WHEN A CHIMP GRINS... IT’S TERRIFIED!

Chimpanzee society is patriarchal. Male chimpanzees are dominant over the smaller, less aggressive females. But females have their own hierarchy, and in exceptional circumstances they may join forces to get rid of an alpha male.

The hierarchy decides which females feed first, and who gets preferential access to the alpha male when they come into season and are ready to breed. Lower-ranking animals – both males and females – display submissive behaviour, such as offering their hands in supplication, or turning around and showing their rear end. Facial expressions are very important in chimp society, as they are for humans too. The commonest ones include a hostile expression, with the hairs raised, which is used during conflicts with a rival; a relaxed ‘play face’, in which the top lip conceals the chimp’s upper teeth, used when two young chimps are indulging in play fights; and a series of what look like pouting faces, mostly used when one chimp is begging for food from another.

The well-known ‘grin’, in which a chimp opens its mouth and bares its teeth (inset), is not, as sometimes thought, a sign of pleasure or happiness, but indicates fear.

Chimpanzees display several key

traits that we once considered unique to humans, including the use of tools such as a stick to fish termites out of their mounds and rudimentary forms of sign and verbal language. They also have a highly developed social structure, which includes the ability to hunt cooperatively.

Also like us, they are very dextrous, and well able to delicately manipulate objects with their hands, fingers and thumbs – and, unlike us, with their feet and toes as well. They also have relatively big brains – with a volume of roughly 300-400 cubic cm. This is roughly one-third to one-quarter the size of our brains, which means they’re able to solve quite complex problems.