London, Sept 23 : Researchers from Emory University have found that chimps have a unique ability to recognize the faces of group members with photos of their behinds.
They found that the primates carry a mental representation of "whole body" of the chimps they meet.
During the study, primatologists Frans de Waal and Jennifer Pokorny of the Yerkes National Primate Research Centre at Emory University in Atlanta, Georgia recruited six adult chimpanzees and focused on how wre these animals able to link pictures of various chimpanzee behinds, either male or female, with photos of individual chimp faces.
A chimp was first shown 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 higher probability.
However, there was a loophole. The chimps succeeded in recognizing only if the faces were of chimps they knew.
"They were not only seeing the photographs as representations of chimps they knew, but linked the face and behind by drawing upon a mental representation of the whole body of those chimps," New Scientist quoted de Waal, as saying.
Primatologist Agnes Lacreuse of the University of Massachusetts in Amherst, said 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," Lacreuse added.
In another experiment, de Waal and Pokorny studied the chimps' ability to recognise the sex of other chimps from photos of their faces alone.
The primates were first shown 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 made to select the face of the same sex as the rear end.
The study showed that the chimps by far succeeded, but again only if the faces belonged to chimps they knew.
According to the De Waal, the findings suggest 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.
The study appears in Advanced Science Letters.