Abstract
This longitudinal study spans two generations of rhesus monkeys. First, the study investigates the effects of early rearing experiences on the maternal behavior of first-generation mothers (rates of premature infant rejection) and, second, the study investigates the effects of maternal rejection on the behavior of second-generation infants. Rhesus macaque mother-infant dyads (Macaca mulatta-N = 176) were observed twice weekly, with each session lasting 300 s. First-generation mothers were raised in one of three conditions: as mother-reared controls (MR; [n = 95]), in peer groups (PR; raised without adults but with constant access to three same-aged peers [n = 49]), or with an inanimate surrogate (SPR; raised with an inanimate fleece-covered, surrogate mother and limited daily peer-group interactions [n = 32]). Second-generation infants were all raised by their differentially reared mothers and statistically grouped into one of two groups: those that were rejected by their mothers beginning at a more-typical weaning age (controls), starting in the third month of life (n = 108), and those that were prematurely rejected, with mothers showing rejections before the third month of infant life (n = 68). Overall, PR mothers exhibited the highest rates of premature infant rejection, except for month 1 of infant life, when SPR mothers exhibited the highest rates of rejection. Intriguingly, after month 1, SPR mothers showed high rates of infant cradling and seldom rejected their infants. Independent of their mothers' early rearing environment, prematurely rejected infants displayed more aggression and passive vigilance, and were cradled and groomed less by their mothers, and there was evidence that the overall rates of rejection after the first 2 months of life had a cumulative negative effect on the developing infant. Post hoc analyses of plasma cortisol levels showed that the prematurely rejected infants had higher cortisol concentrations, suggesting a high level of stress in the prematurely rejected infants. These results suggest that maternal presence during infancy has long-term effects on a female's future maternal skills which, in turn, have intergenerational consequences for the socioemotional development of second-generation infants.