Habitat
Bighorn sheep occur in mesic to xeric, alpine to desert grasslands or shrub-steppe in mountains, foothills, or river canyons (Shackleton et al. 1999, Krausman et al. 1999). Many of these grasslands are fire-maintained (Geist 1971, Erickson 1972). Suitable escape terrain (cliffs, talus slopes, etc.) is an important feature of the habitat. In winter, Rocky Mountain Bighorns spend as much as 86% of their time within 100 meters of escape terrain (Oldemayer et al. 1971, Erickson 1972), and usually stay within 800 meters of escape terrain throughout the year (Pallister 1974). Mineral licks are more important in the range of Rocky Mountain Bighorn than in the range of "California" Bighorn, presumably because the soils in the range of the former are generally lower in mineral content (Van Dyke 1978). Distribution is correlated with low precipitation levels, especially in winter and spring. Elevation varies considerably, both geographically and seasonally, from as low as 450 meters to over 3,300 meters (Shackleton et al. 1999).
Winter ranges of northern populations are relatively snow-free because of light snow, steep south aspect, and/or high winds; bighorns generally avoid snow deeper than 30 centimeters (Stelfox 1975). The solar heat on south aspects also reduces cold stress on sheep (Shackleton et al. 1999).
In the north, bighorn sheep usually are not dependent on free-standing water, getting water instead from succulent vegetation in the summer and snow or ice in the winter (Van Dyke 1978). However, in some southwestern deserts they may be dependent on access to free water during summer (Turner 1979, Turner and Weaver 1980, Seegmiller and Ohmart 1981).
Bighorn Sheep - Peninsular Ranges
Habitat includes steep slopes and cliffs, rough and rocky topography, and sparse vegetation (Monson and Summner 1980), "with use of alluvial fans and washes, and valley floors depending on environmental conditions and dispersal requirements" (USFWS 2011; see also Monson and Sumner 1980 and USFWS 2000). Females about to give birth "seek secluded sites with shelter, unobstructed views, and steep terrain, ...while rams may be found in less steep or rugged terrain" (USFWS 2011). In the Peninsular Ranges, bighorn sheep generally stay elevationally below the pinyon pine-juniper and chararral vegetation zones (USFWS 2011). Vegetation of occupied habitat often includes Agave deserti (agave)-Fouquieria splendens (ocotillo), Opuntia spp. (cholla)-Cercidium floridum (palo verde), and Larrea tridentata (creosote)-Prosopis spp. (palo verde-mesquite). Bighorns use open areas and generally avoid areas where visibility is obstructed by thick vegetation.
Valley floors provide important linkages between adjacent mountainous regions and allow bighorn sheep access to resources (e.g., forage, water, lambing habitat) in neighboring areas; these lowlands also allow gene flow to occur between subpopulations (USFWS 2011). Bighorns may forage or access water on gentle terrain (e.g., alluvial fans and washes) but may travel quickly through valley floors to reach nearby mountains (Simmons, in Monson and Sumner 1980). Reproductive females may make heavy use of the food resources on alluvial fans and washes, and these areas can be important in summer and during droughts, when nutritious forage is scarce elsewhere (Andrew 1994).
North-facing slopes provide cooler refuges during the hottest weather, and south-facing slopes may be used on cold winter days (Andrew 1994).
In summer, bighorn sheep may congregate around water sources; most sheep stay wihtin 3-5 km of water (see USFWS 2011). They may abandon areas where no standing water exists. Above-average rainfall and cooler temperatures may allow bighorns to occupy habitats that may be abandoned during droughts (USFWS 2011).
Suitable habitat exists as a narrow north-south continuum from the San Jacinto Mountains all the way to the U.S.-Mexico border (Rubin et al. 2009).
Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae)
Sierra Nevada bighorn sheep inhabit open areas where the land is rocky, sparsely vegetated, and characterized by steep slopes and canyons.. Habitats range from alpine to Great Basin sagebrush scrub. They prefer open ground and areas with good access to steep, rocky terrain (escape habitat) and so generally avoid forests, thick brush, and large expanses lacking precipitous escape terrain. In summer, most live at higher elevations (10,000-14,000 feet; 3,050-4,270 meters) in subalpine and alpine areas. Females occur largely in alpine environments, whereas males often are at somewhat lower elevations in subalpine habitats. In winter, they occupy high-elevation, windswept ridges if forage is available and tend to inhabit south-facing slopes where snow melts more readily, or they migrate to lower elevations in sagebrush-steppe areas to avoid deep snow and to find forage. Low-elevation winter ranges provide an important source of high quality forage early in the growing season. Reproductive female select steep, rugged slopes and canyons for lambing. Sources: McCullough and Schneegas (1966), Wehausen (1980), USFWS (2007, 2008).
USFWS (2008) determined that primary constituent elements (habitat features) for Sierra Nevada bighorn sheep include: (1) Non-forested habitats or forest openings within the Sierra Nevada from 4,000 feet (1,219 meters) to 14,500 feet (4,420 meters) in elevation with steep (greater than or equal to 60 percent slope), rocky slopes that provide for foraging, mating, lambing, predator avoidance, and bedding and that allow for seasonal elevational movements between these areas. (2) Presence of a variety of forage plants as indicated by the presence of grasses (e.g., Achnanthera spp.; Elymus spp.) and browse (e.g., Ribes spp.; Artemisia spp., Purshia spp.) in winter, and grasses, browse, sedges (e.g., Carex spp.) and forbs (e.g., Eriogonum spp.) in summer. (3) Presence of granite outcroppings containing minerals such as sodium, calcium, iron, and phosphorus that could be used as mineral licks in order to meet nutritional needs.
Johnson et al. (2007) used resource selection probability functions to identify important winter and summer habitat characteristics, and to generate predictive models of habitat use in unoccupied ranges. "Characteristics of topography and vegetation were significant in describing bighorn sheep winter habitat use, and only topographic characteristics were significant in describing summer habitat use. Habitat models were used to determine the amount of winter and summer range within each herd unit, the connectivity of seasonal ranges, areas at risk of contact with domestic sheep, and to simulate the effects of prescribed fire on bighorn sheep habitat."
Ecology
Gregarious, but for most of the year adult males live apart from females/young (Shackleton et al. 1999, Krausman et al. 1999). Among mature males, older males (up to an age of not more than 10 years) generally dominate younger males during the breeding season; males older than 10 years decline rapidly in condition. In western Arizona, January-June home range of adult females was 19-27 sq km (Seegmiller and Ohmart 1981). Male annual home range up to 37 sq km in Nevada (Leslie and Douglas 1979).
Carrying capacity for bighorn can be reduced through grazing by other ungulates (cattle, burros, etc.). In the Santa Catalina Mountains of southern Arizona, not limited by forage quantity or quality (Mazaika et al. 1992).
Contact with a stray domestic sheep is believed to have resulted in the death (through bacterial pneumonia) of entire reintroduced herd of 65 in Warner Mountains, California (California Department of Fish and Game 1990). In some areas, lungworm infections may predispose bighorn to respiratory infection by opportunistic bacteria; lungworm life cycle involves gastropod intermediate host.
In desert, can survive 10 or more days in summer without drinking; may meet water needs in part by eating barrel cacti (Warrick and Krausman 1989).
PREDATION: Probably live in groups primarily to reduce predation (Shackleton et al. 1999). Coyotes may be a significant predator on young in some areas, killing up to 80% of the year's lambs (Hebert and Harrison 1988, Harper 1984, Hass 1989). Cougars can be important predators as well (Harrison and Hebert 1988, Krausman et al. 1999), and can have significant impacts on remnant or transplant herds (Krausman et al. 1999). Direct losses to predation are not generally as important as the fact that predation has forced females and young to use less productive habitats in and near escape terrain (Festa-Bianchet 1988, Demarchi et al. 1999a).
Reproduction
The timing of the mating season varies throughout the range. Bighorns in southwestern deserts have an extended season encompassing several months (Krausman et al. 1999), but the season is relatively later and shorter elsewhere, generally November in the northern part of the range (Shackleton et al. 1999), November-December in some southern California mountains (DeForge (1980). Gestation lasts about 175 days (Geist 1971, Shackleton et al. 1999). Lambing generally peaks in March in desert populations (Rubin et al. 2000), May (occasionally April or June) in the remainder of range (Krausman et al. 1999, Shackleton et al. 1999). Litter size is 1, rarely 2 (Geist 1971, Turner and Hansen 1980).Young are weaned in 4-6 months. Females first breed usually in second year in south, third year in north; occasionally in first year in some areas (Krausman et al. 1999, Shackleton et al. 1999); fecundity generally declines only slightly after eight years of age (Caughley 1977).
In the mating season, mature males battle over access to females through vigorous head butting contests, but during most of the rest of the year they live amiably in small bands apart from the females.