Mother-infant relations, conflict, stress and illness among free-ranging chimpanzees.

In a naturalistic study in the Gombe National Park, Tanzania, five chimpanzee mothers with offspring under 24 months of age were observed. Earlier studies have shown that young chimpanzees pass through distinct periods of developmental reorganization and that these episodes are characterized by regressive behaviour. The episodes in turn trigger changes in the mother and mother-infant conflict, which then lead to the infant developing a new level of independence. Monthly observations were made of mother-infant interactions before and during the illness of two infants. Using a measure of the attachment system which controls the regulation of proximity between mother and infant, it was possible to quantify a stressor in the mother-infant interaction when this proximity-control system was overstretched. When a labile developmental transition period occurred in addition to this stress, two of the infants became ill. The curve quantifying the proximity-control system seems to be self-corrective. Illness occurred between the age at which the discrepancy from the 'target curve' was maximal and the age at which the curve returned to the normal range.