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Bighorn sheep

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Bighorn Sheep
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Mammalia
Order:
Artiodactyla
Family:
Bovidae
Subfamily:
Caprinae
Genus:
Ovis
Species:
O. canadensis
Binomial name
Ovis canadensis
Shaw, 1804

Bighorn Sheep (Ovis canadensis) are one of two species of mountain sheep in North America; the other species being Ovis dalli, that includes Dall Sheep and Stone's Sheep. The taxononomy continues to be modified as new genetic and morphologic data becomes available but most scientists currently recognize the following subspecies of bighorn (see Wehausen and Ramey 2000, Wehausen et al. 2005):

In addition, there are currently 2 federally endangered populations:

  • Sierra Nevada Bighorn Sheep (Ovis canadensis sierrae), recognized as a unique subspecies
  • Peninsular Bighorn Sheep, a distinct population segment of Desert Bighorn Sheep (Ovis canadensis nelsoni)

Origin

Wild sheep crossed the Bering land bridge from Siberia during the Pleistocene (~750,000 years ago) 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 -- Dall 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).

Mythology

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.

Characteristics

Bighorn Sheep

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). In contrast, Sierra Nevada bighorn females weigh about 140 pounds (63 kg) with males weighing around 200 pounds (90 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 live in large herds, but because they do not have the strict dominance hierarchy of the mouflon, they cannot be domesticated. This is because bighorns do not automatically follow a single leader ram as the Asiatic ancestors of the domestic sheep did and do.

Prior to the mating season or "rut", the rams attempt to establish a dominance hierarchy that determines access to ewes for mating. It is during the prerut period that most of the characteristic horn clashing occurs between rams, although this behavior may occur to a limited extent throughout the year (Valdez and Krausman 1999). Ram's horns can weigh more than 40 pounds (18 kg), and frequently exhibit damage from repeated clashes. Bighorn ewes exhibit a 6 month gestation. In temperate climates, the peak of the rut occurs in November with lambs being born in May.

Bighorn sheep are highly susceptible to certain diseases carried by domestic sheep such as scabies and pneumonia; additional mortality occurs as a result of accidents involving rock fall or falling off cliffs (a hazard of living in steep, rugged terrain).

Scientific Analysis

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 Sierra Nevada bighorn occupy the cooler mountainous 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 [1]:

  • 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.
A well-fed Bighorn ram

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 3 subspecies O. c. canadensis, O. c. nelsoni and O. c. sierrae. O. c. sierrae is a genetically distinct subspecies that only occurs in the Sierra Nevada. O. c. nelsoni occur throughout the southwestern desert regions of the U.S. and Mexico, whereas O. c. canadensis occupy the U.S. and Canadian Rocky Mountains and the northwestern U.S.

References

  • Template:IUCN2006
  • "Ovis canadensis". Integrated Taxonomic Information System. 18 March. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)
  • 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.
  • Valdez, R., and P. R. Krausman. 1999. Mountain Sheep of North America. The University of Arizona Press, Tucson.
  • Wehausen, J. D., and R. R. Ramey II. (1993) A morphometric reevaluation of the Peninsular bighorn subpecies, Trans. Desert Bighorn Council, 37:1-10.
  • Wehausen, J. D., V. C. Bleich, and R. R. Ramey II. (2005) Correct nomenclature for Sierra Nevada Bighorn Sheep. California Fish and Game 91:216-218.
  • 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
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