The culture of eyeball poking in some capuchin monkeys

One group of capuchin monkeys has developed a culture of eyeball poking. For twenty years scientists have been studying the traditions of white-faced capuchin monkeys. Behaviour includes finger-sucking, handsniffing and ear-sucking.

But more recently they have noticed a new fashion: eyeball-poking. In this ritual, one participant inserts its finger in the other’s eyeball, slipping the finger deep between the eyelid and the bottom of the eyeball up to the first knuckle.

As in handsniffing, the pair remains in this posture for up to several minutes, and often the one being poked in the eye inserts fingers in the partner’s nostrils or mouth during the eyeball-poking.

http://www.sscnet.ucla.edu/anthro/faculty/sperry/ctp.html