Individual differences in response to sibling birth among free-ranging yearling rhesus monkeys (Macaca mulatta) on Cayo Santiago

The birth of a younger sibling is a normal event in the life of a nonhuman primate, yet commonly it is thought to be a stressful transition for the older sibling. In our previous research, we found that yearling rhesus monkeys (Macaca mulatta) experienced increases in one mild form of distress but no significant increases in overt forms of distress, in spite of significant reductions in mother-yearling interaction. Nevertheless, some individual yearlings were distressed by this transition and here we examine variables that may structure individual differences in distress. We observed 31 yearling rhesus monkeys on Cayo Santiago, Puerto Rico, during the month before and month after their siblings' births using focal animal sampling methods. Attachment theory (Bowlby, 1969), parent-offspring conflict theory (Trivers, 1974), and dynamic assessment models (Bateson, 1994) all predict a relationship between reduction in maternal care and increase in offspring distress, yet no previous study of sibling birth in primates has examined this relationship. We found that the reduction in the proportion of time on the nipple from the month before sibling birth to the month after was related to the rate of geckering (a distress vocalization) after sibling birth, and that the increase in time out of sight of the mother was related to the proportion of time yearlings spent in a tense state after sibling birth. Maternal aggression after sibling birth also was related to the yearlings' rate of geckering. Yearling distress was related to qualities of the mother-yearling relationship, in that yearlings that had relatively greater responsibility for maintaining proximity with their mothers before sibling birth were relatively more tense afterwards. Yearlings displayed increases in play, grooming, and contact with group members other than the mother after sibling birth, suggesting a marked shift toward greater maturity in their social relationships.