Video: Howler Monkeys Who Howl The Loudest Don't Have Well-Endowed Sexual Organs

By R. Siva Kumar - 24 Oct '15 12:25PM

Even though howler monkeys are among the loudest animals on Earth, scientists say that monkeys with the "most impressive" howls are "overcompensating for something", according to HNGN.

The small-big equation is pretty interesting. In a recent study, researchers found that male howler monkeys who called louder seemed to have smaller testes and produced less sperm than the quieter ones, according to the University of Cambridge.

There seems to be an "evolutionary trade-off between the size of the male hyoid (the hollow throat bone that allows the guttural call to resonate), and the size of the reproductive organs", which correspond with their mating systems.

Moreover, males who had large hyoids and small testes usually lived in smaller social groups, in which one male was the king of many females.

Those howlers who had smaller hyoids and larger testes lived in groups of five to six males, in which the females mated with all the males in the particular social circle. As the males in the group do not have access to only females, they have to rely on "sperm competition," in which the quality and quantity of the sperm is important for insemination.

"In evolutionary terms, all males strive to have as many offspring as they can, but when it comes to reproduction you can't have everything," said study leader Jacob Dunn, from the University of Cambridge's Division of Biological Anthropology. "There is evidence in other animals that when males invest in large bodies, bright colours, or weaponry such as horns or long canines, they are unable to also invest in reproductive traits. However, this is the first evidence in any species for a trade-off between vocal investment and sperm production."

Hence, scientists collected information on average testes size among various howler species. using 3-D laser scans to assess and judge the size of more than 250 hyoids. Scientists also looked at the in-depth acoustic analyses of the howler's roars.

"The results of our acoustic analyses show that howler monkeys produce roars at a similar frequency as tigers, which is far lower than we would have predicted from their body size, yet exactly what would be predicted from measuring their giant vocal folds," Dunn said.

Scientists reveal an interesting formula. Those who yell at volumes equivalent to animals 10 times bigger than their body size, have invested their energy into their powerful vocal organs, without enough energy for their highly productive testes.

The findings were published in a recent edition of the journal Current Biology.