Heritable and sex-specific variation in the development of social behavior in a wild primate

Affiliative social bonds are linked to fitness components in many social mammals. However, despite their importance, little is known about how the tendency to form social bonds develops in young animals, or if the development of social behavior is heritable and thus can evolve. Using four decades of longitudinal observational data from a wild baboon population, we assessed the environmental determinants of an important social developmental milestone in baboons--the age at which a young animal first grooms a conspecific--and we assessed how mother-offspring grooming reciprocity develops during the juvenile period. We found that grooming development differs between the sexes: female infants groom at an earlier age and reach reciprocity in grooming with their mother earlier than males. Using the quantitative genetic animal model, we also found that age at first grooming behavior for both sexes is weakly heritable (h2 = 4.3%). These results show that sex differences in grooming emerge at a young age; that strong, reciprocal social relationships between mothers and daughters begin very early in life; and that age at first grooming is heritable and therefore can be shaped by natural selection.