Deeper calls, smaller balls

Across the animal kingdom, males hoot and holler to attract females and ward off competing suitors. Now, a new study finds that male howler monkeys with deeper calls have smaller testicles – and vice versa, according to researchers from universities of Utah, Cambridge and Vienna and other institutions.

In the cover story of Current Biology’s new issue, the research team provides the first evidence of a tradeoff between two traits evolved by different howler monkey species so that males could mate with females and pass their genes to offspring: large gonads to make more sperm, or a large vocal-tract hyoid bone to produce a deep call that scares competitors and attracts females – similar to how women like deep male voices.

“They are different solutions to the same problem,” says...

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