An experimental study on developmental changes of maternal discrimination of infants in crab-eating monkeys (Macaca fascicularis).

Individually reared mother-infant dyads of crab-eating monkeys (Macaca fascicularis) were observed cross-sectionally in their mother-infant relationship. In infants aged from 0 to 5.0 months, rather drastic changes were found both at 0.5-1.0 and 2.0-3.0 months of age. For an explanation of these changes, developmental processes of discrimination between mother and infant were analyzed cross-sectionally by exchanging mother-infant combinations. The results indicated that the first 0.5 months postpartum was characterized as a behaviorally nondiscriminating stage where nipple discrimination by infants was the only exception. The latter half of the first month was the beginning of a nonaggressive discrimination stage by mothers, indicated by lipsmacking and sniffing and in infants by clinging. When infants reached the age of 2.0-3.0 months, the mothers' nonaggressive discrimination with lipsmacking decreased, and her aggressive discrimination of alien infants increased. In addition to maternal visual discrimination of infants' physical appearances, differences in infants' odor and/or their method of nipple contact were suggested to affect the mothers' differential behaviors.