Some monkeys have undergone a drastic facial evolution in order to avoid interbreeding with different species.
The news study provides the best evidence of visual cues being used as a "barrier" for breeding across different species, an NYU news release reported.
"Evolution produces adaptations that help animals thrive in a particular environment, and over time these adaptations lead to the evolution of new species," James Higham, an assistant professor in NYU's Department of Anthropology and the senior author of the study, which appears in the journal Nature Communications, said in the 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 findings provide evidence that visual signals are key in ensuring species recognition. Species may evolve to look differently from species they are at risk of breeding with.
"In other words, how you end up looking is a function of how those around you look. With the primates we studied, this has a purpose: to strengthen reproductive isolation between populations," Higham said.
The researchers looked at guenons, which include more than two dozen species and are indigenous to the forests of Central and West Africa. Many of these monkeys live in close quarters with other species, sometimes even sleeping and eating side-by-side. This puts the primates at a high risk of interbreeding, which can lead to infertile offspring.
The team photographed about two dozen species of guenons in various settings over a period of eight months. They then used something called the eigenface technique, which is used for facial recognition, in order to identify the species' distinct features. The method predicted that these species have evolved to have distinct facial appearances, especially between those that overlap geographically.
"These results strongly suggest that the extraordinary appearance of these monkeys has been due to selection for visual signals that discourage hybridization," lead author William Allen, said in the news release. "This is perhaps the strongest evidence to date for a role for visual signals in the key evolutionary processes by which species are formed and maintained, and it is particularly exciting that we have found it in part of our own lineage."