Mutual (simultaneous) grooming occurs in a variety of mammals and is characterized by a distinct payoff structure compared to unidirectional grooming, as in mutual (but not in unidirectional) grooming both partners simultaneously get the benefits of the receipt of grooming. Mutual grooming, however, has been very little investigated. We studied a captive group of ringtailed lemurs, Lemur catta, to test the hy- pothesis that mutual grooming functions to prolong grooming received. Lemurs were more likely to initiate grooming when they were being groomed than when they were simply approached by a group mate. They were also less likely to terminate grooming if their partner was simultaneously grooming them. These results support the hypothesis that mutual grooming represents a form of immediate repayment of cooperation received that facilitates further investment by the partner. It remains to be ascertained how natural selection may favour mutual over unidirectional grooming in different social and ecological conditions.
