“We think that there’s some kind of trade-off going on,” says Knapp. They discovered a negative correlation between the size of males' hyoids and the size of their testes, which points to the calls having a role in reproduction, not just communication and scaring away predators. They also analyzed the monkeys' group dynamics based on literature published on each of the species they studied. (Credit: Jacob Dunn) Researchers used laser surface scanning to build 3-D anatomical models of hyoids from 255 male and female howler monkey specimens housed in various museums. Researchers found a link between the size of a howler monkey's hyoid (red) and the size of their testes. “If you thought of the howls as only being for communication it would seem you would have more similarity in size between their hyoid bones,” says senior author Leslie Knapp, professor and chairperson of the anthropology department at the University of Utah. But a closer look at the hyoids reveals an unexpected level of variation in between sizes. Loudest in the Room Howler monkeys produce one of the loudest vocalizations of any animal on the planet, and males use their voices to shout down competitors, attract mates and rope off their territory. The disparity tipped researchers off that something else was going on. There was significant variation in hyoid size between howler species, but also between male and females within a species. Animals with louder and lower calls had larger hyoids, a bone in the front of the neck that appears to act as an amplifier, but smaller testes. A research team looking into the monkeys’ incredibly loud and low vocalizations, and the physical structures that support the calls, discovered the correlation. For howler monkeys, it appears the louder the calls, the smaller the - ahem - balls.
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