Chimpanzees spend a lot of time grooming one another. You might think they have a pressing interest in cleanliness, but grooming is less about hygiene and more about strengthening social ties. Chimpanzees groom to make up after a fight, to build new relationships, or just to catch up with old friends. One evolutionary psychologist has hypothesized that chimpanzee grooming is the equivalent of human gossip. It allows individuals to build and reinforce social networks, and gain social capital. And of course, it just feels good to spend time with friends.