Why Do Bighorn Sheep Butt Heads? How Rams Avoid Injury

Head-butting is about mating rights

Bighorn sheep live in herds where the rams compete fiercely for access to females. A ram’s rank in the male hierarchy decides how close he can get to ewes in season, and the top rams father most of the lambs. Most rams do not mate successfully until they are around seven years old, when they are big and strong enough to win contests.

Rams settle rank in several ways before they ever collide. They display their horns by turning their heads to show them off, kick each other with a stiff front leg, push and shove, and rub their faces on rivals. When two rams are badly mismatched, these displays usually end the matter quickly. Full head clashes happen mainly between rams of similar size and horn size, when neither will back down.

Most clashes take place in the weeks before and during the rut, which peaks in November and December in northern herds. Rams in bachelor groups also spar more gently at other times of year, which helps them keep track of each other’s strength.

What a clash looks like

A typical clash follows a ritual. The two rams walk a few steps apart, turn to face each other, rear up on their hind legs and then drop forward and lunge, slamming their horns together with the full weight of their bodies behind them. The crack of the impact echoes across the mountainside.

Each ram may be running at roughly 30 km/h (20 mph), so the two heads meet at a closing speed of about 60 km/h (40 mph). With rams weighing up to 140 kg (300 lb) or more, and horns weighing up to about 14 kg (30 lb), the forces are enormous.

Contests can be repeated again and again. Rams often clash several times an hour, and one well-known observation described a contest lasting more than a day before one ram gave up. Afterwards, the loser typically behaves submissively toward the winner. See How much do bighorn sheep horns weigh? for more on the horns themselves.

How rams avoid serious injury

A collision like this would badly hurt most animals. Bighorn rams have a set of adaptations that spread and absorb the shock:

• A double-layered skull. The bone over the forehead is made of two layers separated by bony struts and air-filled spaces (sinuses). These work rather like crumple zones, absorbing energy before it reaches the brain.
• Horns as shock absorbers. The horns have a bony core covered by a thick sheath of keratin, the same protein as fingernails. Keratin can flex and absorb energy, and the large curled shape helps spread the force over the horn rather than concentrating it on the skull.
• A massive neck. Thick neck muscles and a broad, strong tendon linking the skull and spine help brace the head and absorb the recoil.
• Good aim. Rams hit horn to horn, head-on, so the impact runs through the strongest parts of the skull rather than striking at an angle.

Computer models of ram skulls and horns support the idea that the bone and horn layers together soak up much of the energy of a hit. Scientists study these structures as inspiration for helmets and impact-resistant materials.

Do they ever get hurt?

Yes, though serious injuries are much rarer than the violence of the clashes suggests. The tips of rams’ horns are often worn or broken off, a condition called “brooming,” partly from fighting and partly from rubbing the tips on rocks. Rams can break horns, crack bones or suffer other injuries, and a few are killed or fatally weakened.

The rut is exhausting in other ways too. Rams eat little while competing and following ewes, and they enter winter thin and tired. This is one reason rams usually live shorter lives than ewes in the wild.

Do ewes butt heads too?

Ewes also have horns, though theirs are short and only slightly curved. Females form their own rank order, and young ewes may butt and shove as they find their place in the herd, usually at around one to two years old. They do not stage the dramatic running clashes of the rams.

Is head-butting unique to bighorns?

Head clashing is common among wild sheep and goats. Dall sheep, argali, mouflon and ibex all fight with their horns, and male musk oxen collide head-on at high speed during their rut. Bighorn rams are among the most famous because their horns are so massive and their clashes are easy to watch in national parks such as Yellowstone and Glacier. Other animals that fight for mates include elk, which bugle during the rut, and male American bison, which also butt heads.