Howler monkeys are the loudest land animals on Earth, capable of bellowing atvolumes 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.
More here – Motherboard
PS : My guess is there is no need to study this any further since it is probably true of all primates…..