Monkeys' faces evolved to avoid crossbreeding
Guenon monkeys' colourful and varied faces have evolved as a way to avoid crossbreeding, scientists have found.
Many different species of guenons live side-by-side meaning mating with other species, which could lead to infertile offspring, is a possibility.
The researchers used human facial recognition technology to identify primate features from photographs.
They found that guenons' looks have evolved to become more distinctive from their relatives living close by.
The findings are reported in the journal Nature Communications and researchers say they are the best evidence to date of visual signs acting as a barrier to breeding across species.
Guenons - Cercopithecini - are a group of more than 25 species of monkeys which originated in the forests of Central and West Africa.
Meet the Old World monkeys
Multiple species often travel, feed and sleep together increasing the likelihood of mating with each other.
Guenons have markings which include differently coloured eyebrow patches, ear tufts, nose spots and mouth patches.
In the 1980s, Oxford zoologist Jonathan Kingdon suggested the guenons' variety of facial appearances was due to their need to identify their own species and avoid mating with others. But he failed to find firm evidence for his theory.
The researchers from New York University, US, and Exeter University, UK, wanted to test Mr Kingdon's explanation by using modern identification methods.
They spent 18 months photographing 22 species of guenons in zoos in the US and UK and in a wildlife sanctuary in Nigeria.
Primates' features were distinguished and compared with other species using a computer programme developed for human face recognition (known as the eigenface technique). It is the first time such techniques have been applied to non-human faces.
End Quote James Higham New York University
How you end up looking is a function of how those around you look.”
The results showed the face patterns of guenon species have evolved to become more visually distinctive from other guenons, especially those they come into contact with and therefore are at risk of crossbreeding with.
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."
Scientists have previously shown examples of species being differentiated (called character displacement) by acoustic and electric signals but the authors believe their research is the best example of visual variety across a broad group.
"These results strongly suggest that the extraordinary appearance of these monkeys has been due to selection for visual signals that discourage hybridisation," lead author William Allen said.
"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."
The team is now carrying out more research to find out whether guenons are more distinctive from those species they now live beside or those they were living with at the time their species emerged.