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The bighorn sheep is a species of sheep native to North America named for its large horns. These horns can weigh up to 30 lb (14 kg), while the sheep themselves weigh up to 300 lb (140 kg) By 1900, the population had crashed to several thousand, due to diseases introduced through European livestock and overhunting.

Bighorn sheep are named for the large, curved horns borne by the rams (males). Ewes (females) also have horns, but they are shorter with less curvature.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 127–316 lb (58–143 kg), are 36–41 in (91–104 cm) tall at the shoulder, and 69–79 in (180–200 cm) long from the nose to the tail. Females are typically 75–188 lb (34–85 kg), 30–36 in (76–91 cm) tall and 54–67 in (140–170 cm) 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 500 lb (230 kg) and females that exceed 200 lb (90 kg). In contrast, Sierra Nevada bighorn males weigh up to only 200 lb (90 kg) and females to 140 lb (60 kg). Males' horns can weigh up to 30 lb (14 kg), as much as the rest of the bones in the male's body.

The Rocky Mountains and Sierra Nevada bighorn sheep 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 Southwestern United States and Mexico. Bighorn sheep generally inhabit alpine meadows, grassy mountain slopes, and foothill country near rugged, rocky cliffs and bluffs. Since bighorn sheep cannot move though deep snow, they prefer drier slopes, where the annual snowfall is less than about 60 inches a year. A bighorn's winter range usually lies at lower elevations than its summer range.

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 falls or falling off cliffs (a hazard of living in steep, rugged terrain). Bighorns are well adapted to climbing steep terrain where they seek cover from predators. Predation primarily occurs with lambs, which are hunted by coyotes,bobcats, lynxes and golden eagles.

Bighorn sheep of all ages are threatened by bears, wolves and especially cougars, which are perhaps best equipped with the agility to prey on them in uneven, rocky habitats. They 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.

Bighorn sheep graze on grasses and browse shrubs, particularly in fall and winter, and seek minerals at natural salt licks. Females tend to forage and walk, possibly to avoid predators and protect lambs, while males tend to eat and then rest and ruminate, which lends to more effective digestion and greater increase in body size.

Bighorn sheep live in large herds, and do not typically follow a single leader ram, unlike the mouflon, the ancestor of the domestic sheep, which has a strict dominance hierarchy. Prior to the mating season or "rut", the rams attempt to establish a dominance hierarchy to determine access to ewes for mating. During the prerut period, most of the characteristic horn clashing occurs between rams, although this behavior may occur to a limited extent throughout the year. Bighorn sheep exhibit agonistic behavior: two competitors walk away from each other and then turn to face each other before jumping and lunging into headbutts. Rams' horns can frequently exhibit damage from repeated clashes. Females exhibit a stable, nonlinear hierarchy that correlates with age. Females may fight for high social status when they are integrated into the hierarchy at one to two years of age.

Rocky Mountain bighorn rams employ at least three different courting strategies. The most common and successful is the tending strategy, in which a ram follows and defends an estrous ewe. Tending takes considerable strength and dominance, so ewes are more receptive to tending males, feeling they are the most fit. Another tactic is coursing, which is when rams fight for an already tended ewe. Ewes typically avoid coursing males so the strategy is not effective. Rams will also employ a blocking strategy. They will prevent a ewe from accessing tending areas before she even goes into estrus.


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