We recently reported on the howler monkeys in Central and South American rainforests, which were found to have an embarrassing secret in a recent study: the louder they were, the smaller their testes.

But if that’s the case, why are these creatures so loud in the first place? Scientists say they’re the loudest land animals on the planet — even louder than a lion. It’s how they got their name, after all.

The purpose of the howl is obvious: to attract females and to scare off males. But a recent study found that males with the deepest and loudest calls typically had the smallest testes and thus were less reproductively blessed, so to speak. They did this by comparing the hyoid — a bone above the larynx that amplifies their calls — with the size of their testes to see if there was any correlation. So basically, a howler monkey can have big testes or a big call, but not both.

But why do they have to be so loud to begin with?

They can be heard clearly from a distance of three miles, something no other land animal can match.

The answer is that howler monkeys are very territorial. These monkeys need to communicate clearly and over large distances, often covered by dense Amazonian rainforest, that this territory is theirs and other males would be wise to stay away. Every morning the howler monkeys get noisy to keep reminding everyone in the vicinity of their boundaries.

These howls are often answered by other monkeys that can hear them. The monkeys check on each other at the beginning and ending of each day so everyone knows where everyone is.

Howler monkeys must subsist off of leaves, and since it’s not a very nutritious food, it is important to protect every bit of food they can.

The most recent study was 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.