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, analysed 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
"If closely related species overlap geographically, they run the
risk of hybridising," 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 hybridisation. 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
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
Higham and his colleagues say their results suggest guenons owe
their colourful faces to selection for visual signals that
discourage hybridisation. 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
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
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.
This article originally appeared on Wired.com