This is Scientific Americans 60 Second Science. I'm Jason Goldman. Got a minute?

Halder monkeys.

From their Central and South American Rainforest home, they produce some of the loudest

animal calls in the world.

They're about the size of an adult cocker spaniel, but they can sound as large as a tiger. Biologists have long suspected that the monkey's howls played a role in attracting mates.

The sounds are like the auditory version of peacock feathers or deer antlers that say,

hey, check me out. And now we know that the quality of the howl depends on a bone near their

throats called the hyoid. The bigger the hyoid, the deeper the howl. Males with deeper

calls sound more attractive to females, which means that

males with smaller hyoids had to come up with another strategy.

When you're in the field and you look up at the monkeys, you notice that in Alawatopiata, that Mantled Howler Monkey, that the testes are huge and they're

white, they're really obvious.

University of Utah Anthropologist Leslie Knapp. She and her team found that howlour monkeys face an anatomical

trade-off, a species can either have a really big hyoid bone or it can be very well

endowed, but not both.

That finding is in the journal Current Biology.

Those species with bigger hyoids live in single male groups,

so it makes sense to invest in what the researchers call

pre-copulatory traits, that is, their courtship strategy of attracting

the female's attention.

But in a species where groups include multiple males, the best way to ensure passing on more genes

is to make more sperm by building bigger testes.

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