Credit: C. BOISVIEUX/CORBIS

Proc. R. Soc. Lond. B doi:10.1098/rspb.2009.1256 (2009)

What good is a horn on a female yak (pictured)? Naturalists have long known that competition for mates drove the evolution of horns among male ungulates, but the presence of prongs on some females has defied coherent explanation ever since Charles Darwin puzzled over them.

Now, Theodore Stankowich of the University of Massachusetts and Tim Caro of the University of California at Davis propose that, in the case of large animals living in open territory, the 'weaponry' on female bovids arose mainly for defence against predators. In smaller bovids, they say, female horns evolved for territorial battles among females of the same species. These two hypotheses agree with a phylogenetic reconstruction, and explain the presence or absence of horns in 80 of the 82 bovid species compared.