ABOVE: Chimpanzees lack fancy ornamentation, but have large testes in relation to their body size.
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Birds are well known for extravagant, sexually selected features such as peacocks’ tails, but primates too go out of their way to show off their good looks to potential mates. Some species display flashy ornaments designed to signal their dominance or attract females. Take the large cheek flanges of orangutans or the oversized noses of proboscis monkeys. But the ornaments don’t always match what’s under the hood, so to speak.
A comparative study of more than 100 primate species finds that males tend to have either large testes or flashy ornaments, but not both—suggesting an evolutionary trade-off between the two. The situation is different for canine teeth, which some species use as weapons: in fact, the larger the testes, the longer the canines tend to be, scientists reported April 10 in Proceedings of ...
