The existence of these so-called ‘secondary sexual characters’ on the ‘passive’ female was therefore something of a mystery to Darwin. Horns are just as costly for the female to grow as they are for the male, after all. So why do the females of some species have them? The idea that females might be using their horns to fight other females is never considered in the many pages of Darwin’s thoughtful conjecture. Instead he concludes that although such armaments must be a ‘waste of vital power’, their presence or absence in females depends not on them being ‘of any special use, but simply in
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