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IT WAS one of the many mysteries pondered by Darwin: why do some female animals have horns? Horns on cloven-hooved mammals are thought to have evolved for fighting each other, but most female cattle and deer don鈥檛 do this.

Now of the University of Massachusetts and from the University of California, Davis, have a solution. They noted the presence or absence of horns in 117 species of bovid and set up competing mathematical models to examine whether evolution of horns was likely to have been driven by body size, openness of habitat, territorial behaviour, group size or conspicuousness.

This showed that horns were most likely in conspicuous species 鈥� those living in open habitats and large enough to be clearly visible to predators 鈥� suggesting that they evolved as defensive weapons (Proceedings of the Royal Society B, ).

Behavioural ecologist of the University of Liverpool, UK, is not convinced. 鈥淭hey haven鈥檛 shown that female competition for food could not be the reason why horns evolved.鈥�