http://www.cell.com/current-biology/fulltext/S0960-9822(15)01109-4

A new study finds that Howler monkeys scream louder when they have smaller testicles.

The study published yesterday in the journal Current Biology said that howler monkeys with deep roars — used to attract females, deter love rivals and scare off predators — tend to also have smaller testicles and a shortage of sperm compared to their peers.

The Howler Monkey has one of the loudest vocalizations in the animal kingdom. This new study finds that the smaller monkeys with smaller testicles have the loudest voices in an attempt to compensate for the reproductive advantage held by larger monkeys.