Dall sheep, Dall's sheep, Stone sheep, Stone's sheep
The Thinhorn sheep (Ovis dalli) is a wild sheep native to North America. These sheep live in dry alpine areas where they browse grasses and sedges and acquire minerals to supplement their diet from mineral licks.
Thinhorn sheep are off-white in color, and their coat consists of a fine wool undercoat and stiff, long, and hollow guard hairs. Their winter coats can be over 2 inches (5 cm) thick. They are sexually dimorphic, which means males and females look different. Males are larger than females. Thinhorn sheep begin growing horns at about two months old. Females have small, slender horns compared to the massive, curling horns of males. Young males resemble females until they are about 3 years of age. At this point, their horns begin to grow much faster and larger than females' horns. Adult males are easily distinguished by their horns, which continue to grow steadily from spring to early fall. This results in a start-and-stop growth pattern of rings called annuli. Annuli can be used to help determine age.
Thinhorn sheep inhabit northwest Canada, including Yukon, the Northwest Territories, and British Colombia, as well as Alaska in the United States. They live in mountainous regions with dry, hostile, and rugged areas. In spring, they occupy grassy mountainsides, moving in summer up to high alpine pastures. In winter, they migrate to areas at lower elevations that have less snow.
Thinhorn sheep are diurnal and gregarious. Females live in a flock with other ewes, immature rams, lambs, and yearlings. Rarely, there are aggressive interactions between females, due to conflict over bedding or feeding sites. Older males live in groups of bachelors that do not interact with ewes except during the mating season. Males will establish a hierarchy where rank depends on horn size. The ranking is for access to females as well as for social order. Males occupy as many as six different home ranges during the year and females up to four, though some populations of thinhorn sheep are sedentary. When seasons change, the sheep move to areas that are more suitable. Females move to particular areas to lamb, generally in secluded areas of high elevation. Being able to navigate upland meadows as well as near-vertical cliffs provides Thinhorn sheep with an advantage when seeking safe areas for birthing and to escape from predators.
The diet of Thinhorn sheep is mainly grasses, shrubs, and sedges, with the addition of lichens, willows, and mosses in autumn when there is a short supply of other vegetation. ‘Mineral licks’, which are rich in essential minerals such as calcium, are also important in certain seasons.
Thinhorn sheep are polygynous, dominant males breeding with females most often. Late November to early December is the mating season, and births take place from mid-May to early June. After a gestation of about 175 days, one lamb is born. Lambs are precocial and are able to travel with their mother only 24 hours after being born, and within days the two rejoin their group. Lambs are fully weaned after three to five months. Rams are sexually mature at 18 months of age but do not generally mate successfully until 5-7 years of age when they achieve adult size and social dominance. Females are sexually mature at 30 months, giving birth for the first time by the age of 3 or 4.
Thinhorn sheep are under threat of the trophy harvest (adult rams especially), hunting in reserves and parks, and hunting by native peoples. Other threats are high population density, low-quality forage, and disease. Avalanches and accidental falls are the cause of some deaths. Human activities, such as mineral exploration and road building within the Thinhorn sheep’s range, are further threats.
Canada has a population that is about 41,500 animals. Of this total, 27,000 are Dall’s sheep subspecies: with 19,000 sheep in the Yukon; 7,500 Dall’s sheep in the Northwest Territories, and 500 Dall’s sheep in British Columbia. 14,500 Stone’s sheep subspecies: with 3,000 Stone’s sheep in the Yukon and 11,500 Stone’s sheep in British Columbia. The total U.S. population of Dall's sheep is estimated at 70,000 to 75,000 animals. Overall thinhorn sheep numbers are stable today and they are classified as least concern (LC) on the IUCN Red List.