Chimps can match up the faces of group members with photos of their behinds. The ability, researchers say, shows that chimps carry around mental representations with "whole body" detail of chimps they have encountered.
Primatologists Frans de Waal and Jennifer Pokorny of the Yerkes National Primate Research Centre at Emory University in Atlanta, Georgia, tested how well six adult chimpanzees could link pictures of various chimpanzee behinds, either male or female, with photos of individual chimp faces.
They showed a chimp, first, a photo of a chimp's behind, including genitals, then the faces of two chimps, both of the same sex as that behind. Each of three male and three female chimps were able to make the correct face-with-behind pairing with a probability significantly higher than chance.
But the chimps succeeded only if the faces were of chimps they knew. This suggests, the researchers say, that the chimps weren't simply detecting generic visual cues in the faces that would link them to the behind in question. Rather, it seems that the chimps must be capable of what psychologists call "whole body" integration.
"They were not only seeing the photographs as representations of chimps they knew, says de Waal, "but linked the face and behind by drawing upon a mental representation of the whole body of those chimps."
Earlier experiments had hinted that some non-human primates might have this capability, but this is the first time "whole body knowledge" has been convincingly demonstrated.
Primatologist Agnes Lacreuse of the University of Massachusetts in Amherst, says that more experiments are needed before we can conclude that chimps identify other chimps using a "gender construct" method. "We know that macaques are able to categorize faces as males or females, so it would be very surprising if chimpanzees were unable to do so."
In other experiments, de Waal and Pokorny also tested the chimps' ability to recognise the sex of other chimps from photos of their faces alone.
They first presented chimps with a photo of either a generic male or female chimp rear end – a sexually charged stimulus. The chimps were then shown closely cropped photos of two chimps, one male and one female, and encouraged to select the face of the same sex as the rear end.
The chimps tended to be successful at this test too, but again only if the faces belonged to chimps familiar to them.
This suggests, de Waal speculates, that chimps may operate with a "gender construct" – that is, the chimps recognise the sex of other chimps based, not just on physical attributes, but on other information from their previous experience with those individuals, such as their roles in the larger group.
This would be similar to how humans recognise gender, de Waal points out. In experiments with sex cues such as facial shape and hair removed, for example, people can identify faces as male or female more rapidly if they are faces of familiar people.