Field Guide to Mammals
of Southern California
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Nelson's Bighorn Sheep (Ovis canadensis nelsoni)
Populations of Nelson's bighorn sheep are found in four areas on National Forest System lands in southern California. The San Rafael Peak/Cobblestone Mountain population is a small, reintroduced population that was established in the 1980s. This population inhabits an area in the southern part of the Los Padres National Forest. Nelson's bighorn sheep in the San Bernardino Mountains are considered to constitute two separate populations: the larger population (San Gorgonio Herd) occurs in the vicinity of Mount San Gorgonio in wilderness; the other population (Cushenbury Herd) occurs on the northern edge of the range in desert-facing canyons (e.g., Furnace, Bousic, Arctic, and Marble Canyons). Finally, the population in the San Gabriel Mountains is concentrated primarily in the Bear Creek drainage; the upper East Fork of the San Gabriel River and Cattle Canyon (both in the Sheep Mountain Wilderness); San Antonio Canyon; Cucamonga Canyon; and the South and Middle Forks of Lytle Creek (Stephenson and Calcarone 1999).
The term "desert bighorn sheep" is used to describe those subspecies of bighorn sheep inhabiting dry and relatively barren desert environments. In the past, these subspecies included Nelson's bighorn sheep (O. c. nelsoni), Mexican bighorn sheep (O. c. mexicana), Peninsular bighorn sheep (O. c. cremnobates), and Weems bighorn sheep (O. c. weemsi) (Manville 1980). However, new genetic evidence and a recent reanalysis of morphometric data resulted in changes in the accepted taxonomy for Nelson's bighorn sheep and the Peninsular bighorn sheep (U.S. Fish and Wildlife Service 2001). After their analysis, Wehausen and Ramey (1993) placed Peninsular bighorn sheep within the Nelson subspecies.
Desert bighorn sheep inhabit dry, relatively barren, desert mountain ranges throughout North America. Escape terrain is identified as the single most important habitat component for bighorn sheep in these mountains. Escape terrain is defined as steep slopes (80 percent or steeper) with abundant rock outcrops and sparse shrub cover (canopy cover of 30 percent or less). Nelson's bighorn sheep in the San Gabriel Mountains occur at elevations of 3,000-10,064 feet (914-3,068 meters [i.e., to the summit of Mount San Antonio]). During the winter and spring, Nelson's bighorn sheep occur primarily in escarpment chaparral in the lower canyons at 3,000-6,000 feet (914-1,829 meters).
The breeding season of Nelson's bighorn sheep generally begins in November with the rutting season. Following a six month gestation period, ewes give birth to single lambs (occasionally twins) from late April through early July. During the first few weeks after giving birth, ewes remain alone with their lambs in steep terrain until they join a nursery group. Lambs are weaned at 1–7 months, and juveniles remain with the ewes until they reach sexual maturity (U.S. Fish and Wildlife Service 2000b). Rams are believed to be sexually mature at 6 months of age (U.S. Fish and Wildlife Service 2000a). In the San Gabriel Mountains, the duration of the rut was from mid-September to late December with a peak in late October through the first half of November (De Forge 1980). During the height of the rutting period, mature rams seemed to have little fear of humans and made movements up to 2.5 miles to find ewes.
In general, bighorn sheep feed in the early morning, at midday, and in the evening, lying down and chewing their cud at other times, and bedding down for the evening. Foraging and bedding spots may be used for years (McMahon 1985). Daily foraging and resting cycles also vary depending on forage quality (U.S. Fish and Wildlife Service 2000a). Seasonal activity depends on availability of water, forage, and escape cover. Typically, bighorn sheep congregate near dependable water sources from May through October, when temperatures are highest. This aggregation of individuals also corresponds with breeding activities. Young bighorn sheep learn locations of escape terrain, water sources, and lambing habitat from older individuals in the group (U.S. Fish and Wildlife Service 2000b). Bighorn sheep migrate between winter and summer ranges, generally moving downslope in winter and spending summer in alpine habitats. Water restricts movement of the species during hot summer months (Zeiner and others 1990).
Diet and Foraging
Nelson's bighorn sheep graze and browse on succulent grasses and forbs. Sedge, grasses, and small alpine forbs constitute the chief food (Ingles 1965). Browse is an important component of the diet for populations in arid habitats (Zeiner and others 1990).
Young ewes learn home range boundaries from their mothers and/or older females and demonstrate a high degree of philopatry to these traditional home ranges throughout their lives (Geist 1971). Rams do not exhibit the same site fidelity as ewes and tend to move among ewe groups. Home ranges in one study were found to average 9.8 square miles (25.5 square kilometers) and 7.8 square miles (20.1 square kilometers) for rams and ewes, respectively (U.S. Fish and Wildlife Service 2000a). De Forge (1980) working in the San Gabriel Mountains found ewe home ranges from 1.84 and 3.06 sq. km (.71-1.8 sq. mi.) and a ram home range to be 17.9 sq. km (6.9 sq. mi.).
Recently, mountain lion predation has been documented as a threat to some ewe groups in the Peninsular Ranges. Predation by other species such as coyotes and bobcats may also reduce lamb recruitment (U.S. Fish and Wildlife Service 2000a). Holl (2002) and Holl and others (2002) both implicated predation by mountain lions as a primary cause of the huge decline of the San Gabriel Mountain sheep herd.
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Information gathered from California DFG - California Interagency Wildlife Task Group