Response to sibling birth in juvenile cotton-top tamarins (Saguinus oedipus)

Juveniles' reaction to the birth of siblings was investigated in a cooperatively breeding primate, the cotton-top tamarin (Saguinus oedipus). Twelve captive juvenile cotton-top tamarins were observed from 8 weeks before until 12 weeks after the birth of their first set of younger siblings. The birth of younger siblings was an extremely disruptive event for juveniles, and was accompanied by large decreases in play, increases in proximity to the parents, and increases in overt conflict with the parents. However, these effects were relatively transient. There: was little evidence that juveniles lost parental care following the birth, and their behavior appeared to be primarily directed at gaining access to the new infants. Carrying of infants by juveniles was best predicted by the number of other potential carriers in the social group, whereas juvenile age and sex were of little importance. The intense interest in infants displayed by juveniles of both sexes is not surprising in the context of a cooperative breeding system in which both sexes participate in caring for infants that are not their own and in which experience with infants improves later reproductive success.