CAMBRIDGE, England, Oct. 22 (UPI) -- Howler monkeys can't have it all. Evolution offers a choice: big balls or big calls. The choice dictates not only physical attributes, but a style of living.
In a recent study published in the journal Current Biology, researchers at the University of Oxford found that howler monkeys with larger throats and louder calls possess smaller testicles and produce less semen -- evidence of a unique evolutionary trade-off.
Of course, the choice isn't really a choice at all, but the work of evolution. The trade-off is real, however, and the results are seen not just in the size of a monkey's reproductive organ, but in the manner of social organization.
Dozens of howler monkey species populate the jungles of South America, and different howler monkey species adopt different ways of living. Species with smaller testicles and louder calls tend to adopt the "harem" social model, whereby a single male dominates a group of females.
Species boasting larger testes tend to organize in larger groups, some with as many as five or six males living together. The males don't have exclusive sexual access to any of the females, and some females may reproduce with two or more of the males in a group. Competition is less a matter of vocal domination and intimidation. Sex determines superiority, specifically quantity and quality of sperm.
"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," lead study author Jacob Dunn, a researcher with the University of Cambridge's Division of Biological Anthropology, said in a press release.
"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," Dunn explained. "However, this is the first evidence in any species for a trade-off between vocal investment and sperm production."
Dunn and his colleagues say the new findings highlight a theory first proposed by Darwin, which referred to a trade-off between "pre- and post-copulatory reproductive strategies."
When it comes to reproductive adaptations, Darwin theorized, evolutionary capital can be invested in one of two ways: in traits that secure access to mates or in traits that boost reproductive abilities.