Monkeys are social animals, and frequently engage in play with one another, as well as mutual grooming and non-reproductive sexual activities. This promotes bonds among members of the monkey troop.
Grooming especially is used by monkeys to create and solidify friendships. Monkeys groom one another by combing through fur with their fingers and picking off lice and other debris. Grooming is not only sociable, but it is also hygienic. In addition, the bonds created by grooming can create helpful alliances that aid in raising infants and young offspring. Female baboons with strong bonds in the troop have more offspring surviving to adulthood as a result of other baboons helping them care for their young.
Young monkeys especially play with one another, whether rough wrestling or chasing one another through the trees. Their play helps them learn motor skills and how to fit into the social hierarchy when they become adults. Play that is too aggressive is discouraged, but a monkey that does not play with anyone out of passivity will have a harder time fitting in and finding a mate. Even adult monkeys engage in frequent play.
Some monkeys have been observed engaging in sexual behavior that is not reproductive, the bonobo being the chief example. Bonobo sex is engaged in regardless of gender and appears to be another way the monkeys establish social hierarchies and bonds.
Monkeys also engage in solitary play, batting about pebbles and playing with twigs. This develops their ability to manipulate tools.