Howler monkeys may have deep voices or larger testicles, but not both according to the findings of a new research published in the Current Biology journal.

With 30 species of Howler monkey spanning a wide region from Argentina to Mexico, the findings apply to approximately nine of those that have been under the scrutiny of the biologists.

The international team of researchers looked at how these evolutionary trade-offs developed in the monkey species. Even if the evolutionary trade-offs were observed in humans as well, it is a surprising pattern for the howler monkey species. Largely, these depend on the social organization of the Howler monkey species.

Howler monkeys are known for the impressive roars they utter either to intimidate rivals or to woo mating partners. The impressive roars (terrifying at times) are produced due to the hyoid bone present in their throat. The hyoid bone acts as an amplifier of the otherwise less impressive howls. The bowl depth also plays a role in tuning the sounds. The vocal  folds act as strings would with a musical instrument.

Observing the hyiod bone led the scientists to understand that different species have significantly different sizes. This is where the evolutionary trade-offs come in place. Howler monkeys may have deep voices or larger testicles, but not both.

The deeper the voice and the more impressive the roar, the higher the likelihood to find a mating partner. To no avail though, as the impressive roar eclipses the smaller testicles and thus the lower quantity of sperm a male Howler monkey may produce.

The research is really a breakthrough for the studying of evolutionary trade-offs with view to reproduction. It is the first time where a relation has been observed between sperm production, fertility and vocalising.

Doctor Jacob Dunn from the Cambridge University, one of the researchers involved in the study explained that in no species of Howler monkeys did they find both large testicles and lower frequency sounds. Which indicates that the trade-off is necessary.

While in some of the species the female Howler monkey are attracted to the male with the most impressive roar, with others things are different. A female monkey will mate with several males to boost the chances of being fertilized.

Doctor Dunn added that similar trade-offs concerning reproduction have been observed with chimpanzees or gorillas.

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