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

Grooming hand movements in free-ranging rhesus macaques (Macaca mulatta) are recognizably foraging movements, with comb, brush, and pincer-grasp actions co-opted from geophagy.

• Why it matters: The study tests whether skilled hand movements evolved for feeding or instead evolved independently across behaviors in catarrhine primates.
• What they did: Video recordings from the Cayo Santiago colony (CPRC, University of Puerto Rico) compared allogrooming, self-grooming, and rhythmic scratching with geophagy (mining and eating soil).
• The result: Grooming bouts were visually mediated and used comb/brush searching plus thumb–index pincer grasp to place targets in the mouth, while rhythmic scratching lacked visual guidance but used a comb digit movement.