Alouatta caraya vocal chorus. Credit: Mariana Raño

Well, this is awkward.

It turns out that the male howler monkeys that shout the loudest are compensating for something. Exactly what that is requires a rather delicate explanation. It's sexual in nature.

Let's just say big howls used to lure mates come with small packages. Namely, the scrotum. It's smaller and makes less semen, according to a study released Thursday by the University of Cambridge.

"The bigger a male howler's vocal organ, and the deeper and more imposing roar they possess, the smaller their testes and the less sperm they produce," the study authors found.

So female howler monkeys beware: Some suitors have a Napoleon complex of sorts. Their magnificent audibles write checks that their bodies can't cash.

Things go downhill from there. Males that are all howl live in small social groups where a single dominant male mates with a number of females lured by his sound and fury. By contrast, the other guys with bigger unmentionables live in larger communities of about six males and lots of females and engage in jungle love as a group.

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Other than having an opportunity to, yes, howl with laughter, why would anyone want to know any of this? Lead researcher Jacob Dunn said it's yet another example of sexual selection as described by Charles Darwin nearly 150 years ago.

Canelo, a male red howler who was rescued from animal trafficking lives within Senda Verde Animal Refuge and Eco Lodge, Bolivia. He howls to identify his territory. (Senda Verde)

"There is evidence in other animals that when males invest in large bodies, bright colors or weaponry such as horns or long canines, they are unable to invest in other reproductive traits," said Dunn, who has worked with howler monkeys for a decade. "However, this is the first evidence in any species for a trade-off between vocal investment and sperm production."

The study, based on data from museums, zoos and research in many countries, appears in the journal Current Biology. Tecumseh Fitch, an evolutionary biologist and cognitive scientist at the University of Vienna, and Leslie Knapp, a biological anthropologist at the University of Utah, were among the other co-authors.

A natural question is whether this has any carry over to the monkey's closest relative, humans. A thousand women in a thousand bars might say yes, men who talk a good game are often duds where the rubber meets the bed, but the study offers no answers.

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The researchers stuck to what they knew. And they really got into the work. They measured testes size across a range of howler monkey species and collected the data to back their findings. They also analyzed the size of the hyoid, a curvy bone in the neck that supports the tongue to power howling, in 250 males.

Developing a larger vocal organ to produce a bigger roar might have limited development in a more sensitive area. Dunn theorized that "roaring is so costly that there is simply not enough energy left to invest in the testes." On the other hand, roaring loudly might make bigger males back off, producing fear while making females amorous.


Alouatta arctoidea Group howls. Credit: Carolyn M. Crockett

Howler monkeys weigh about as much as a little dog, but the acoustics of their roar is as loud as a tiger's -- and among the loudest of any animal on earth. Anyone who's visited Tikal National Park in Guatemala and tried to sleep through the night in the surrounding jungle can attest to that. The party goes on until dawn.

"The vocal folds of a howler monkey are three times longer than a human's, yet they are 10 times smaller," Dunn said.

The study came to one other conclusion, too: The most powerful roarers might live in small groups for a reason.

"In evolutionary terms, all males strive to have as many offspring as they can," Dunn said. But with bodies that generate impressive sound volume but little reproductive volume, "you can't have everything."

Correction: An earlier version of this post misidentified Leslie Knapp's current affiliation as the University of Cambridge. 

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