Relationship with the mother modulates the response of yearling Japanese macaques (Macaca fuscata) to the birth of a sibling.

The authors investigated the changes induced by the birth of a sibling in the relationship of 1-year-old Japanese macaques (Macaca fuscata) with their mothers and group companions. After the birth of a sibling, mother-yearling contact, proximity, and grooming decreased dramatically. Yearlings responded to such a reduction in maternal care in either of 2 radically different ways. Yearlings either sought attention from group companions and showed no sign of depression or did not compensate for the mother's reduced availability and became depressed. The modality of response was predicted by the quality of the relationship with the mother before the sibling birth. Yearlings that had spent a larger amount of time in contact with their mothers were less likely to become depressed. Security of the attachment relationship with the mother may be the factor mediating the link between the time in contact and the yearling's response to the birth of a sibling.