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http://www.newscientist.com/article/dn14783-chimps-never-forget-a-bum.html?DCMP=ILC-hmts&nsref=news2_head_dn14783

Chimps never forget a bum

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.
Body knowledge

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.
Concept of sex?

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.

Journal reference: Advanced Science Letters (DOI:10.1166/asl.2008.006)