Humans and chimps are pretty good at recognizing faces. But when faces are upside down, we both find it much more difficult to identify them than other inverted objects. This is thought to be because we use spatial cues to recognize faces. But how well do you think you would recognize a butt? Well, it seems chimps are just as confused at recognizing inverted faces as they are when recognizing inverted cracks – the same cannot be said for humans however.

Known as the “face inversion effect”, it has been found that while everyday objects shown upside down are easy to identify because we look at the item as a whole, it is not the same with faces. When faces are shown inverted, it takes slightly longer for our brains – which have a specific facial recognition zone – to process. 

When scientists tested this phenomenon with other body parts, they found no similar effect, until now. A new piece of research from the University of Leiden has found that chimps take slightly longer to recognize inverted butts, just as they do with faces. “This is a good indication that this category has priority over other categories of objects,” says Mariska Kret, who co-authored the study published in PLOS One.

^{Chimpanzee butts (left) show similar symmetrical features to human faces (right). Kret et al. 2016}

It implies that something similar is going on in the cognitive processing of faces and bums, and could even provide insight into our own evolution. In chimpanzees, the females show they are sexually receptive through the enlarging of their arses, which also flush red. The authors suggest, perhaps rather controversially, that throughout our evolutionary history, faces and butts have played a similar role, and that humans are using a similar cognitive process to recognize them.

“The findings suggest an evolutionary shift in socio-sexual signalling function from behinds to faces,” write the authors, “two hairless, symmetrical and attractive body parts, which might have attuned the human brain to process faces, and the human face to become more behind-like.” They go on to state that the lack of hair may enable color to become more obvious, something that they claim is enhanced by the addition of blusher and red lipstick.

So while chimps may be seeing butts in the same way as they see faces, we’ve lost this aspect as focus shifted primarily to our "behind-like" faces.