Grooming for infant handling in tufted capuchin monkeys: a reappraisal of the primate infant market

Newborn infants can affect female social dynamics, and provide ideal conditions to test the biological market theory and its assumptions. In infant markets, infants are the desired commodity, mothers control access to them, and other females (potential handlers) trade grooming for infant handling. The supply/demand ratio corresponds to the number of available infants per potential handler. Variation in the number of infants causes changes in the supply/demand ratio that can alter the market equilibrium. We investigated whether grooming was interchanged for handling in wild tufted capuchin females, Cebus apella nigritus. Behavioural observations were conducted on 10 mothers in three groups. Potential handlers were strongly attracted to infants and grooming the mothers (specifically, its occurrence rather than its duration) increased their probability of handling infants. However, the number of infants in the group did not affect the amount of grooming needed for access to infants. At least three nonmutually exclusive hypotheses can explain differences from previous findings. First, grooming in capuchin monkeys may represent a signal of benign intent with no market value. Second, dominance relations among handlers may have prevented competition by outbidding among handlers and thus hindered free trade. Third, the increased grooming required by mothers when infants were scarcer may reflect the need to calm more stressed mothers that were the subject of more frequent harassment; since in our study mothers did not appear to be more stressed when fewer infants were available, no effect of the number of infants on grooming by handlers emerged.