BATTLE OF THE BIGHORNS
Male BIGHORN SHEEP are built to fight, explains BBC zoologist Ben Garrod.
Some animals lead a perilous life. Though bighorn sheep are the largest wild sheep in North America, size alone is no guarantee of protection against attack by mountain lions, wolves and coyotes. So they have made their home in some of the steepest, rockiest, most inhospitable places imaginable, from Mexican deserts to the Rocky Mountains.
One might assume that herbivores that spend so much time on sheer rock faces would be timid creatures preoccupied with keeping their footing. Yet cliff-living bighorn sheep indulge in some of the most violent, attention-grabbing displays in the animal kingdom. Unlike the ancestor of domestic sheep, the European mouflon, bighorn sheep do not have a clear hierarchy in their groups. Instead, males battle for dominance at the start of each breeding season.
Two males will approach, then walk away from one another. They turn, rear up on their hind legs and rush forwards, crashing horns together at an estimated 20–40kph. And they repeat this head-banging – sometimes for hours – until one combatant submits. Signs of damage can often be seen on males’ horns, but they rarely suffer serious injuries, even though scientists have estimated that the force generated in these fights might be up to 60 times greater than that needed to fracture our skulls.
The bighorn sheep skull has undergone several adaptations to survive such brutal clashes. The bone is extremely thick, so harder to damage. To help cushion mighty blows, the sinuses at the front of the skull are enlarged, and there are extra little sections of scaffolding-like bone called septa. Every mammalian skull is made from many smaller bones joined together, but the skull of the bighorn sheep does not fuse fully; instead these lines of fusion, or sutures, remain partly open and act as natural shock absorbers. Force from the impact is distributed along these unfused sutures, making the entire skull slightly flexible yet incredibly strong.
Ben Garrod is an evolutionary biologist and BBC science presenter.
Rival mature male bighorn sheep face off during the breeding season. The species’ brutal battles require a suite of cranial adaptations.