Among howler monkey species, loud calls come at the expense of testicle size and sperm production—or to put it another way, monkeys with the largest testes don't make as much noise
Howler monkeys. From their Central and South American rainforest home, they produce some of the loudest animal calls in the world. They’re about the size of an adult cocker spaniel, but they can sound as large as a tiger.
Biologists have long suspected that the monkeys’ howls played a role in attracting mates. The sounds are like the auditory version of peacock feathers or deer antlers that say, hey, check me out.
And now we know that the quality of the howl depends on a bone near their throats called the hyoid. The bigger the hyoid, the deeper the howl. Males with deeper calls sound more attractive to females. Which means that males with smaller hyoids had to come up with another strategy.
“When you’re in the field and you look up at the monkeys, you notice that in Alouatta palliata, the mantled howler monkey, that the testes are huge. And they’re white, they’re really obvious.”
University of Utah anthropologist Leslie Knapp. She her team found that howler monkeys face an anatomical trade-off: a species can either have a really big hyoid bone, or it can be very well endowed. But not both. That finding is in the journal Current Biology. [Jacob C. Dunn et al, Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys]
Those species with bigger hyoids live in single-male groups, so it makes sense to invest in what the researchers call “precopulatory traits,” that is, their courtship strategy of attracting the female’s attention.
But in a species where groups include multiple males, the best way to ensure passing on more genes is to make more sperm—by building bigger testes.
The researchers say the finding is the first evidence for an evolutionary trade-off between mating calls and sperm production. And they warn that while human women do tend to prefer males with deeper voices, we men advertise ourselves in many other ways as well. So don’t go clicking that pop-up ad that promises to enlarge your hyoid.
[The above text is a transcript of this podcast.]
[Howler monkey sounds from a video courtesy of La Senda Verde Animal Refuge, Bolivia]