Deep-voiced monkeys, known as howlers (genus Alouatta), have a low growl that attracts females. The males often live with a harem of females and no other males.
There’s another interesting fact about these alpha males: They have smaller testicles than their higher-pitched peers. A study published in the Current Biology journal said monkeys with higher voices had larger testicles and were among the lovers of polyamorous females.
Leslie Knapp, professor and chair of anthropology at the University of Utah, referred to the published work on monkeys as her “calls and balls” paper and talked about how the findings demonstrate evolutionary principles.
“The idea has been around since Charles Darwin, but this is the first time that anyone actually has demonstrated a trade-off between vocal characteristics before mating and sperm competition after mating.”
Lead researcher Jacob Dunn of Cambridge University said none of the deep-voiced primates had large testes. It is always an either/or situation.
Knapp said the hyoid bone, which is located in the throat and produces sound, contributes to the tone of the deep-voiced primates’ howls. Researchers used museum samples of hyoid bones and compared their size to the monkey’s testicular mass.
Nine species were studied, and researchers created 3-D scans of 255 hyoid bones, from museums in the United States and Europe. Data was also collected on the testicle size of 86 deep-voiced monkeys. The testicles of 21 more monkeys from zoos in Brazil and Germany were measured with calipers. Knapp said measurements were taken when the animals were sedated, and “[i]t’s not the easiest thing to do.”
In Central and South America, male monkeys mate with several females and a deep growl scare other suitors away. In this situation, testicle size doesn’t matter. There are other scenarios where females copulate with many males. Testicle size is significant because the larger the testes, the greater the ability to produce sperm, which means bigger testicles increase the odds of sperm reaching an egg and siring a baby monkey.
Although there are differences in species, larger hyoid bones are synonymous with bigger bodies across all types. Per the study, most mammals that are deep-voiced have larger bodies and hyoid bones.
Where does evolution come to play in all of this? Dunn explained that howler monkeys as well as their counterparts with larger testicles expend energy throughout the course of their lives. Both the voice and testicle size contribute to the survival of the species.
“Bigger testicles are also vulnerable to being caught on a branch or scratched by another monkey.”
Knapp said the tradeoff between a deep voice and large testicles in monkeys derived from “different solutions to the same problem.”
The study presents evidence that a “vocal competition hypothesis” suggests that deep voices among the howler monkeys is sexually selected. Female monkeys have large hyoids for, seemingly, similar reasons as males, such as defending the group and keeping predators away. It wasn’t clear why the volume of female hyoids correlated with the males, and further research will be done on this.
Per Knapp, deep-voiced monkeys are preferred by females. Likewise, human females prefer men with deeper voices.
[Image: Courtesy of Nick Shields/Flickr]