Bighorn sheep are iconic inhabitants of North American mountain ranges, known for their impressive horns. Unlike deer and other ungulates, bighorn sheep do not shed their horns; these structures are permanent throughout their lives. This difference is due to how horns develop and are maintained.
Horns Versus Antlers
The distinction between horns and antlers explains why bighorn sheep retain their headgear. True horns, like those on bighorn sheep, are permanent structures. They consist of a bony core extending from the skull, covered by a tough outer keratin sheath, the same material found in human fingernails. The horn grows continuously from its base throughout the animal’s life. Both male rams and female ewes possess horns, though males typically develop much larger, more curled horns, each containing a living core with a continuous blood supply.
In contrast, antlers, seen on species like deer and elk, are made entirely of bone and are typically found only on males (except caribou). Antlers are shed and regrown annually. This regenerative process means antlered animals discard their headgear each year, while horned animals, like bighorn sheep, maintain their structures permanently.
The Lifelong Growth of Bighorn Sheep Horns
Bighorn sheep horns exhibit continuous growth from the base, accumulating new keratin layers throughout the animal’s lifespan. This ongoing growth leads to characteristic spiral curls, especially prominent in rams. As horns grow, they also form distinct annual growth rings, known as annuli.
These rings are created due to seasonal variations in nutrition and physiological stress, such as during winter or mating season, when growth rates slow. Each ring represents approximately one year of growth, allowing scientists to determine a bighorn sheep’s age by counting them. Over time, horn tips may show wear, a process called “brooming,” resulting from rubbing or impact.
The Role of Horns in Bighorn Sheep Life
Horns serve multiple functions for bighorn sheep, particularly for rams. Their primary role is establishing dominance hierarchies during the rut, or mating season. Rams engage in head-butting contests, colliding their horns at high speeds to assert strength and fitness. The size and condition of a ram’s horns indicate its age, health, and social status within the herd.
Horns also provide a means of defense against predators. The bighorn skull’s unique structure, including enlarged sinuses, helps absorb impact forces during clashes, protecting the brain from injury.