Morphology and Taxonomy

Wild sheep crossed the Bering land bridge from Siberia during the Pleistocene and, subsequently, spread through western North America. In North America, wild sheep have diverged into two extant species: thin horn sheep (Ovis dallli) that occupy Alaska and northwestern Canada, and bighorn sheep (Ovis canadensis) that range from southern Canada to Mexico. For more than a half century Sierra Nevada bighorn sheep have been classified as California bighorn (Ovis canadensis california), which were distributed, north to south, between British Columbia and the southern Sierra Nevada and west to east, between the Cascade and Rocky Mountains. But recent genetic and morphological research and reanalysis of historic data do not support the traditional classification. In recent years cranial morphometric research and analysis of mitrochondrial DNA variation suggest bighorn sheep from the Sierra Nevada are a distinct subspecies, qualifying them as an "evolutionary significant unit" (Moritz 1994).

Biological Characteristics

Bighorn sheep have a generally stocky build. As adults, Sierra Nevada bighorn sheep stand about three feet tall at the withers and weigh up to 140 pounds for females and 220 pounds for males. Coat color is variable from almost white to dark brown with a distinctive large white rump patch and a short dark tail. Females carry small narrow horns which rarely exceed 12 inches in length, while males carry more massive horns used in ritual jousting matches for dominance. The horns of male Sierra Nevada bighorn sheep are notably wide and flaring, but relatively small as bighorn sheep go.

Sierra bighorn sheep breed in the fall and have slow population growth rates, because of their life history attributes. Unlike many desert populations, bighorn sheep in the Sierra Nevada have a restricted lambing season in spring and early summer. The timing of lambing has been found to vary with patterns of nutrient intake (Wehausen 1996). Nutrition also strongly influences growth rates of lambs, age at sexual maturity, frequency of lambing, and lamb survival to adulthood, all of which affect population growth rates. At best, females bear their first lamb at two years of age and bear only one lamb per year. When nutrition is poor, they may not bear their first lamb until they are four years of age and breed only in alternate years. Wehausen (1980, 1996) showed a great influence of winter range use on nutrition and lamb survival.

Habitat

Two adaptations of bighorn sheep substantially define basic habitat requirements. First is their agility on precipitous slopes, which is their primary means of escaping predators. Second is their keen eyesight, which is their primary sense for detecting predators. Relatively short legs and a stocky build allow agility on rocks, but preclude fleetness necessary to outrun coursing predators in less rocky terrain. Consequently, bighorn sheep select open habitats that allow them to detect predators at distances sufficient to provide adequate lead time to reach the safety of precipitous terrain. In short, optimal bighorn habitat is visually open and contains steep and generally rocky slopes.

Forest and thick brush usually are avoided to the extent possible, and fire can play an important role in creating bighorn habitat as well as making existing patches safer relative to predators. Large areas that lack precipitous escape terrain, such as the Owens Valley, represent substantial barriers to movement. Even within mountain ranges like the Sierra Nevada, bighorn sheep habitat is frequently patchy and the population structure is one of natural fragmentation (Bleich et al. 1990). This structure can include multiple independent female subpopulations within what are commonly designated single populations (Bleich et al. 1996), and male sheep travel to, from, and between these subpopulations.

Bighorn sheep exhibit a variety of seasonal movement patterns to meet varying needs relative to food, water, safety, and exposure to environmental conditions. Of these, water is not a factor for Sierra Nevada bighorn. In high mountains, like the Sierra Nevada, altitudinal migration is a common pattern: the sheep minimize their exposure to environmental extremes and maximize their nutrient intake by occupying lower elevation in winter and spring, and then following the plant growing season up the mountain (Wehausen 1980, 1983).

Because of the naturally fragmented nature of their habitat, bighorn sheep have developed conservative behaviors with long histories that each generation learns from the previous one by following adults (Geist 1971). Because of these conservative behaviors, natural colonization is slow. Thus, it has been necessary to capture and move bighorn sheep to locations deemed suitable to speed up and assure re-occupancy of ranges considered lost as a result of human activities (Sierra Nevada Bighorn Sheep Interagency Advisory Group 1997).