From Wikipedia, the free encyclopedia.
Bighorn Sheep (Ovis canadensis) is a species of sheep in North America with two endangered subspecies:
- Desert Bighorn Sheep (Ovis canadensis nelsoni)
- California Bighorn Sheep (Ovis canadensis californiana).
Wild sheep crossed the Bering land bridge from Siberia during the Pleistocene and, subsequently, spread through western North America as far south as Baja California and northern mainland Mexico (Cowan 1940). Divergence from their closest Asian ancestor (Snow Sheep) occurred about 600,000 years ago (Ramey 1993). In North America, wild sheep have diverged into two extant species -- thinhorn sheep that occupy Alaska and northwestern Canada, and bighorn sheep that range from southern Canada to Mexico. However, the status of these species is questionable given that hybridization has occurred between them in their recent evolutionary history (Loehr et al. 2005).
Bighorn sheep were amongst the most admired animals of the Apsaalooka, or Crow, people, and what is today called the Bighorn Mountain Range was central to the Apsaalooka tribal lands. In the Bighorn Canyon National Recreation Area book, storyteller Old Coyote describes a legend related to the bighorn sheep. A man possessed by evil spirits attempts to kill his heir by pushing the young man over a cliff, but the victim is saved by getting caught in trees. Rescued by bighorn sheep, the man takes the name of their leader, Big Metal. The other sheep grant him power, wisdom, sharp eyes, sure footedness, keen ears, great strength and a strong heart. Big Metal returns to his people with the message that the Apsaalooka people will survive only so long as the river winding out of the mountains is known as the Bighorn River.
Bighorn Sheep are named for the large, curved horns borne by the males, or rams. Females, or ewes, also have horns, but they are short with only a slight curvature. They range in color from light brown to grayish or dark, chocolate brown, with a white rump and lining on the back of all four legs. Rocky Mountains bighorn females weigh up to 200 pounds (90 kg), and males occasionally exceed 300 pounds (135 kg).
Bighorn sheep graze on grasses and browse shrubby plants, particularly in fall and winter, and seek minerals at natural salt licks. Bighorns are well adapted to climbing steep terrain where they seek cover from predators such as coyotes, eagles, and pumas. They are susceptible to disease such as lungworm, and sometimes fall off cliffs.
During the mating season or "rut" the rams butt heads in apparent sparring for females. Rams' horns can weigh more than 40 pounds (18 kg), and frequently show broken or "broomed" tips from repeated clashes. At the same time, rams are known to actively engage in sex with each other, including group sex of up to ten participants. This behavior happens year-round, but ewes show interest in sex only during the mating season.
Bighorn Sheep are considered good indicators of land health because the species is sensitive to many human-induced environmental problems. In addition to their aesthetic value, Bighorn Sheep are considered desirable game animals by hunters. The Rocky Mountain and California races of bighorn occupy the cooler western and northwestern regions of Canada and the United States. In contrast, the Desert Bighorn Sheep subspecies are indigenous to the hot desert ecosystems of the Southwest United States.
In 1940, Cowan taxonomically split the species into seven subspecies :
- Rocky Mountain Bighorn Sheep Ovis canadensis canadensis. Habitat: from British Columbia to Arizona.
- California Bighorn Sheep Ovis canadensis californiana. Owens defined the habitat from British Columbia down to California and over to North Dakota. The definition of this subspecies has been updated (see below).
- Nelson's Bighorn Sheep Ovis canadensis nelsoni, the most common desert bighorn sheep, ranges from California through Arizona.
- Mexicana Bighorn Sheep Ovis canadensis mexicana, range from Arizona and New Mexico down to Sonora and Chihuahua.
- Peninsular Bighorn Sheep Ovis canadensis cremnobates. Habitat: the Peninsular Ranges of California and Baja California.
- Weems' Bighorn Sheep Ovis canadensis weemsi. Habitat: Baja California.
- Audubon's Bighorn Sheep Ovis canadensis auduboni. Habitat: North Dakota, South Dakota, Montana, Wyoming, Nebraska. Extinct since 1925.
However, starting in 1993, Ramey and colleagues, using DNA testing, have shown that this division into seven subspecies is largely illusory. The latest science shows that Bighorn Sheep is one species, with two subspecies O. c. nelsoni and O. c. californiana. O. c. californiana is a genetically distinct subspecies that only occurs in the Sierra Nevada.
- Cowan, I. McT. (1940) Distribution and variation in the native sheep of North America, American Midland Naturalist 24:505-580.
- Loehr, J., K. Worley, A. Grapputo, J. Carey, A. Veitch and D. W. Coltman. (2005) Evidence for cryptic glacial refugia from North American mountain sheep mitochondrial DNA, Journal of Evolutionary Biology, in press.
- Ramey, R. R. II. (1993) Evolutionary genetics and systematics of North American mountain sheep, Ph.D. Thesis, Cornell University, Ithaca, NY.
- Wehausen, J. D., and R. R. Ramey II. (1993) A morphometric reevaluation of the Peninsular bighorn subpecies, Trans. Desert Bighorn Council, 37:1-10.
- Description of Bighorn Sheep at Yellowstone Park (public domain source)
- U.S. Fish and Wildlife Service draft recovery plan for the Sierra Nevada Bighorn Sheep (public domain source)
- Ovis canadensis