Bigger Calls and Smaller Balls: Howler Monkeys Compensate for Small Testes

First Posted: Oct 23, 2015 08:16 AM EDT

Deeper calls may mean smaller balls for monkey species. Scientists have found that male howler monkeys have adapted different sex tactics in order to attract mates.

In this latest study, the researchers found that there's a tradeoff between two traits evolved by different howler monkey species. If the males have larger gonads, they produce more sperm but have a less-deep howl to attract mates. If the howler monkey has a large vocal-tract hyoid bone to produce a deep call to scare competitors and attract females, they have smaller gonads.

"They are different solutions to the same problem," said Leslie Knapp, one of the researchers, in a news release. "It is not possible to produce a large hyoid and large testes. This probably arose because individuals within one species produced more offspring if they had large hyoids. And in another species they were more successful if they had large testes."

In this latest study, the researchers used 3D laser scans to calculate the volumes of 255 cup-shaped howler monkey hyoid bones from museums in the United States and Europe. To confirm the scans were accurate, the scientists made CT and MRI images of two adult male howlers. In addition, the researchers visited zoos to measure testes of 21 more monkeys while the animals were sedated for health exams.

The hyoid is an energetically expensive organ to make. This means that males that grow large hyoids may have to compensate by having smaller structures elsewhere.

"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," said Knapp. "Males with smaller hyoids live in multimale groups and have larger testes."

The findings reveal that howler monkey males have several adaptations when it comes to attracting females. Whether it's larger testes or larger hyoids, these mammals have adapted to their particular environment.

The findings are published in the journal Current Biology.

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TagsEvolution, monkeys, Monkey, reproduction, Sex, Mammals, Male