Howler monkeys are fairly small animals, but can roar as powerfully as tigers. However, this ability comes at an evolutionary "trade-off." Essentially, those individuals that have larger throats and vocal organs are able to produce deeper calls but often have smaller reproductive organs, specifically testes--and less chance for reproductive success, a recent study revealed. This hardly seems fair for these monkeys that rely on their roar to attract females and scare off other rival mates.

Researchers from Cambridge University discovered that this trade-off directly impacts the size of their social groups. For instance, males with deeper roars but more diminutive testes lived in small social groups with often only one male dominating a number of females, according to a news release. On the other hand, males with bigger testes and smaller vocal organs were seen living in larger groups of up to five or six males. In this case, males did not have exclusive access to females, and instead females chose their mate based on sperm quality and quantity.  

"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," Dr. Jacob Dunn, the study's leader from the University of Cambridge's Division of Biological Anthropology, explained in the release. "There is evidence in other animals that when males invest in large bodies, bright colors, 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."

Howler monkeys can be seen in Latin American rainforests, ranging through eastern Bolivia, southern Brazil and Paraguay, and northern Argentina. The animals also use their deep howls to protect their territory and food resources, which is mainly comprised of young, nutritious leaves, as well as the occasional fruit or flower. Even in a dense forest their bellowing sounds can travel up to three miles. They use their roars as a form of communication and males routinely check in with one another at the beginning and end of each day to scope out their nearest competitors. (Scroll to read more...)

Biological trade-offs generally exist when one trait cannot increase without a decrease in another, researchers explained. However, this theory does not directly explain why howler monkeys either develop larger vocal organs or testes.  

"It may be that investment in developing a large vocal organ and roaring is so costly that there is simply not enough energy left to invest in testes. Alternatively, using a large vocal organ for roaring may be so effective at deterring rival males that there is no need to invest in large testes," Dunn explained.

In proportion to the size of their bodies, researchers were surprised that the howler monkeys were able to produce such loud roars. This means that the monkeys are able to give predators the impression they are much bigger creatures and ultimately defend themselves better.

Their study was recently published in the journal Current Biology

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