Effect of mother's dominance rank on offspring temperament in infant rhesus monkeys (Macaca mulatta).

In humans, temperament plays an important role in socialization and personality. Some temperaments, such as behavioral inhibition are associated with an increased risk for psychopathology. Nonhuman primates can serve as a model for neurobiological and developmental contributions to emotional development and several recent studies have begun to investigate temperament in nonhuman primates. In rhesus monkeys, dominance rank is inherited from the mother and is associated with social and emotional tendencies that resemble differences in temperament. The current study assessed differences in temperament in infant rhesus monkeys as a function of maternal dominance rank. Temperament was assessed in 26 infants (13 males) from birth until 6 months of age with a battery that included Brazelton test, human intruder test, human intruder-startle, cortisol stress reactivity, and home cage observations of interactions with peers and the mother. Throughout testing, infants lived with their mothers and a small group of other monkeys in indoor/outdoor runs. Dominance rank of the mothers within each run was rated as either low/middle (N = 18, 9 male) or high/alpha (N = 8, 4 female). Infants of high-ranking mothers displayed more intruder-directed aggression and reduced startle potentiation in the human intruder tests. Dominant offspring also had reduced levels cortisol and startle across development and spent more time away from mothers in the interaction tests. These results suggest that dominance of the mother may be reflected in behavioral reactivity of infants early in life. These findings set up future studies, which may focus on contributing factors to both dominance and temperament such as genetics, rearing, and socialization. Such factors are likely to interact across development in meaningful ways. These results also suggest future human-based studies of a similar relationship may be warranted, although social dominance is clearly more complex in human than macaque societies.

Developmental changes of rhesus monkeys in response to separation from the mother.

The development of separation response behaviors in infant rhesus macaques across the first 6 months of life was assessed. Seventeen infants underwent a neonatal assessment at 7, 14, 21, and 30 days of age which included a brief period of social isolation. At 3 and 6 months of age these same monkeys and four additional subjects were again subjected to a period of brief social isolation and also exposed to a novel environment with their sedated mother. Results indicate a developmental increase followed by a steady decline in the frequency of separation vocalizations. A modest relationship between early-infancy locomotor profiles and separation responses was also observed at several time points suggesting a possible relationship between these measures. However, stable inter-individual measures of separation distress did not emerge until late in the infantile period. This could suggest that high levels of maternal contact-seeking behavior early in infancy are context specific and not a reliable index of enduring temperament.