For the monkeys in the Guenon genus, breeding with another guenon species can lead to trouble. The offspring of these unions tend to be infertile, and thus a dead end from an evolutionary standpoint. If many monkeys from a single population interbreed, it could cause a sharp decline in the population. The problem is that many guenon species live in close proximity to each other, heightening the risk for interbreeding. As a result, species that live in close contact have evolved certain facial patterns to prevent any unwanted hookups, researchers report today in Nature Communications. The scientists snapped photographs of 2 dozen species of guenons (pictured) for 18 months and used face recognition algorithms to determine key features that demonstrated stark differences between neighboring species—white fur patches that cover the nose, a well-defined unibrow, or colorful ear tufts, just to name a few. The findings put to bed an alternative hypothesis that suggested environmental factors such as the lighting of a species’s habitat could be the cause of guenon facial diversity.