Do I know you from somewhere? Chimpanzees recognise bums like people recognise faces

Much of the headline-grabbing research about chimpanzees, humans’ closest animal relatives, is framed in terms of how good chimps are at doing things we do. Well, here’s a new finding on something those great apes trounce us at: recognising each other’s butts.

They’re good at this in the same way we people are at recognising individual faces. Unlike other common objects, we tell one face from another in a holistic way, processing the eyes, nose, lips and other features together. When we see images of faces turned upside-down, we’re disproportionately worse at recognising them than we are at recognising, say, a flipped car.

This is called the “inversion effect,”and the authors of a new study in PLOS One found that chimpanzees have it when it comes to buttocks.

It was already known that chimps sometimes demonstrate an inversion effect with faces and bodies. But the researchers, based in the Netherlands and Japan, spotted a gap in the literature: “Previous studies included almost all body parts, except the most obvious one, which is the behind.”

Why would this be obvious? Because rear ends serve a big purpose in the chimp world. Female chimps’ buttocks grow redder and swollen when they are ovulating, signalling to males that it’s business time. And it’s important to know whose bottom it is, in part to prevent inbreeding. The buttocks have, in scientific parlance, a “high socio-sexual signalling function”.

But when we began walking upright, our bottoms became fleshier and no longer broadcast our ovulation status, possibly to discourage casual hook-ups in favour of pairing up and sticking together for the children’s sake. On the other hand, humans – “especially females,” the researchers write – developed ruddier and thicker lips, as well as fattier faces.