Why is mutual grooming rare despite its function? A hypothesis for cognitive constraints.

Mutual grooming, where both participants groom each other simultaneously, is a type of allogrooming and probably also plays an important role as a social and hygienic function; however, it has attracted little attention, probably due to its rarity compared to prevailing unidirectional grooming in many social animals, such as primate species. While previous studies have shown that mutual grooming has functional significance, such as promoting social bonds and maximizing short-term benefits, we do not know why mutual grooming is much rarer than unidirectional grooming, and its restrictive factors have been mostly uninvestigated. In this article, we propose a hypothesis of cognitive constraints in which cognitive complexity is assumed as an underpinning mechanism for mutual grooming, thereby restricting its prevalence across animal species. To achieve joint action and its symmetric nature, mutual grooming is considered to require behavioural coordination between participants, which could be facilitated by active communication and, furthermore, by joint commitment based on a mutual understanding of intention between the two. The fact that joint commitment has been proved only in a limited number of animals may support this cognitive demand hypothesis, although it definitely calls for further in-depth investigation and comparative studies across primate and non-primate species.