cowan in 1940 was the Kenai Sheep (Ovis dalli kenaiensis) of the Kenai Peninsula, Alaska. Today, this is considered another form of Dall’s Sheep. The type locality of Dall’s Sheep is the mountains west of the Yukon River, Alaska. For Stone’s Sheep, the type locality is the Che-on-nee Mountains at the headwaters of the Stikine River, northwestern B.C., but as Dr Ian MacTaggart Cowan pointed out, this name is not marked on any maps, and the locality is more likely the Rainbow Mountains between the Stikine and Iskut rivers, B.C.
The possessive “Dall’s” and “Stone’s” are the correct terms for the subspecies, because both were named after people. Dall’s Sheep is named for the American zoologist W.H. Dall, and Stone’s Sheep for the Montana naturalist A.J. Stone who discovered this sheep.
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2021. E-Fauna BC:
Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab
for Advanced Spatial Analysis, Department of Geography, University of British
Columbia, Vancouver. [Accessed:
2022-05-16 8:50:46 AM]
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