In other words, the facial features of Old World monkeys have evolved to separate themselves from other geographically proximate species, preventing crossbreeding.
"Evolution produces adaptations that help animals thrive in a particular environment, and over time these adaptations lead to the evolution of new species," explained study author James Higham, an assistant professor in New York University's Department of Anthropology. "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 answer -- as researchers from NYU and Exeter University, in the United Kingdom, found out -- is precise facial feature differentiation. Features like various face markings, colorful eyebrow patches, ear tufts, nose spots, mouth patches and more.
The researchers arrived at their conclusion after first photographing and then analyzing the facial features of some 22 types of guenons, or Cercopithecini -- a group of monkeys which first evolved in the forests of Central and West Africa. By plotting and comparing specific facials features, the researchers we able to show that each species' markings and features became more distinct over time. They were also able to show that those species that spent time in close proximity were more likely to stand out from each other than species that spent less time together.
"These results strongly suggest that the extraordinary appearance of these monkeys has been due to selection for visual signals that discourage hybridization," said lead author William Allen, who has since left NYU's Department of Anthropology for the University of Hull in England. "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."