Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae)
“The wild sheep ranks highest among the animal mountaineers of the Sierra. Possessed of keen sight and scent, and strong limbs, he dwells secure amid the loftiest summits, leaping unscathed from crag to crag, up and down the fronts of giddy precipices, crossing foaming torrents and slopes of frozen snow, exposed to the wildest storms, yet maintaining a brave, warm life, and developing from generation to generation in perfect strength and beauty.” —John Muir
Image above: Bighorn sheep rams, Langley herd, 2016. Credit: California Department of Fish and Wildlife
Naturally limited by the harsh terrain in which it lives, the bighorn was likely never very abundant in numbers, and it is estimated that approximately 1,000 of the animals have lived in the Sierra Nevada at any given time historically. The primary reasons for the decline of bighorn sheep have been identified as a combination of disease transmission from domestic sheep, predation by mountain lions, and unregulated hunting. Bighorn sheep have survived in the Sierra Nevada through at least three ice ages, or approximately 300,000 years, after diverging from the desert bighorn sheep approximately 300-400 thousand years ago.
Contact between wild bighorn sheep and domestic livestock—particularly domestic sheep—has long been one of the greatest threats to the bighorn’s survival since the arrival of Europeans and their livestock in the Sierras. Transmission of respiratory and other diseases has resulted in mass die-offs of bighorns, who have no immunity to domestic livestock diseases. Domestic sheep, goats, and cattle are also much more destructive to plant resources than the nimble bighorn. In the winter when most of the bighorn herds retreat to lower elevation regions, the likelihood of contact with domestic sheep increases. When coupled with predation from mountain lions and loss of genetic diversity from small population size, the threats from other natural causes (weather, climate, avalanches and other random events) present an increased risk for their survival.
By the 1970s, approximately three remaining Sierra bighorn sheep herds were living at the very highest reaches of their range, along the crest of the mountains in the vicinity of Mt. Whitney on the eastside of the Sierra, above Owens Valley (see historic distribution map). Despite heroic efforts led by John Wehausen, founder of the Sierra Nevada Bighorn Sheep Foundation, to translocate sheep to reestablish herds in historical locations, the herds continued to decline. By 1995, there were only about 100 sheep left.
In the Sierra Nevada, 96 percent of deaths of bighorns from predation were found to be from mountain lions. In order to avoid predation by mountain lions, some bighorns appear to be abandoning their winter range to remain at high elevation through the winter. This is a strategy that may be lethal during extreme winter conditions. From 1975-2000, at least 54.5 percent of bighorn sheep deaths overall were from mountain lion kills, a rate of loss that could not be sustained as the herds dwindled, necessitating emergency listing and predator control efforts to reduce the lion kills. Today about 200 bighorn are wearing radio telemetry collars, giving good insight into demographic rates. Five mountain lions are currently outfitted with radio collars, and their movements are closely monitored. As a result, over the last year, CDFW biologists know that 11 sheep were killed by mountain lions. Nevertheless, these kills were distributed among several herds, and scientists do not considered this excessive and no mountain lions had to be killed.
Habitat occurs from the eastern base of the range as low as 4,790 feet to peaks above 14,100 ft . Sierra Nevada bighorn sheep prefer open, rocky areas characterized by steep slopes and canyons that naturally have sparse vegetation where they can see clearly. With keen eyesight, bighorn sheep need to be able to see long distances to escape from predators, especially mountain lions. Fire suppression has been identified as a threat to bighorn because of this, and also because fire is necessary to rejuvenate plants browsed by the sheep. Bighorn sheep are ruminant herbivores, and their diet includes grasses, herbaceous plants, and shrubs. They are very selective feeders and choose the most nutritious forage available. New plant growth is the most nutritious, although this varies by species. Consequently, diet composition varies by season.
In the winter, most bighorn sheep move to lower elevation winter range, where they face the risk of contact with domestic livestock, as well as greater mountain lion predation. At least one herd has begun to remain at high elevation throughout the winter. It is thought that this may be an attempt to evade the heavy mountain lion predation that many herds are facing, however, the harsh winter environment in the alpine regions of the Sierra Nevada present grave risks to the bighorn as well. These risks are becoming more unpredictable with climate change.
Breeding takes place in late fall, generally November and December and most lambs are born in May and June. The lifespan for both Sierra Nevada bighorn sheep males and females has been observed at 8 to 12 years. Adult male rams weight between 120-220 pounds; adult female ewes weight between 110 and 155 pounds. They are five feet long and are approximately 2 ½ to 3 ¼ feet tall at the shoulders. The oldest known ewe documented in the SN lived to be 17 years old, but maximum age is 10-12 years for rams and 12-20 years for ewes.
Both rams and ewes have horns, though the horns of rams are much larger and more curved. The horns of the Sierra Nevada bighorn sheep are more widely spaced than the desert bighorn sheep, whose horns make a tight curl at the ends, while the horns of the Sierra Nevada bighorn continue to flare outwards as they grow.
Rams engage in battles to determine dominance during the breeding (rutting) season. Horn clashes are the most widely known of ram interactions. Rams can be heard for long distances during dominance butting matches.
Thanks to successful translocations, and at least two natural colonizations into new areas, there are currently 14 herd units, with occupied habitat that extends from Yosemite National Park to the bottom of the Owens Valley. Although the most recent published population estimate, from April, 2016, indicated 623 bighorns extant, the winter of 2016-17 was hard on the bighorns, with record precipitation in some areas of the mountains. Staff at CDFW estimate that approximately 100 bighorn sheep may have died. See map of current herd distribution here.
In 1999, with a successful petition for emergency listing under the federal and state Endangered Species Acts, dedicated funding was secured and the Bighorn Sheep Recovery Program was staffed. In 2007 the Sierra Nevada Bighorn Recovery Plan (PDF) was completed and implementation was underway. The recovery actions include (1) Management of disease risk from domestic sheep, (2) translocations (augmentations and reintroductions) to increase bighorn numbers and their geographic distribution, (3) predator management to limit predation on bighorn sheep, and (4) monitoring and management of genetic variation.
Vigilant monitoring combined with a successful program of capture and translocation has resulted in a mostly steady upward trend for the imperiled species. Failing herds have been bolstered with additions of camptured rams and ewes, and the reintroduction to historical herd areas has also been successful. Captured sheep are ear tagged and fitted with radio telemetry collars, they are given a rapid physical assessment, and genetic data is being compiled for almost every individual sheep. Genetic data are obtained from fecal samples, blood samples, carcasses, and horns painstakingly gathered for decades, providing a wealth of genetic information that is used to inform current and future translocations in respect to available nutritional sources and threats such as mountain lion predation. The California Department of Fish and Wildlife is the lead agency responsible for managing recovery of the bighorn sheep, but the Bighorn Sheep Recovery Program is an interagency effort with support from US Fish & Wildlife Service, the National Park Service, BLM, US Forest Service, and USDA Wildlife Services.
Critical habitat was designated by US Fish and Wildlife Service in 2008, encompassing 417,577 acres in mountainous regions of Tuolumne, Mono, Fresno, Inyo, and Tulare Counties, California. The recovery area encompasses two national parks (Yosemite and Sequoia-Kings Canyon), four national forests (Inyo, Humboldt-Toiyabe, Sierra, and Sequoia), and lands managed by the Bureau of Land Management. The recovery area is almost entirely federal land and most of it is in federally designated wilderness areas (John Muir, Ansel Adams, Hoover, and Golden Trout). While critical habitat designation does not apply to private land, grazing permits issued by the BLM and Forest Service allow private individuals to graze livestock on publicly owned lands. Thus, designation of critical habitat for endangered species is frequently contested by the ranching/livestock industry.
When John Muir first visited the Sierra Nevada, he was shocked by the level of destruction from the “hooved locusts” that were pastured by the millions in the mountains of California. Writing in his journal in the fall of 1873 in the Yosemite region, he wrote: “It is almost impossible to conceive of a devastation more universal than is produced among the plants of the Sierra by sheep. The grass of meadows is eaten close and trodden until it resembles a corral…9/10ths of whole surface of the Sierra is swept by this scourge. Demands legislative interference.” Legislative interference took the form of designation of Yosemite Park, but despite this, trespass by scofflaw livestock herders continued for decades after the park was established.
The Inyo National Forest Plan Revision
The draft Inyo National Forest is currently undergoing forest plan revision. The Forest Service is required to follow the recovery plan for the bighorn, since it is a federally protected species. The goal of the recovery plan is to attain population sizes and geographic distribution of bighorn sheep in the Sierra Nevada that assure long-term viability of the overall population and thereby allow its delisting as an endangered species. To that end, the Inyo NF forest plan revision defers to the recovery plan, but lacks specifics for what the agency might do to protect the sheep from contact with domestic sheep and other livestock. Read the 2016 Inyo NF draft plan here and the draft environmental analysis here.
For additional research and other information, visit the California Department of Fish and Wildlife's website. The SN Bighorn Recovery Program has a wealth of research materials, maps, and media.
Clifford, D.L. et al. 2007. Modeling risks of disease transmission from domestic sheep: implications for the persistence and restoration of an endangered endemic ungulate. U.C. Davis Wildlife Health Center DFG Resource Assessment Program: Final Report.