Bighorn sheep

The bighorn sheep (Ovis canadensis) is a species of sheep native to North America. It is named for its large horns. A pair of horns may weigh up to 14 kg (30 lb); the sheep typically weigh up to 143 kg (315 lb). Recent genetic testing indicates three distinct subspecies of Ovis canadensis, one of which is endangered: O. c. sierrae.

Sheep originally crossed to North America over the Bering Land Bridge from Siberia; the population in North America peaked in the millions, and the bighorn sheep entered into the mythology of Native Americans. By 1900, the population had crashed to several thousand due to diseases introduced through European livestock and overhunting.

Ovis canadensis is one of two species of mountain sheep in North America; the other species being O. dalli, the Dall sheep. Wild sheep crossed the Bering land bridge from Siberia into Alaska during the Pleistocene (about 750,000 years ago); subsequently, they spread through western North America as far south as Baja California and northwestern mainland Mexico. Divergence from their closest Asian ancestor (snow sheep) occurred about 600,000 years ago. In North America, wild sheep diverged into two extant species — Dall sheep, which occupy Alaska and northwestern Canada, and bighorn sheep, which range from southwestern Canada to Mexico. However, the status of these species is questionable given that hybridization has occurred between them in their recent evolutionary history.

In 1940, Ian McTaggart-Cowan split the species into seven subspecies, with the first three being mountain bighorns and the last four being desert bighorns:

Starting in 1993, Ramey and colleagues, using DNA testing, have shown this division into seven subspecies is largely illusory. Most scientists currently recognize three subspecies of bighorn. This taxonomy is supported by the most extensive genetics (microsatellite and mitochondrial DNA) study to date (2016) which found high divergence between Rocky Mountain and Sierra Nevada bighorn sheep, and that these two subspecies both diverged from desert bighorn before or during the Illinoian glaciation (about 315–94 thousand years ago). Thus, the three subspecies of O. canadensis are:

In addition, two populations are currently considered endangered by the United States government:

Bighorn sheep are named for the large, curved horns borne by the rams (males). Ewes (females) also have horns, but they are shorter and straighter. They range in color from light brown to grayish or dark, chocolate brown, with a white rump and lining on the backs of all four legs. Males typically weigh 58–143 kg (128–315 lb), are 90–105 cm (35–41 in) tall at the shoulder, and 1.6–1.85 m (63–73 in) long from the nose to the tail. Females are typically 34–91 kg (75–201 lb), 75–90 cm (30–35 in) tall, and 1.28–1.58 m (50–62 in) long. Male bighorn sheep have large horn cores, enlarged cornual and frontal sinuses, and internal bony septa. These adaptations serve to protect the brain by absorbing the impact of clashes. Bighorn sheep have preorbital glands on the anterior corner of each eye, inguinal glands in the groin, and pedal glands on each foot. Secretions from these glands may support dominance behaviors.

Bighorns from the Rocky Mountains are relatively large, with males that occasionally exceed 230 kg (500 lb) and females that exceed 90 kg (200 lb). In contrast, Sierra Nevada bighorn males weigh up to only 90 kg (198 lb) and females to 60 kg (132 lb). Males' horns can weigh up to 14 kg (30 lb), as much as all the bones in the male's body.