Utah researcher: The louder the howler monkey, the smaller its testicles

Salt Lake Tribune ^ | 10/22/2015 | By RICH KANE

[Red Badger]

A University of Utah researcher co-authored a study released Thursday that reveals size does indeed matter — at least when it comes to howler monkey testicles.

Primatologist Leslie Knapp, professor and chair of the university's Department of Anthropology, found that the smaller a howler monkey's testicles are, the deeper and louder his mating calls.

Overcompensating? Perhaps. The monkeys with the deeper howls also had larger hyoid bones, which enable them to have some of the loudest voices in the animal kingdom, an eardrum-busting call that's been compared to a tiger's roar.

They don't "howl" in the way your dog might, though. Instead, their call sounds more like a non-stop belch.

The point of the deep howls is twofold: to attract a female and to frighten away any competing males, as the deeper howls give a sonic impression of a large body size.

"We noticed there were significant size differences in the monkeys' hyoids and testicles, and were really surprised at the great range," said Knapp. "We discovered that the hyoid can be as much as 10 times larger than that of the smaller howler monkeys."

Males with smaller hyoids, she added, "live in multi-male groups and have larger testes, but monkeys with smaller testes live in single-male groups with a harem of a few females."

The study — which Knapp refers to as her "calls-and-balls paper" — had its genesis in trips she took to Central and South America, where she would hear howler monkeys giving noisy examples of how they earned their name and wonder what the sounds were all about.

[poobear]

“The study — which Knapp refers to as her “calls-and-balls paper”

You can’t make this stuff up Red!