Dr James Higham, senior author, said: "Evolution produces adaptations that help animals thrive in a particular environment, and over time these adaptations lead to the evolution of new species.
"A key question is what mechanisms keep closely related species that overlap geographically from interbreeding, so that they are maintained as separate species.
"Our findings offer evidence for the use of visual signals to help ensure species recognition: species may evolve to look distinct specifically from the other species they are at risk of interbreeding with," Dr Higham said.
"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."
"Monkeys' faces evolved to avoid crossbreeding"
BBC: Guenon monkeys' colourful and varied faces have evolved as a way to avoid crossbreeding, scientists have found.