Humans are unique in developing permanent breasts that send sexual signals, says Stuart Blackman. However, there is another primate with an ostentatious chest.
Ethiopian geladas have bald chest patches that are bright red. As the sole grass-eating primates, geladas spend much of their time shuffling around. Squatting hides their backsides, so there’s no point having scarlet, swollen rumps, which are a social signal in many other Old World primates.
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Instead, male geladas use their chests to communicate social status – the more dominant the male, the redder he is – while the chests of female geladas swell and blister to indicate fertility. The chest patches might have evolved to mimic buttocks, tapping into an existing psychological sensitivity to red swellings among potential mates and rivals.
Intriguingly, human female breasts might also have evolved to imitate bottoms after we started walking upright, though in our case it’s more about shape than colour.
