Getting to the bottom of face processing. Species-specific inversion effects for faces and behinds in humans and chimpanzees (Pan Troglodytes).

For social species such as primates, the recognition of conspecifics is crucial for their survival. As demonstrated by the ‘face inversion effect’, humans are experts in recognizing faces and unlike objects, recognize their identity by processing it configurally. The human face, with its distinct features such as eye-whites, eyebrows, red lips and cheeks signals emotions, intentions, health and sexual attraction and, as we will show here, shares important features with the primate behind. Chimpanzee females show a swelling and reddening of the anogenital region around the time of ovulation. This provides an important socio-sexual signal for group members, who can identify individuals by their behinds. We hypothesized that chimpanzees process behinds configurally in a way humans process faces. In four different delayed matching-to-sample tasks with upright and inverted body parts, we show that humans demonstrate a clear face, but no behind inversion effect and that chimpanzees show a behind, but no face inversion effect. The findings suggest an evolutionary shift in socio-sexual signalling function from behinds to faces, 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.