Bighorn sheep (Ovis canadensis)

Also known as: Mountain sheep
French: Mouflon D'Amérique, Mouflon Du Canada, Mouflon Pachycère
Spanish: Borrego Cimarrón, Carnero Del Canadá, Carnero Salvaje
GenusOvis (1)
SizeLength: 1.5 – 1.8 m (2)
Male weight: 57 – 140 kg (3)
Female weight: 56 – 80 kg (3)

Classified as Least Concern (LC) on the IUCN Red List (1). The Mexican population is also listed on Appendix II of CITES (4).

The bighorn sheep is named for its massive, spiral horns, which in the male can reach lengths of over a metre (5) and weigh up to 14 kilograms, equalling the weight of the entire skeleton (2). The coat of this species is hairy rather than woolly, and coloured glossy brown in the summer, becoming paler in the winter (2). The hooves are black, the tail is short, and the rump has a conspicuous pale patch (6). The female’s horns, though large compared to some species of sheep, are much smaller than the male’s, and less curved (2). 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) (7) (8)

The bighorn sheep ranges from south-west Canada, south through the western and central USA to northern Mexico (2).

The bighorn sheep is found in a variety of open, rocky habitats, including alpine meadows, grassy mountain slopes, desert, and sparsely vegetated foothills, close to rocky cliffs and bluffs (3) (5).

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 (2) (5). This species is generally active during the cooler parts of the day, resting in a shallow, scraped-out depression during hotter periods (5) (6). While nocturnal activity has also been recorded (5), this species typically spends the night huddled in groups on rocky slopes that offer a wide view of the surroundings to guard against predation (6). During the day, the herds, which may number more than 100 individuals, forage over home ranges spanning several kilometres (5). The diet varies according to location, but mainly comprises grasses, along with forbs, and the leaves, shoots and twigs of available shrubs and trees (1). Outside the breeding season, there are two distinct types of herds, which comprise either sexually mature males or females and mixed-sex, immature offspring (1) (5). 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 (5).

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 (2) (5). These contests may last for hours until one of the rams give in (2). After a gestation period of around 174 days, the female usually gives birth to a single young (5), though litters of two and three offspring have also been recorded (2). After the first few weeks, the young spend little time around the mother, and instead form groups with other juveniles, returning only to suckle (5). 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 (5). Within the herds, the younger members are taught seasonal pathways and suitable habitats by the adults (2). Females usually mate and produce offspring at around 18 months old, whereas males do not usually breed until over 3 years old (1), when developed enough to successfully compete for a mate (5). On average, the bighorn sheep lives for around 9 years, with males rarely exceeding 12 years, and females 15 years (5).

During the late 19th and early 20th century, bighorn sheep numbers suffered a dramatic decline as result of overhunting and competition with domestic sheep and goats. This was further compounded by outbreaks of disease, such as mange and pneumonia, which were generally transferred from domestic livestock. Today, thanks to extensive conservation and management efforts the bighorn sheep has made a substantial recovery, and with a relatively large, stable total population, is no longer considered to be threatened. Nevertheless, the situation for some regional populations is much less favourable, as the small herd sizes are particularly vulnerable to extirpation. Without conservation action, the ongoing habitat degradation, poaching, and spread of disease affecting these populations could potentially lead to the loss of unique races of the bighorn sheep (1).

A significant proportion of the bighorn sheep’s total population can be found within the many Canadian and US protected areas that occur within its range. These areas provide a valuable refuge from hunting and habitat degradation, although poaching remains problematic. Outside the protected areas, populations are widely hunted, but generally well managed through well-enforced hunting laws, requiring permits and specifying annual quotas (1). Collaboration between hunters and conservationists has led to the formation of numerous conservation groups, which are working to ensure that the bighorn sheep remains abundant (9). Various programmes have been initiated such as habitat improvement and the translocation of bighorn sheep to new areas, along with research into this species’ biology. Nevertheless, it is important that the small, declining herds that are found in some regions receive specific attention in order to prevent extirpation (1).

In Mexico, the bighorn sheep is listed on Appendix II of the Convention on International Trade in Endangered Species, which restricts all international trade in this species through the use of trade permits and export quotas. Nevertheless, populations in Mexico are seriously affected by poaching and a lack of protected areas. This situation may be changing, however, as private landowners, capitalising on the high prices that hunters are willing to pay to hunt wild sheep, are now investing funds into monitoring and management programmes for the bighorn sheep (1).

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  1. IUCN Red List (July, 2014)