Howler Monkeys Have a Deep Voice or Big Balls But Not Both

In these very interesting primates we find either low sexy calls or giant testes

Posted Oct 22, 2015

All biologists know that there are many trade-offs in evolution (please also see) in anatomy, physiology, and behavior. I just came across a very interesting essay in the New York Times by Jim Gorman called "Seductive Bass Tones Enough to Seal the Deal in Some Monkey Species," in which he summarizes recent research that shows that among nine species of howler monkeys living in Central and South America, "species evolved either to make lower-frequency sounds, or have larger testicles, but none had both a very low sound and very large testicles."

The research paper with which Mr. Gorman is concerned titled "Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys," was published in the journal Current Biology by Dr. Jacob Dunn of the Division of Biological Anthropology at University of Cambridge (UK) and his colleagues. It's available online so here are a few snippets from the abstract of this fascinating and extremely important research project (the researchers write (the numbers refer to essays in the references section of the paper). "Males often face a trade-off between investments in precopulatory and postcopulatory traits [1], particularly when male-male contest competition determines access to mates [2]. To date, studies of pre-copulatory strategies have largely focused on visual ornaments (e.g., coloration) or weapon morphology (e.g., antlers, horns, and canines). However, vocalizations can also play an important role in both male competition and female choice [3–5]... We examined the relative male investment in hyoids [vocal apparatus] and testes among howler monkey species in relation to the level of male-male competition and analyzed the acoustic consequences of variation in hyoid morphology... Species characterized by single-male groups have large hyoids and small testes, suggesting high levels of vocally mediated competition... Across species, as the number of males per group increases, testes volume also increases, indicating higher levels of postcopulatory sperm competition, while hyoid volume decreases. These results provide the first evidence of an evolutionary trade- off between investment in precopulatory vocal characteristics and postcopulatory sperm production."

You can't have a low sexy voice and big balls

In a nutshell, so to speak, a low sexy voice means small balls and vice versa. There are similar trade-offs in other animals, including chimpanzees and gorillas. In groups of gorillas there is one dominant male who is huge but also has tiny testicles. Mr. Gorman notes, "In chimpanzees, mating is more of a free-for-all and sperm from several males may compete for success in fertilizing a female’s eggs. Chimpanzee testicles, he [Dr. Dunn] said, are 'massive.'”

It should be noted that the researchers measured hyoid bones in museum specimens and testes size in monkeys in zoos. It would be nice to get some data on free-living monkeys but Dr. Leslie Knapp, of the University of Utah correctly noted, “We have to come up with another way to measure testes size if we’re working in the field.” Indeed they do.

While the research "has no practical application to human mate choice," Mr. Gorman writes, "some research has suggested that men with deep voices have more sex partners, and therefore more opportunities to reproduce. But another study showed lower sperm quality in deeper-voiced men." Oh wouldn't it be nice if ... or would it?

Please stay tuned for more on the fascinating nonhuman animals with whom we share out magnificent planet. 

Marc Bekoff's latest books are Jasper's Story: Saving Moon Bears (with Jill Robinson), Ignoring Nature No More: The Case for Compassionate ConservationWhy Dogs Hump and Bees Get Depressed, and Rewilding Our Hearts: Building Pathways of Compassion and CoexistenceThe Jane Effect: Celebrating Jane Goodall (edited with Dale Peterson) has recently been published. (; @MarcBekoff)