Mutual medication in capuchin monkeys – Social anointing improves coverage of topically applied anti-parasite medicines

Wild and captive capuchin monkeys will anoint themselves with a range of strong smelling substances including millipedes, ants, limes and onions. Hypotheses for the function of the behaviour range from medicinal to social. However, capuchin monkeys may anoint in contact with other individuals, as well as individually. The function of social anointing has also been explained as either medicinal or to enhance social bonding. By manipulating the abundance of an anointing resource given to two groups of tufted capuchins, we tested predictions derived from the main hypotheses for the functions of anointing and in particular, social anointing. Monkeys engaged in individual and social anointing in similar proportions when resources were rare or common, and monkeys holding resources continued to join anointing groups, indicating that social anointing has functions beyond that of gaining access to resources. The distribution of individual and social anointing actions on the monkeys' bodies supports a medicinal function for both individual and social anointing, that requires no additional social bonding hypotheses. Individual anointing targets hard-to-see body parts that are harder to groom, whilst social anointing targets hard-to-reach body parts. Social anointing in capuchins is a form of mutual medication that improves coverage of topically applied anti-parasite medicines.