Howler Monkeys Who Roar the Loudest Have Smaller Testicles

According to a study published on Thursday October 22 in the journal Current Biology, howler monkeys whose roars are the deepest and loudest have smaller testicles.

Research was conducted by Jacob Dunn at Cambridge University, Leslie A. Knapp at the University of Utah, and W. Tecumseh Fitch, at the University of Vienna.

9 species of howler monkeys were assessed, based on the dimensions of their testicles, and the size of their hyoid bone, which influences how deep their vocalizations are.

Experts conducted this analysis because that had noticed a large variety of sizes when it came to hyoid bones and primate testicles, and they wanted to see if they would identify a connection between the two.

It was determined that there are anatomy disparities, based on the functions which male monkeys have in their communities.

For example, among monkeys in Central and South America, deep bass tones are preferable, because they allow the animals to scare away potential rivals, while drawing females closer.

On the other hand, testicle size, which influences sperm production, is inconsequential in these courting rituals. This is also the case of gorillas, where having an imposing, threatening stature is much more important that factors related to virility.

On the other hand, in other species where communities are larger and females have several sexual mates, the situation is the opposite. There, it’s extremely important to have larger testicles and thus produce higher quantities of sperm, in order to increase the chances of passing on genes to potential offspring.

Chimpanzees are such species which have to compete in order to fertilize the female’s eggs, so they rely on the size of their testicles.

After analyzing 3-D scans of hyoid bones and comparing them against vocalization habits, researchers have also noticed that none of the primates that have been studied make low-frequency calls, if they already have larger testes for sperm production.

Usually, it’s either one characteristic, or the other, thus avoiding overly high energy consumption as the animal matures and grows.

“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”, concluded the study authors.

This evolutionary trade-off between sperm production and vocalization allows less-endowed primates to still attract potential mates, using enticing bass tones, which make them appear larger than they actually are.

Such unusual studies have also been conducted on human participants in the past, but results have been inconclusive.

While some researchers have indicated that men with lower voices tend to have more sexual partners, thus increasing their chances of perpetuating their genes, other scientists have pointed out that being deep-voiced can be associated with diminished sperm production.

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