Movements for Grooming in Rhesus Macaques Co-opted from Feeding Movements

Abstract Catarrhine primates exhibit a wide range of skilled hand movements, including those used for foraging and grooming, and their evolution attracts ongoing attention. Competing hypotheses are that skilled hand movements evolved for feeding, or that the hand movements of various behaviors evolved independently. The present study addressed these questions by asking whether the movements of grooming of rhesus macaques (Macaca mulatta) are co-opted from feeding movements. We video-recorded free-ranging rhesus macaques in the Cayo Santiago colony of the Caribbean Primate Research Center (CPRC), University of Puerto Rico. Allogrooming (grooming a conspecific), self-grooming, and rhythmic scratching with a hand or foot were compared with a common foraging behavior, geophagy, mining and eating soil. Allogrooming, self-grooming, and foraging occurred in visually mediated bouts that featured numerous comb and brush hand movements directed toward searching for a target in the fur or soil. Once located, the target was grasped between the thumb and index finger in a pincer grip to be placed in the mouth - or less often bitted or licked from the fur. Hand movements were generally directed toward the actor but were also influenced by object-surface features, including fur direction during grooming and the rocks and debris covering soil targets during geophagy. The grooming movements of comb, brush, and grasp were recognizably those used during foraging. Rhythmic scratching with a hand or foot was not visually guided but did employ a comb movement of the digits of the hand or foot. Thus, with the exception of the rhythmicity of scratching, the hand movements of grooming are fundamentally foraging movements. It is proposed that grooming movements are coopted from oromanual feeding movements in macaques, a relationship that may be widespread in other species.