Males of most even-toed ungulates have horns or antlers, which biologists agree evolved as weaponry to compete for mates. The origin of headgear in females, however, has remained enigmatic. Now, two evolutionary biologists think they’ve figured out why some species’ females have horns and others don’t.
Theodore Stankowich at the University of Massachusetts Amherst and Tim Caro at the University of California, Davis, studied the question in 117 species of bovids—a family that includes antelopes, cattle, goats, and sheep. They statistically tested the relative ability of several hypotheses to predict the occurrence of female horns in each species. Did they evolve in large bovids unable to easily hide or flee from predators? In group-living bovids with intense competition for food? Or in bovids whose females compete for territory?
The answer: none of the above—at least not completely. Only when they considered how conspicuous a species is in its natural habitat did they find a strong correlation. Large species living in open habitats are visible to faraway predators; in almost every one, females have horns, presumably for defense. But in small species, or species living in brush or forest, females go bareheaded. In African duikers and a few other bovids, females fight each other for land; they, too, have horns.
The pattern holds for eighty of the eighty-two cases of horned females, and the researchers think it may apply to other hoofed mammals as well.
The research was detailed in the Journal Proceedings of the Royal Society B.
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