Behavioral and hormonal responses to separation in infant rhesus monkeys and mothers.

Effects of social stimuli on behavioral and physiological responses were examined in infant rhesus monkeys at 4 and 9 months of age. Infants and mothers were removed from the social group and housed as dyads. Following this period, infants were removed and separated under four counterbalanced conditions: (a) totally isolated--placed in a holding cage for 24 hr; (b) mother present, no contact--housed in a single cage in view of their mother, no contact; (c) mother present, contact--similar to above, with mother in proximity to the infant; and (d) peer present--separated but in proximity to a peer. In the first experiment, the infants rarely vocalized when totally isolated but showed high rates of vocalization in the presence of the mother, both with and without contact. In the mother-present conditions, they failed to show a plasma cortisol response. In contrast, totally isolated infants showed a significant elevation in plasma cortisol. At 9 months of age, these infants were separated for 3 days under two different conditions: mother present and totally isolated. Once again, the infants that were totally isolated showed little vocalization but significant elevations in plasma cortisol. In contrast, infants separated in the presence of their mothers showed high vocalization rates but no cortisol response. The concepts of protest and despair are discussed as they relate to behavioral and physiological differences observed following different separation paradigms.