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FROM peacocks’ tails to deer’s antlers, males often flaunt their assets as a
guide to their quality as a mate. Now there is a female example: the baboon’s
backside. When female baboons are fertile, their bottoms swell up and turn red
to advertise the fact. Leah Domb, formerly of Harvard University, and Mark Pagel
of the University of Reading have found that the size of the swelling is an
accurate indicator of how fertile a baboon really is. Big-bottomed females
reproduce earlier and have more and healthier offspring. Males turn out to be
sensitive even to small differences in size, and fight harder to win females
with the biggest bottoms (Nature, vol 410, p 204).