Study: The louder the monkey, the smaller its balls

A new study finds that animals can make a lot of noise or a lot of sperm — but trying to do both just takes too much energy. A team based out of Cambridge University came to this conclusion by comparing the size of dozens of monkeys’ testes with the hyoid bones located in their voice boxes, which revealed a negative correlation between decibel levels and testicular endowment. The results are published in the journal Current Biology.

The findings shed important new light on the kind of evolutionary tradeoffs animals must engage in to ensure the survival of their species.

The researchers used both new and published data on hyoid and testes size of 144 male howler monkeys.

Howler monkeys are the loudest land animals on Earth, capable of bellowing at volumes of 140 decibels, which is on the level of gunshots or firecrackers. Their vocalizations, resonate across the jungle, and can be heard from up to 5 kilometers away. Primatologists have observed that the roars can go on for more than 40 minutes, a considerable investment in energy. Most researchers think that the racket helps fend off competition from other males.

“We found that males with larger hyoids, who can make lower-pitch vocalizations, have smaller testes and live in single-male groups with a harem of a few females,” anthropologist Leslie Knapp, a senior author of the study, said in a statement. “Males with smaller hyoids live in multimale groups and have larger testes.”

Putting all of these results together, the team comes up with the following evolutionary scenario: Species that live in smaller groups invest more energy in fending off males from other groups, simply by howling louder; if they are successful, then they can get away with smaller testicles because they have more exclusive access to the females around them.

But males in larger groups, who are not alone in trying to mate with local females, invest more energy in competing with each other: They produce as much sperm as possible to increase their chances of being the lucky guy to impregnate the gal, a phenomenon known as sperm competition.

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