Monkeys With Lower Calls Have Smaller Balls (And Less Sperm)

Howler monkeys with lower calls have smaller testes; here a "chorus" of howlers can be seen vocalizing. Mariana Raño

Howler monkeys with lower calls have smaller testes, according to new research.

Amongst howlers, more resonant and lower calls are thought to signal better genetic quality and larger body size, and those males with more impressive "howls" tend to intimidate more males and attract more females to mate with.

Leslie Knapp, a primatologist and anthropologist at the University of Utah, and colleagues found that males that tended to live solo, but that are surrounded by a harem of females, indeed have larger hyoid bone structures, which are situated in the monkeys' throats and create the deep calls of the males of the genus. The bigger the hyoid, the deeper the call.

The researchers wanted to explore anatomical differences between these males and others: Are there any trade-offs to having a larger hyoid, and thus having a deeper call and higher likelihood of having a harem?

It turns out these solo male monkeys with louder calls have smaller testes, compared to those males with softer calls, and which live in groups with other males and don't have harems. These communal monkeys have significantly larger testes and thus more sperm.

Knapp explains that these two groups of monkeys are pursuing two different mating strategies. A howler that attracts a harem can mate with any of a number of females, while the lady monkeys won't breed with any other male. On the other hand, a number of male monkeys in multiple-male groups may mate with the same female; in this case, it's useful for an individual male to have larger testes and more sperm, to increase the chance that it's his swimmer that fertilizes the female's eggs and passes on his genes.

Monkeys, it seems, face a trade-off between a larger hyoid and testes. "You can't have everything," she says.

Reproductive "trade-offs" are common in nature. For example, red deer bellow during mating season to attract mates, but in doing so, in open fields, risk predation. Peacocks with larger tail feathers attract more mates, but similarly make themselves more visible to predators and may have a harder time getting around.

The researchers examined 144 male museum specimens of nine different species of howlers, measuring the testes and hyoid bones. They also went through field notes relating to each specimen to determine what sort of group each animal lived in and whether it had a harem or not. The results were published today in the journal Current Biology.