Researchers have found that howler monkeys in Central and South American rainforests with particularly guttural calls have a secret.

Howler monkeys are so named because they have among the loudest calls of any land mammal, with roars meant to attract females and ward off male rivals — but as it turns out, the males with the deepest calls actually have the smallest testes and therefore the lowest potential when it comes to reproducing, according to a Reuters report.

Researchers looked at the hyoid, which is a bone above the larynx that amplifies its calls. They studied nine howler monkey species and found that those with the biggest hyoid had the deepest calls, but also had small testes and therefore weren’t the best sperm producers

It’s an indication that there is some sort of evolutionary trade-off for male howler monkeys Some get a better vocal tract, others get bigger testes. Some species invest in one rather than the other, with none of them having both positive qualities.

Howler monkeys can be found from Mexico in the north all the way down to Argentina. They lie in the trees most of the time, and their howls are so loud they have been heard up to three miles away. They also are quite intimidating, resembling that of a lion.

The findings were published in the journal Current Biology.

A news release on the findings was published on the University of Utah’s website, which can be found here.