Bighorn Sheep

Built to Ram

The bighorn sheep (Ovis canadensis) has a compact, muscular body, and it is one of two wild sheep species in North America with large horns. Most weigh 73 to 113 kilograms (160 to 250 pounds), but males may weigh more than 159 kilograms (350 pounds) and stand around 102 centimeters (40 inches) at the shoulder. A bighorn’s lifespan is about 10 years, and the record in captivity is 20.6 years.

The Headbutt Physics

Mating season comes in the fall, when rams fight for dominance or mating rights. They rear up on their hind legs and pitch forward at each other at up to 64 kilometers per hour (40 mph), and the crash of horns can be heard up to a mile away. The ritual repeats until one ram concedes and walks away. The skull is built for it: thick and bony, it absorbs the repeated impact with little injury to the ram. It is double-layered and shored with struts of bone, while a broad, massive tendon linking skull to spine helps the head pivot and recoil from blows.

Horns that can weigh as much as the rest of his skeleton A ram’s curved horns can weigh up to 14 kilograms (30 pounds), as much as the rest of the bones in his body. An older ram’s horns can grow over 0.9 meters (three feet) long.

Living on the Edge

Bighorns live in the Rocky Mountains from southern Canada to Colorado, and a desert subspecies ranges from Nevada and California to west Texas and south into Mexico. The species has three living subspecies: the Rocky Mountain, Sierra Nevada and desert bighorns. In warmer months they eat grasses, clover and sedges, switching to woody plants like willow and sage in colder months, and desert sheep often eat holly and cactus. A complex four-part stomach lets them eat large portions fast, then retreat to cliffs or ledges to rechew their food, safe from predators.

Though not as agile as mountain goats, bighorns are well equipped for climbing the steep terrain that keeps their predators at bay. The outer hoof is a modified toenail shaped to snag any slight protrusion, while a soft inner pad grips and conforms to each surface. Their mountain habitat has ledges sometimes only five centimeters (two inches) wide, and it gives cover from predators such as coyotes, golden eagles, mountain lions, bears and Canada lynx. A 2013 to 2017 GPS-collar study of two Arizona desert populations found that sheep using more rugged areas and steeper slopes were less likely to be killed by a mountain lion. Bigger groups also cut the risk, though the benefit became marginal at about ten sheep a group.

Shrinking Trophies

In one wild population, sport hunting drove evolution: rams’ body weight and horn size declined significantly over time. A 30-year study of that population, where trophy hunting targeted rams with rapidly growing horns, found both traits were highly heritable. The rams hunters shot had higher genetic value for weight and horn size than the rams they spared, and they were shot at an early age, so they did not achieve high breeding success. Under unrestricted trophy hunting, the population’s genetic averages for both traits fell, leaving smaller-horned, lighter rams and fewer trophies.