The Snow Sheep (Ovis nivicola)...
The grayish brown coat is accented by a small patch of light hair on the
buttocks. The wooly winter coat is a light, milky coffee colour. The fronts
of the legs are dark chocolate brown, while the rear edges may have whitish
markings. A dark band, which runs across the nose between the eyes and
muzzle, contrasts greatly with the bright white rostrum. The ears are small
and dark grey in colour. The horns, found in both sexes, are considerably
lighter than those of the related Bighorn sheep, with up to 35% less horn
substance. Growing to 89 cm / 35 inches long in males, the horns curl
backwards, downwards, and upwards around the ears, corkscrewing outwards in
old males as the horns begin their second revolution. While the base of
a male's horns may be up to 38 cm / 15 inches in circumference, those in
females are significantly thinner and shorter, curving backwards in a
sabre-like fashion.
The snow sheep is a well adapted mountain dweller - extremely agile and
nimble, and able to move quickly over steep, uneven terrain. Within bachelor
herds, a dominance hierarchy is formed based primarily on horn size. These
hierarchies remain relatively stable, even in the breeding season, with larger
males getting the majority of the mating rights. However, if two males have
approximately equal sized horns, the dominant/subordinate relationship is
decided in combat. Facing each other from a distance, they run towards each
other with heads lowered, rearing up and crashing their horns together in an
attempt to throw their rival off balance.
The snow sheep is classified as low risk, conservation dependent by the IUCN
(1996). Also, O. n. borealis is considered vulnerable, and O. n. nivicola as
a low risk, near threatened subspecies.
More closely allied to the North American sheep than the Asian or European
sheep, the snow sheep is thought to have evolved in the Americas, diverging
from the two American sheep via a movement across the then-existent Bering
land bridge.
G-band patterns of the Siberian snow sheep, Ovis nivicola alleni, 2n=52, were compared with the patterns reported in 2n=54 wild Asiatic mouflon and 2n=54 North American sheep and those in domestic sheep with 2n=54, 53, and 52. The three largest pairs of biarmed autosomes displayed indistinguishable, presumably homologous, G-banding patterns in all types of sheep. The banding and morphology of the fourth pair of biarmed autosomes in O.nivicola differed from those of the three translocation variants described in domestic sheep. Wild sheep with 2n=54 may have evolved monophyletically from an ancestral 2n=58-56-54 population or polyphyletically by a series of independent, nonrandom fusions. In contrast, the fouth pair of biarmed autosomes in O.nivicola and in 2n=52 domestic sheep variants may have resulted from random fusions of different chromosomes.
Based on mitochondrial cytochrome b gene sequence analysis, the history of true sheep (Ovis) began approximately 3.12 million years ago (MYA). The evolution of Ovis resulted in three generally accepted genetic groups: Argaliforms, Moufloniforms, and Pachyceriforms. The Pachyceriforms of the subgenus Pachyceros comprise the thin-horn sheep Ovis nivicola (snow sheep), Ovis dalli (Dall and Stone sheep), and Ovis canadensis (Rocky Mountain and desert bighorn). North America wild sheep (O. canadensis and O. dalli) evolved separately from Eurasian wild sheep and diverged from each other about 1.41 MYA. Ancestral stock that gave rise to snow sheep, Moufloniforms, and Argaliforms occurred 2.3 MYA, which then gave rise to two different extant lines of snow sheep that diverged from each other about 1.96 MYA. The more recent nivicola line is genetically closer to the North American wild sheep and may represent a close association during the refugium when Alaska and Siberia were connected by the Bering land bridge. The earlier period of evolution of the Pachyceriforms suggests they may have first evolved in Eurasia, the oldest ancestor then giving rise to North American wild sheep, and that a canadensis-like ancestor most likely gave rise to nivicola. Cytogenetic analysis further validates that the standard diploid number for modern nivicola is 52.