Mutual grooming among adult male chimpanzees: the immediate investment hypothesis

Unidirectional grooming is a low-cost behaviour for which the groomer is repaid via kin selection or reciprocity. Return benefits can come in the form of increased probability of being groomed or social benefits such as coalitionary support. By contrast, the reasons for mutual grooming, which occurs when two individuals simultaneously groom each other, are not understood. In this study, we test three hypotheses regarding the function of mutual grooming among wild male chimpanzees, using 16 years of data. The social bonding hypothesis posits that mutual grooming promotes a return benefit by serving to strengthen and maintain social bonds, whereas the immediate investment hypothesis states that it functions as a signal to indicate willingness to invest in (continue) the grooming bout. The switching hypothesis states that mutual grooming results from overlap created when the direction of the grooming interaction is switched. The social bonding hypothesis was not supported: measures of association were not correlated with the probability of mutual grooming. We also found no support for the switching hypothesis, as mutual grooming was equally likely to occur without a switch in the direction of grooming as when a switch occurred. The immediate investment hypothesis was supported by our finding that bouts with mutual grooming (1) were longer, (2) contained a more equitable distribution of unidirectional grooming and (3) had more unidirectional grooming switches than bouts without mutual grooming. We conclude that male chimpanzees use mutual grooming to obtain short-term benefits in the form of prolonging a grooming bout, and suggest that mutual grooming thus represents a form of overlapping parcelling.