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. Not surprisingly, male howlers frequently use this power to advertise their sexual fitness, catcalling females with their ear-splitting roars.
But in a beautiful twist of expectations, scientists have now found that the louder the monkey’s calls, the smaller the monkey’s balls. 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 today in the journal Current Biology.
“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.”