A group of Old World monkeys called guenons have undergone an incredible diversification in facial appearance to avoid interbreeding with closely related species.
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.
"If closely related species overlap geographically, they run the risk of hybridizing," says Higham. "You need a good way to make sure you don't make bad mate choice decisions."
One way to avoid interbreeding and maintain separate species is through character displacement: 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.
The researchers studied character displacement in guenons, a group of more than two dozen species of monkey native to the forests of Central and West Africa. Many guenons form mixed-species groups that travel and forage together. Interbreeding, which could result in less fit offspring, is possible, though it doesn't often occur in the wild.
Guenon species have a variety of different facial markings, including ear tufts, eyebrow patches, and nose spots. In the 1980s, Oxford zoologist Jonathan Kingdon suggested the diversity of guenon faces evolved to visually reinforce differences between species and avoid hybridization. However, his ideas were based on observations with the naked eye, and he struggled to quantify all the variation in guenon faces. Allen, Stevens and Higham set out to test Kingdon's hypotheses using more sophisticated tools available today — facial recognition algorithms that can identify and quantify many different facial features.
The researchers used the eigenface technique, one of the first algorithms created for computer face recognition. Using this technique, they could distinguish specific facial features and then determine whether the appearance of each guenon species was related to the appearance of other species that overlapped geographically.
They found that, as predicted, the faces of guenon species evolved to become more visually distinctive from those other guenon species with which they share space, and hence are at the most risk of hybridization.
Higham and his colleagues say their results suggest guenons owe their colorful faces to selection for visual signals that discourage hybridization. The distinct faces of different species help ensure species recognition and prevent guenon species that live together from interbreeding.
Now, the team are trying to go back in time and figure out if each guenon species' distinctive facial markings are more dependent on the species they overlap with currently or the species they overlapped with when they first evolved.
"We have good molecular data suggesting when each species arose," Higham says. "We can ask questions about which guenon species overlapped at the time of speciation events, and we can compare that to those that each species geographically overlaps with now."
This study provides some of the strongest evidence to date for the role of visual signals in character displacement. It's also unique in that it looked at a complex trait, whole facial appearance, in a large number of species. The results show that in guenons, distinct faces help form and maintain separate species.
Allen, W. L., Stevens, M., and Higham, J. P. (2014). Character displacement of Cercopithecini primate visual signals. Nature Communications 5:4266. doi: 10.1038/ncomms5266.