Monkeys Evolved Unique Faces to Avoid Interbreeding with Close Species

It turns out that Old World monkeys have evolved a clever way to avoid interbreeding with closely-related species. Scientists have found that monkeys possess remarkable differences in facial appearances that act as an efficient barrier to breeding across species.

In order to better understand how monkeys avoid interbreeding, the researchers examined guenons, a group of more than two dozen species of monkeys that are indigenous to the forests of Central and West Africa. Many of these species live in close proximity to one another, but interbreeding almost never occurs. In the past, researchers believed that this was partly due to the fact that there is a large diversity in the facial appearance of guenons.

In order to see whether facial appearance might impact interbreeding, the researchers used facial recognition algorithms that could identify and quantify detailed features in faces. They photographed nearly two dozen species of guenons in various settings over an 18-month period. More specifically, they employed what is known as the eigenface technique 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 findings revealed that the face patterns of guenon species actually have evolved to become more visually distinctive, especially in species that overlap geographically.

"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," said James Higham, the senior author of the study, in a news release. "In other words, how you end up looking is a function of how those around you look. With the primates we studied, this had a purpose: to strength reproductive isolation between populations."

The findings reveal that the extraordinary appearance of these monkeys has a purpose. By having defined facial features, these monkeys have evolved in order to encourage genetic separation and prevent interbreeding between species.

The findings are published in the journal Nature Communications.