Bighorn sheep are named for their large, curling horns. The weight of the males' horns can reach 30 pounds, which is more than the combined weight of the sheep's bones. Several subspecies of bighorn sheep inhabit the high country and deserts from Canada to northern Mexico. Rocky Mountain bighorn sheep are the largest, with the males occasionally weighing up to 500 pounds. The desert subspecies tend to be smaller.
Mountain bighorns inhabit the cool regions of the Rocky Mountains and the Sierra Nevada range. They prefer steep, dry slopes where they can spot and flee from predators such as cougars, bears and wolves. Desert bighorns inhabit the dry ranges of the southwestern United States and the Baja Peninsula. Bighorn feed on grass, sedges, leaves, twigs and shoots. Desert bighorns also remove spines from cacti and eat the moist insides. Bighorns live in flocks that are usually separated into same-sex groups of mature males and females with young. Before mating season, males assert dominance by launching themselves at each other and clashing horns. The thick double layer of bone in their skulls usually prevents serious injury, but they continue the conflict until one ram concedes defeat and walks away.
In the 19th century, millions of bighorn sheep were spread over North America. By the early 20th century, their numbers had been reduced to several thousand. Programs of conservation, including reduced hunting, protection in national parks and reintroductions of populations in certain areas, have increased their numbers, but several types of bighorn sheep are considered endangered species.