Monkeys Avoid Interbreeding with Differing Faces

Monkeys have undergone incredible facial evolution in order to differentiate themselves and avoid interbreeding with closely related and geographically proximate species, according to new research from New York University (NYU) and the University of Exeter.

"Evolution produces adaptations that help animals thrive in a particular environment, and over time these adaptations lead to the evolution of new species," senior author James Higham, an assistant professor in NYU's Department of Anthropology, explained in a news release. "A key question is what mechanisms keep closely related species that overlap geographically from inter-breeding, so that they are maintained as separate species."

The researchers looked at guenons - a group of more than two dozen species of monkeys native to Central and West African forests. Many different species of guenons are sympatric, meaning they live very close to one another, often feeding, traveling and sleeping side-by-side. Therefore interbreeding is a worrisome likelihood, and can result in infertile offspring.

Guenons have great facial diversity, including differently colored eyebrow patches, ear tufts, nose spots, and mouth patches. Oxford zoologist Jonathan Kingdon previously tried to explain this differentiation, suggesting it was an evolutionary tactic to tell one another apart, given they all live in close proximity.

NYU and University of Exeter scientists attempted to prove Kingdon's theory in this study by using facial recognition algorithms that can identify and quantify detailed features in faces.

Over 18 months, they took over 1,400 photographs of nearly two dozen species of guenons in various settings, including US zoos and a Nigerian wildlife sanctuary.

As predicted, the results, published in the journal Nature Communications, showed that the face patterns of guenon species have distinctly evolved from other neighboring species to avoid overlap and therefore hybridization.

"In other words, how you end up looking is a function of how those around you look," added Higham. "With the primates we studied, this has a purpose: to strengthen reproductive isolation between populations."