Grooming and group cohesion in primates: implications for the evolution of language

It is well established that allogrooming, which evolved for a hygienic function, has acquired an important derived social function in many primates. In particular, it has been postulated that grooming may play an essential role in group cohesion and that human language, as verbal grooming or gossip, evolved to maintain group cohesion in the hominin lineage with its unusually large group sizes. Here, we examine this group cohesion hypothesis and test it against the alternative grooming-need hypothesis which posits that rates of grooming are higher in species where grooming need (i.e. the motivation to groom for hygiene and its associated psychological reward) is more pronounced. This alternative predicts that the derived social function of grooming evolved mostly in those lineages that had the highest exposure to ectoparasites and dirt, i.e. terrestrial species. A detailed comparative analysis of 74 species of wild primates, controlling for phylogenetic non-independence, showed that terrestriality was a highly significant predictor of allogrooming time, consistent with the prediction. The predictions of the group cohesion hypothesis were not supported, however. Group size did not predict grooming time across primates, nor did it do so in separate intra-population analyses in 17 species. Thus, there is no comparative support for the group-cohesion function of allogrooming, which questions the role of grooming in the evolution of human language.