Do males with bigger testicles have deeper voices? The jury is still out on humans, but if you’re a howler monkey, forget about it. A new study finds that the animals can make a lot of noise or a lot of sperm—but trying to do both just takes too much energy. The findings shed important new light on the kind of evolutionary tradeoffs animals must engage in to ensure the survival of their species.
The study is “long overdue,” says Dawn Kitchen, a physical anthropologist at Ohio State University, Columbus, who was not involved with the work. The results, she says, are “robust and clearly point to a trade-off.”
It’s a man’s world in many ways, but among primates, being a male still has its challenges. You’ve got to attract mates, fend off competing males, and keep your sperm count high enough to finish the reproductive job. Howler monkeys, members of the genus Alouatta and native to Central and South America, are famous for their powerful roars. Their vocalizations, among the loudest produced by any animal, resonate across the jungle, and can be heard from up to 5 kilometers away. Primatologists have observed that the roars can go on for more than 40 minutes, a considerable investment in energy. Most researchers think that the racket helps fend off competition from other males.
The sizes of howler monkeys’ larynxes—the noise-producing voice box common to most mammals—vary widely among the 10 universally recognized species. The best measure of larynx size is the hyoid, a U-shaped bone that supports the tongue and larynx; its size varies from only 8 cubic centimeters in some species to 110 cubic centimeters—14 times larger—in others. Likewise, the size of a howler monkey’s testes varies about seven times between the species with the smallest pair, about 3.5 cubic centimeters, and the one with the largest, about 23 cubic centimeters. This wide size range holds up even when corrected for differences in body size.
A team led by Jacob Dunn, an evolutionary biologist at the University of Cambridge in the United Kingdom, decided to investigate the exact relationship between these two indicators of male virility. The researchers used both new and published data on hyoid and testes size of 144 male howler monkeys from specimens of nine of the 10 species, collected in nine museums in the United States, Brazil, and Europe. For the new data, the group used laser surface scanning to produce 3D models of the hyoids. And to verify previous assumptions that hyoid size determined how deep and resonant the howls were, the team analyzed the frequencies of previously recorded howler monkey calls.
The researchers found that, as expected, there was a linear correlation between the size of a species’s hyoid and the so-called “formant spacing” of its calls, a measure of the deepness and resonance of the vocalizations; so having a big hyoid generally means producing louder roars. But as they report online today in Current Biology, there is a clear inverse linear relationship between the size of a monkey’s hyoid and the size of his testes. In other words, species with deeper calls had smaller balls.
Another important finding, Dunn and his colleagues report, was how these two body measures are related to the number of males in a howler monkey’s immediate social group, which ranges from one to three animals depending on the species. Thus, species in which the males congregate into larger groups had smaller hyoids and larger testes, whereas species living in groups with just a single male had larger hyoids and smaller testes.
Putting all of these results together, the team comes up with the following evolutionary scenario: Species that live in smaller groups invest more energy in fending off males from other groups, simply by howling louder; if they are successful, then they can get away with smaller testicles because they have more exclusive access to the females around them. But males in larger groups, who are not alone in trying to mate with local females, invest more energy in competing with each other: They produce as much sperm as possible to increase their chances of being the lucky guy to impregnate the gal, a phenomenon known as sperm competition.
“You can’t invest in everything at once,” Dunn says, because both howling and sperm production have high energetic costs. “Perhaps surprisingly, given sperm’s reputation as an abundant resource, there is strong evidence that sperm production is actually quite costly.” Similarly, Dunn adds, howling for 40 minutes at a time is “a very strenuous activity.”
Kitchen says that the findings are particularly important for the insights they provide into the monkeys’ social organization. Instead of using their loud roars to compete within their social group, she says, they direct all that noise to males from outside the group who might be wanting to get at the local females. And in larger groups where up to three males are trying to impregnate the females—as opposed to “haremlike” groups where one male has all the gals to himself—“investing in sperm competition would be more cost-effective than investing in energetic vocal displays.”