The Deeper Howler Monkeys Call, The Smaller Their Balls

Males with small testes live in groups where a single male is dominant, and controls the mating to the females in the harem. Carolyn M. Crockett
Josh Davis 22 Oct 2015, 13:46

From Barry White to Johnny Cash, we all know that sometimes a deep, husky voice can appear irresistible to the opposite sex. It seems that this is also true for howler monkeys, which broadcast their presence across the rainforests of South America with deep hollering vocalizations. But their voices could be telling us a little more about the males than they might want to let on. New research has found that the deeper the male's call, the smaller his balls.

“We have been interested for quite some time about the vocalizations howler monkeys make,” University of Utah’s Leslie Knapp explained to IFLScience. “Because if you go to the field, and study these primates in Mexico or other parts of central or South America, you can’t help but notice the loudness of the calls.” What they found was that the species of howler monkeys who had the loudest and deepest calls, also had the smallest testes, and vice versa.

A 3D laser scans a howler monkey hyoid bone. Jacob C. Dunn/University of Cambridge

The trick to the howler monkey's booming voice is all in a single bone called the hyoid. In humans, this forms a horseshoe-like structure that sits in the neck and only really serves for muscle and ligament attachment related to the voice box. But in the howler monkey, the hyoid bone is enlarged and almost balloon-like in its shape. This unusual structure appears to be important in resonance and for maximizing the depths of the calls, because the vocalizations of howler monkeys with large hyoids are much deeper, and much more resonant than those of the smaller-hyoid howlers.

It was while measuring the different sizes of the hyoids and comparing them between species when Knapp and her team started to wonder, as you do, if they had any relation to the monkeys’ sex lives. “Once we started getting into the data, it was extraordinary,” says Knapp. “The range of difference [in hyoid size] we saw across all species of howlers. And then we started looking at testes volume and at group size – the number of males in a group – and all of those things came together to tell us a very interesting story.”

They found that it doesn’t seem possible for howler monkey species to develop a large hyoid bone and have big testes at the same time. It seems that the biological cost of producing such a big bone means that the monkeys have to cut corners elsewhere, and it seems that fell on the testes. The study is to be published in Current Biology

Red howler monkey in Bolivia, possibly compensating for something. Senda Verde/YouTube

And this difference in testicle size is further reflected in how the monkeys organize themselves socially. Those species with the deeper calls and smaller balls live in single male, multi-female groups. This means that he has less competition for mates, and so only needs to produce a limited amount of sperm. For the species that have less deep vocalizations, the opposite is true, in that the monkeys live in multi-male and multi-female groups. This means that the fellas have to compete with lots of other males an thus producing lots of sperm to out compete the others is an advantage. This is a pattern seen time and again within primates across the world.   

What Knapp wants to find out next is how the variation of the hyoid size not just between, but within species affects how many offspring a male then has. “We really need to know if the trend is that the males with the deepest vocalizations and the largest hyoids are more reproductively successful. You would sort of think so, but of course, there may be other things going on that we’re not aware of,” Knapp says. 

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