A bighorn ram's horns can weigh as much as all its bones
Two bighorn rams walk apart, turn, rear up and crash their heads together, again and again, before the autumn rut. Their curled horns can weigh 14 kilograms, as much as the rest of the skeleton combined, and the tips are often worn blunt from years of collisions on North America's cliffs.
Wild sheep reached Alaska from Siberia across the Bering land bridge around 750,000 years ago and spread south as far as Baja California and northern Mexico. North America now has two mountain sheep species: the Dall sheep of Alaska and northwestern Canada, and the bighorn, which ranges from southwestern Canada to Mexico. A 1940 scheme recognised seven bighorn subspecies, but DNA studies since 1993 have cut that to three: Rocky Mountain, Sierra Nevada and desert.
Rocky Mountain males occasionally top 230 kilograms, while Sierra Nevada rams reach only about 90. Bighorns favour alpine meadows and grassy slopes near rugged cliffs, where they escape predators, and they avoid deep snow, preferring slopes with less than about 150 centimetres a year. Unlike the mouflon, ancestor of domestic sheep, their herds have no single leader; rams sort out rank by clashing before the rut, which peaks in November. Lambs arrive in May and can walk within hours of birth.
Two centuries ago an estimated 150,000 to 200,000 bighorns roamed the West. Unregulated hunting, habitat loss, overgrazing and diseases from livestock drove a steep decline, worst between about 1870 and 1950. Pneumonia bacteria that domestic sheep carry without symptoms remain a major threat, with outbreaks sometimes killing up to 90% of a herd.
Recovery efforts started early. In 1936 Arizona's Boy Scouts launched a statewide campaign after Major Frederick Russell Burnham noted that fewer than 150 bighorns remained in the state's mountains. Conservation groups joined in, and on January 18, 1939, over 600,000 hectares became the Kofa and Cabeza Prieta National Wildlife Refuges. Today the Sierra Nevada bighorn and the Peninsular population are listed as endangered by the United States government.
Source: Bighorn sheep
