Dall sheep or thinhorn sheep (Ovis dalli), is a species of wild sheep native to northwestern North America. O. dalli live in mountainous alpine habitats distributed across northwestern British Columbia, the Yukon, Northwest Territories and Alaska. They browse a variety of plants, such as grasses, sedges and even shrubs, such as willow, during different times of the year. They also acquire minerals to supplement their diet from mineral licks. Like other Ovis species, the rams engage in dominance contests with their horns.
Taxonomy and genetics
The specific name dalli, is derived from William Healey Dall (1845–1927), an American naturalist. The common name, Dall's sheep or Dall sheep is often used to refer to the nominate subspecies, O. d. dalli. The other subspecies, O. d. stonei, is called the Stone sheep.
Originally, the subspecies O. d. dalli and O. d. stonei were distinguished by the color of their fur. However, the pelage-based designations have been shown to be questionable. Complete colour intergradation occurs in both O. dalli sheep subspecies (i.e., Dall's and Stone's), ranging between white and dark morphs of the species. Intermediately coloured populations, called Fannin sheep, were originally (incorrectly) identified as a unique subspecies (O. d. fannini) with distributions inhabiting in the Pelly Mountains and Ogilvie Mountains of the Yukon Territory. Fannin sheep have more recently been confirmed as admixed individuals with predominantly Dall's sheep genetic origins. Previous mitochondrial DNA evidence had shown no molecular division along earlier subspecies boundaries, although evidence from nuclear DNA may provide some support. Current taxonomy using mitochondrial DNA information may be less reliable due to hybridization between O. dalli and O. canadensis recorded in evolutionary history.
thumb|alt=Dall Sheep ram (male adult)|Dall sheep ram (male adult)
Description
O. dalli stand about at the shoulder. They 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 thick. O. dalli can live to be 12 to 16 years of age.
O. dalli are sexually dimorphic, which means rams and ewes look different. Rams are larger than ewes and typically weigh between at maturity. Ewes weigh approximately on average. During the winter, adult sheep may lose up to 16% of their body mass, and lambs and yearlings as much as 40% depending on winter weather severity. O. dalli begin growing horns at about two months old. Ewes have small, slender horns compared to the massive, curling horns of rams. Young rams resemble ewes until they are about 3 years of age. At this point, their horns begin to grow much faster and larger than ewes' horns.
Natural history
Ecology
The sheep inhabit the subarctic and arctic mountain ranges of Alaska, the Yukon Territory, the Mackenzie Mountains in the western Northwest Territories, and central and northern British Columbia. O. dalli are found in areas with a combination of dry alpine tundra, meadows, and steep or rugged ground. This combination allows for both grazing and escape from predators.
thumb|alt=Dall Sheep herd|Dall Sheep herd
Social order and dominance rank is maintained in ram groups through a variety of behaviors including head-on collisions. These dramatic clashes involve each ram getting a running start before colliding, horns-first into one another. Other behaviors associated with establishing social order include leg kicks, bluff charges, and dominance mounting. Most of this behavior establishes order year-round, but clashes between males with similar horn sizes intensify as the rut approaches.
Where sport hunting is allowed in Alaska's national preserves, hunters can harvest mature O. dalli rams that have horns that are full-curl or greater, have both tips broken off or are eight years of age or older.
thumb|right|O. dalli ram eating grass
Some species are expected to benefit from climate change while others will not. Shrubs and woody plants typically dominate plant communities at lower elevations. As elevation increases, the dominant plant community transitions to one dominated by low-growing grasses, flowers, and lichens. Warming climate trends, longer growing seasons, and changes in precipitation have the potential to allow woody plant species to find suitable habitat at higher elevations.