Chimpanzees pass down a 'secret handshake' through the generations, according to a new study.
Researchers found that troops of chimps pass on traditions, including the handshake, from one generation to the next - and they differ from group to group.
When chimps are cleaning, they adopt a 'grooming handclasp' - two chimps clasp onto each other's arms, raise them in the air and groom each other with their free hand.
Previous research suggest this handclasp may be a cultural phenomenon - like how people greet each other - but only some chimpanzee colonies practise this grooming behaviour.