depends on the degree to which parental care varies across the two species (or strains) in question. In birds, cross fostering can be relatively benign (e.g., rearing of green finches’ offspring by canaries; Guettinger 1979). In other cases, however, cross fostering may complicate the assessment of stress and distress, as cross-fostered rats, mice, and goats frequently exhibit behaviors more similar to the adoptive mother strains (Ahmadiyeh et al. 2004; Anisman et al. 1998; Kendrick et al. 2001). In rats, female offspring of dams bred for high licking and grooming that have been reared with their biological mothers will themselves provide extensive maternal care of their own pups. In contrast, if female offspring of high licking and grooming dams are instead cross fostered with low licking and grooming (i.e., “poor”) mothers, they will subsequently provide little maternal care to their own offspring, resulting in behavioral and physiological changes that persist into adulthood (Francis et al. 1999). Recent research into the effects of maternal behavior on such developmental traits as DNA methylation, an epigenetic mechanism that alters gene expression, has shown that maternal environmental programming (for example, high or low grooming) affects the glucocorticoid receptor gene and possibly the responses of the offspring to stress. Offspring of high grooming mothers (or those cross fostered to them) appeared less responsive to stressful stimuli and had increased expression of these receptors in the hippocampus compared to those raised by low grooming dams (Fish et al. 2004; Weaver et al. 2004). Microarray analysis has shown that more than 900 genes of the hippocampal transcriptome are stably regulated by maternal care (Weaver et al. 2006).
In species such as primates, however, infants may be nursery-reared because of the infant’s illness, the mother’s failure to care for the infant, or demands of the experimental protocol. The two most common nursery rearing procedures for macaques are peer rearing (i.e., rearing infants together 24 hours a day) and surrogate peer rearing (i.e., rearing infants on inanimate surrogate mothers 24 hours a day with 1-2 hours of daily peer exposure in a playroom setting). Both conditions commence shortly after an animal’s birth before a strong attachment has been formed to the mother, and thus infants show little in the way of separation anxiety.
From a developmental perspective, peer rearing appears to confer the greater risk for distress and social maladjustment. Peer-reared monkeys typically show higher levels of mutual clinging and greater fear responses than surrogate-peer-reared monkeys early in life and have difficulty adapting to larger social groups as juveniles (Ruppenthal et al. 1991). Peer rearing has also been associated with impaired immune responses (Coe et al. 1989; Lubach et al. 1995) and, when combined with repeated separations, appears to promote heightened aggressiveness, impaired impulse control, alcohol abuse, and low levels of 5-hydroxyindoleacetic acid (a serotonin metabolite) in cerebrospinal fluid (Ichise et al. 2006). Although