Expectant parents groom adult sons according to previous alloparenting in a biparental cooperatively breeding primate

In the biparental, cooperatively breeding cottontop tamarin, Saguinus oedipus oedipus, adult sons provide an energetically and reproductively valuable service to their breeding parents by helping to carry infants. Other species may use aggressive coercion or punishment to enforce alloparenting. Yet, affiliation in primates is important in negotiating social relationships and may be exchanged for services. We hypothesized that the rewards of grooming might be used by expectant parents to cultivate strong relationships with and maintain the services of adult son alloparents. We tested for an interchange trade of aggression or grooming for infant carrying according to a biological market. Individual rates of grooming, but not aggression, received by sons were explained by previously demonstrated levels of alloparental infant carrying. However, the direction of the interaction differed according to the sex of the parent. Mothers groomed better carriers at a higher rate, whereas fathers groomed poorer carriers at a higher rate. This sex difference in parental behaviour may reflect differential benefits that male and female breeders receive with increasing help. Market predictions based on alloparent availability were not supported, but results indicated a connection between affiliation and cooperative service. The rates of huddling and contact affiliation between fathers and adult sons suggest a sex-specific set of social relationship traits conducive to both affiliation and infant carrying. These may be necessarily tied to the particular form of infant care by males in this cooperatively breeding species. Fathers may condition sons for the social aspects of carrying, which involve socially coordinated efforts and close physical proximity between adult males.