Image: Wikimedia Commons

Male howler monkeys compensate for smaller testes size with louder vocalizations, according to new research.

Howler monkeys include fifteen different species belonging to the largest of the New World monkeys, native to South and Central American forests. They are aptly named, given their vociferous calls capable of reaching volumes of 140 decibels. Thanks to specialized larynx with an enlarged hyoid bone, they can produce some of the loudest calls in the animal kingdom.

Now, a new study published in the Journal of Current Biology reveals a fascinating correlation between hyoid size and testes size.

Image: Jan Hazevoet, Flickr

A team based out of Cambridge University researched the role vocalizations play in competition between males for mating. Research shows that males with smaller testes and larger hyoids tend to form groups where they mate exclusively with multiple females — an evolutionary trade-off that means these less-endowed males have less competition from other males.

In groups with a large mixture of males and females where there is no exclusive copulation, males tend to have larger testes and smaller hyoids, with competition coming down to sperm production. Larger testes size correlates with higher sperm production and the higher likelihood of impregnation.

Image: Lara Danielle, Flickr

If less endowed monkeys were forced to compete with more potent males, procreation would be unlikely. Therefore they do a better job of impressing females with their louder calls and creating exclusive groups so they get females all to themselves.

“It may be that investment in developing a large vocal organ and roaring is so costly that there is simply not enough energy left to invest in testes. Alternatively, using a large vocal organ for roaring may be so effective at deterring rival males that there is no need to invest in large testes,” stated lead author Dr. Jacob Dunn in a statement.