Changes in yearling rhesus monkeys' relationships with their mothers after sibling birth

The birth of a new sibling is believed to signify an abrupt and important transition in a young primate's relationship with its mother-one that is of potential importance from at least three theoretical perspectives: attachment theory, parent-offspring conflict theory, and dynamic assessment models. This study examines changes in relationships between free-ranging yearling rhesus monkeys (Macaca mulatta) and their mothers concomitant with the birth of the mother's next infant, and tests predictions derived from each theoretical perspective. We observed 31 yearling rhesus on Cayo Santiago, Puerto Rico, 3 months before and 3 months after their siblings' births, using focal animal sampling methods. Changes in measures related to mother-yearling interaction and yearling distress were examined using repeated-measures analysis of variance. After sibling birth, mothers and yearlings abruptly reduced amounts of time in contact and increased amounts of time at a distance and out of sight of one another. Mothers and yearlings played approximately equal roles in bringing about decreases in proximity, and yearlings took the primary roles in bringing about decreases in contact. Rates of maternal aggression toward yearlings increased immediately and markedly after birth, possibly providing yearlings with early cues regarding subsequent decreased levels of maternal care. There were no marked increases in overt signs of yearling distress (e.g., vocalizations or tantrums) following the births. We conclude that yearlings generally acquiesced to reduced levels of care, responding behaviorally with increased independence and maturity. In this sense, our study provides preliminary support for dynamic assessment models over attachment theory and parent-offspring conflict theory models.