The bighorn sheep is named for its massive, spiral horns, which in the male can reach lengths of over a metre and weigh up to 14 kilograms, equalling the weight of the entire skeleton. The coat of this species is hairy rather than woolly, and coloured glossy brown in the summer, becoming paler in the winter. The hooves are black, the tail is short, and the rump has a conspicuous pale patch. The female’s horns, though large compared to some species of sheep, are much smaller than the male’s, and less curved. There is some debate regarding the taxonomy of the bighorn sheep, with variable numbers of subspecies recognised by different authorities. The most recent research indicates that there may be three distinct subspecies, which occupy separate geographical areas: the Rocky Mountain bighorn sheep (Ovis canadensis canadensis); the Sierra Nevada bighorn sheep (Ovis canadensis sierrae), and the desert bighorn sheep (Ovis canadensis nelsoni)
Well adapted to its rocky environment, the agility and keen eyesight of the bighorn sheep help it to detect and evade predators, and when threatened, it will bound away over rocks, climbing nearly vertical rock faces where its pursuer can rarely follow. This species is generally active during the cooler parts of the day, resting in a shallow, scraped-out depression during hotter periods. While nocturnal activity has also been recorded, this species typically spends the night huddled in groups on rocky slopes that offer a wide view of the surroundings to guard against predation. During the day, the herds, which may number more than 100 individuals, forage over home ranges spanning several kilometres. The diet varies according to location, but mainly comprises grasses, along with forbs, and the leaves, shoots and twigs of available shrubs and trees. Outside the breeding season, there are two distinct types of herds, which comprise either sexually mature males or females and mixed-sex, immature offspring. Both herds make seasonal movements, which exceptionally have been known to cover 48 kilometres, dispersing into more expansive upland regions in the summer, and concentrating in sheltered valleys in the winter.
The bighorn sheep breeds during the autumn and early winter, with births taking place in the spring. During the mating period or “rut”, males take part in spectacular battles for dominance and the chance to mate. After pushing and shoving one another, the males back away, before rearing up on the hind legs and lunging forwards and down, bringing the horns together with tremendous force. These contests may last for hours until one of the rams give in. After a gestation period of around 174 days, the female usually gives birth to a single young, though litters of two and three offspring have also been recorded. After the first few weeks, the young spend little time around the mother, and instead form groups with other juveniles, returning only to suckle. Weaning takes place at four to six months old, after which the young join the mother’s herd. Females often remain in this herd for life, whereas males depart at around two to four years old and join a herd of rams. A dominance hierarchy exists in both types of herd, which is defined mostly by age, as well as by horn size amongst the rams. Within the herds, the younger members are taught seasonal pathways and suitable habitats by the adults. Females usually mate and produce offspring at around 18 months old, whereas males do not usually breed until over 3 years old, when developed enough to successfully compete for a mate. On average, the bighorn sheep lives for around 9 years, with males rarely exceeding 12 years, and females 15 years.