Have you ever been accused of staring too long at someone’s butt? Well it might be just part of your primate instincts. Turns out chimpanzees use butts as an identifying trait just as we use faces.

New research by neuropyschologist Marika Kret in the journal PLUS ONE studied how chimpanzees processed information about faces and buttocks. What they found is that the same recognition humans derive from faces, chimpanzees derived from butts. And if you have ever seen a chimp butt, you know it can be expressive.

Also, like human faces, when you invert an image of a familiar butt, the chimpanzees had more difficulty recognizing it. Commonly known as the “face inversion effect,” chimpanzees experienced a so-called “buttocks inversion effect.” Images of a familiar chimp butt displayed upside down did not trigger a response.

The researcher went on to draw some additionally bizarre similarities between buttocks and faces. Both faces in humans and buttocks in chimps have evolved to be more colorful and attractive.

“Colour also plays an important role,” Kret explained in a summary of her work. “It is not without reason that it’s the face and buttocks of female primates that are free of hair, which makes the skin and colour all the more visible.’ Primates’ eyes are perfectly set to distinguishing red tints. That’s why not only emotional expressions, such as blushing when we are angry or shy, become more visible, but also sexual arousal.  It also explains why the buttocks of female chimpanzees are red. ‘That may not apply to humans, but it does explain why  women use lipstick and blusher.”