With bushy beards and funky blue, yellow or brown faces, guenon monkeys have some of the most colorful and quirky faces of all primates. A close study of more than 20 guenon monkey species reveals these sociable animals may rely on their distinctive patterns to prevent interbreeding.
“Animals use coloration and patterning for a whole host of different reasons and we think guenons are using their patterns to recognize different species,” said study lead author William Allen, of the University of Hull in the United Kingdom.
Guenon monkeys live in the forests of Central and West Africa eating mostly fruits, insects and leaves. Sometimes called cheek pouch monkeys, most guenons are the size of cats and dogs and they forage in large groups of two or more different species. Each species has unique chirps, sneezes and whistles but the monkeys learn other species’ warning calls like a foreign language providing protection for all.
William Allen and James Higham of New York University and Martin Stevens of the University of Exeter analyzed the unique faces of 22 guenon species. Their work which provides some of the best evidence for the role of visual signals as impediments to breeding across species is published in the journal Nature Communications.
One way to avoid interbreeding and maintain separate species is through character displacement i.e. species that share the same space evolve to look more different from one another so they can more easily tell their own kind from other species.
“Evolution produces adaptations that help animals thrive in a particular environment and over time these adaptations lead to the evolution of new species,” explains James Higham. “A key question is what mechanisms keep closely related species that overlap geographically from inter breeding, 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 inter-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 i.e. to strengthen reproductive isolation between populations.”
To do this, they photographed nearly two dozen species of guenons in various settings, over an 18-month period: in zoos in the United States and the United Kingdom and in a wildlife sanctuary in Nigeria. Armed with more than 1,400 standardized photographs, the researchers employed what is known as the Eigen face technique, which has been used in the field of computer vision for machine recognition of faces, in order to distinguish primate features and then to determine whether the appearance of each guenon species was related to the appearance of other species.
The new findings were published on June 26 in the journal Nature Communications.
The researchers suspect that the close living style of the guenon species combined with repeated expansion and contraction of the monkeys’ forest habitat drove the development of their incredible facial diversity. For example, isolated groups living in far-flung forests were squeezed together when their habitat shrunk forcing them to live with different species, Allen said.
“Where species came into contact with each other they developed visual signals so they didn’t get confused with each other,” he said.
Researchers studying primates in South America have discovered a similar pattern with more complex facial features among primates who need to quickly recognize members of their own species.