Sierra Nevada Bighorn Sheep

  • Endangered Status: State and federally endangered
  • Size: 3 feet tall at shoulders. Females weigh 140 pounds / males 220 pounds
  • Color: Varies from white to dark brown with a large white rump patch and a short, dark tail
  • Horns: Females have small narrow horns less than 12 inches / males have larger horns
  • Territory: Steep terrain over 10,000 feet elevation on the northeastern edge of the park
  • Reproduction: Females can bear their first lamb at age 2 and then one lamb per year
Collared-sheep runs across rocky landscape with snow

Scientists collar Sierra Nevada bighorn sheep to track their movements.

CA Department of Fish and Game

2010-2013 Interagency Study: In a partnership between Yosemite and the California Department of Fish and Game, GPS satellite collars are placed on captured Sierra Nevada bighorn sheep to view locations.

The Sierra Nevada bighorn sheep (Ovis canadensis sierrae) is the only mammal in Yosemite National Park that is listed as endangered under the federal Endangered Species Act. This designation came in 2000, after the species reached a Sierra-wide critically low population of about 125 animals.

No one knows how many bighorns originally ranged over the Sierra Nevada, but if they occupied all suitable habitat, they probably numbered several thousand. Although bighorns are rarely seen by visitors today, they stand as a true symbol of wilderness at their elevation of 10,000-feet-plus and represent a need to protect wild lands.

sheep bends down toward rocky ground

Approximately 400 Sierra Nevada bighorn sheep exist.

Copyright Ron Wolf

Bighorns inhabit the highest alpine elevations and among the sharp, craggy peaks of the Sierra Nevada and forage for sparse grasses and other alpine plants adapted to the short, windy summers, and long, cold winters. These plants must, however, be located on open plateaus and meadows where approaching predators can be detected by the bighorns, and within sprinting distance of "escape" terrain: steep, rocky terrain where predators would have a hard time keeping up with the bighorns. Bighorns seem to glide effortlessly across steep cliff faces, using even the narrowest ledges as foodholds. This surefootedness is due, in part, to an interesting adaptation. Bighorns have soft pads on the bottoms of their hooves, which give them traction over rocky terrain. This, combined with phenomenal strength and balance, allows even bighorn lambs to race across seemingly featureless, vertical cliffs. John Muir, in his writings, referred to the Sierra Nevada bighorn sheep as "animal mountaineers."

With the arrival of gold miners in the 1850s and sheep grazers soon thereafter, to the Sierra, the stage was set for decimation of Sierra Nevada bighorns. Miners killed large numbers of bighorns for food. The largest factor, however, that persists as a threat to this day, is exposure to the diseases that all domestic sheep carry. Bighorns have no natural resistance to these diseases to which domestic sheep have adapted. The result is that it takes only one nose-nose contact between a bighorn and a domestic sheep to fatally infect the bighorn. The infected bighorn, however, most often carries the diseases back to its main herd, resulting in a "wildfire" of death that sweeps through the herds and can kill hundreds of bighorns in several months. The highest priority for recovery of Sierra bighorns, therefore, is to remove any chance of contact between domestic sheep and bighorns.

So dramatic was this decline that bighorns were granted full protection by the State of California as early as 1882. Despite this protection, the remnant herds continued to dwindle, until only two populations remained in 1979, both in the southern Sierra Nevada, and totaling only 125 animals. This precarious status prompted the formation of the Sierra Bighorn Interagency Advisory Group in 1981 with the goal of reversing this decline by establishing more populations. These efforts included the introduction of 38 bighorns near Tioga Pass called the Yosemite Herd in the late 1980s. (Scroll down to see graphic of herd locations.) Although this herd spends most of its time outside the park on U.S. Forest Service land, its growth and success will likely result in reoccupation of habitats in the park. This population grew to nearly 100 animals by 1993, then unknown factors resulted in a population crash with a loss of more than 60% and a continued decline over the next several years. By 2000, only 20 bighorns remained in the Yosemite Herd. Other herds in the Sierra Nevada were experiencing similar plummets in population. This sudden plunge prompted emergency listing of the Sierra Nevada bighorn sheep as both a state and federal endangered subspecies.
A single bighorn sheep stands erect

Bighorn sheep is Yosemite's only federally endangered animal.

Copyright Ron Wolf

The recovery of the bighorns continues to be an interagency effort with California Department of Fish and Game, U.S. Forest Service, U.S. Fish and Wildlife Service, and the National Park Service joining forces. A fundamental part of this recovery effort has been to determine the causes of mortality that may have resulted in the sudden decline in populations, and could be continuing to limit their growth. Recovery of Sierra Nevada bighorn sheep has been slow but successful, with the current count of the species at approximately 400 with a herd of about 40 near Yosemite. Since 2002, the Department of Fish and Game has followed 29 bighorn sheep in the Yosemite Unit, using radio collars and conventional telemetry units. Although these telemetry units have enabled detection of previously unknown movement patterns, this monitoring system requires extensive fieldwork and signal triangulations to collect relatively few data points.

In 2010, a collaborative study between the Department of Fish and Game and Yosemite seek a more efficient monitoring system of bighorns. Using advanced innovative technology, GPS satellite radio collars will be deployed on a portion of Yosemite's bighorn sheep population over a three-year period (2010 – 2013). These GPS collars, sensed by an orbiting satellite, can provide biologists with detailed data on bighorn movements and habitat use. In addition, acquiring remote locations via GPS collars will improve the efficiency of field observations to determine lambing success and will facilitate the collection of forage and fecal samples for nutritional status and genetic analyses. Habitat-use data is especially critical for the Yosemite Herd because the proximity of active domestic sheep grazing allotments places the bighorns at risk of contact. Data generated from GPS collars will enable wildlife managers to mitigate for potential threats and make well-informed management decisions, as well as promote bighorn expansion into historically occupied areas of the park.
Close-up of mountain lion face

Mountain lions are one of the biggest threats to bighorn sheep.

Copyright Green TV

Data from previous studies indicates mountain lions (Felis concolor) could also be affecting the bighorns in two ways. First is reduction in bighorn numbers through predation. The second effect of the lions was more complex. It appeared that the bighorns had abandoned their low-elevation wintering areas, likely because of the threat of lion predation. More bighorns were wintering at high elevations, where conditions are more severe and forage is more sparse. This could have resulted in higher rates of winter mortality, and lower rates of lamb production. As a result of these data, an intensive program of mountain lion monitoring was developed by the U.S. Fish and Wildlife Service to determine which lions are adversely affecting the bighorns and must be targeted for removal. This was not an easy decision for biologists, but until bighorn numbers reach a level at which they can withstand natural levels of predation, intervention and assistance are needed. No lions have been taken in Yosemite.

The restoration of bighorns to Yosemite's alpine summits will be a monumental achievement, both for the benefit of the ecosystems, and the unique opportunity for visitors intrepid enough to view such a spectacular, finely evolved creature.
Other References

a bighorn sheep stands on rocks

Still from Green TV

  • Documentary Film: View segments of the Counting Sheep documentary, which tells the story behind the endangered listing of the Sierra Nevada bighorn sheep by biologist John Wehausen of the UC White Mountain Research Station
    Overview clip (1 minute and 45 seconds)
    Range Map clip (33 seconds)
    Mating clip (3 minutes and 23 seconds)
Map shows locations of sheep herds

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