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A University of Utah researcher co-authored a study released Thursday that reveals size does indeed matter at least when it comes to howler monkey testicles.
Primatologist Leslie Knapp, professor and chair of the university's Department of Anthropology, found that the smaller a howler monkey's testicles are, the deeper and louder his mating calls.
Overcompensating? Perhaps. The monkeys with the deeper howls also had larger hyoid bones, which enable them to have some of the loudest voices in the animal kingdom, an eardrum-busting call that's been compared to a tiger's roar.
They don't "howl" in the way your dog might, though. Instead, their call sounds more like a non-stop belch. (Find an audio recording here.)
The point of the deep howls is twofold: to attract a female and to frighten away any competing males, as the deeper howls give a sonic impression of a large body size.
"We noticed there were significant size differences in the monkeys' hyoids and testicles, and were really surprised at the great range," said Knapp. "We discovered that the hyoid can be as much as 10 times larger than that of the smaller howler monkeys."
Growing a hyoid takes a relatively "expensive" energy toll, so researchers theorized that males with large hyoids could have smaller structures elsewhere.
Males with smaller hyoids, Knapp added, "live in multi-male groups and have larger testes, but monkeys with smaller testes live in single-male groups with a harem of a few females."
For a male with access to several females, having a louder, deeper voice may make him more attractive to them, she explained, or it may intimidate possible competing males. But a male living in a community with other males benefits from having larger testicles that produce more sperm increasing his chances of reproducing during each mating.
Published in Current Biology, the study is the first to show a trade-off between vocal characteristics, important before mating, and sperm production during mating, Knapp said. It shows that Charles Darwin's theory that the roars of howler monkeys play an important role in reproduction was probably right, she said.
The study which Knapp refers to as her "calls-and-balls paper" had its genesis in trips she took to Central and South America, where she would hear howler monkeys giving noisy examples of how they earned their name and wonder what the sounds were all about.
To obtain the data, which Knapp collected with researchers in the United States and Europe, her team calculated the volumes of 255 howler monkey hyoid bones from museums. Data on testicle sizes for 66 howler monkeys were collected from published works, but Knapp said researchers also partnered with several zoos to measure the testes of 21 monkeys while the animals were sedated during health exams.
"Thankfully, I didn't have to do that," she said.
So can this study be applied to humans? Possibly, Knapp suggests, since monkeys and humans are so closely related.
"Some studies say that women find deeper voices to be more attractive," said Knapp. "Other studies have found that the deeper a man's voice pitch, the more women they've slept with."
So what has Knapp told friends and family when they've asked about her work? She laughs.
"They all know I study lots of different primates, and they'll sometimes say, 'Oh, more info about monkey sex,' or 'Oh, Leslie, she's our friend who picks through monkey poop.' But it's exciting information because it helps us understand primate behavior, and that way, we can learn more about ourselves."