Whitney Maxwell and Dr. Dee Higley, Psychology
Under stressful conditions the activity of the hypothalamic-pituitary-adrenal (HPA) axis increases in both humans and animals leading to an increase in production of adrenocorticotropic hormone (ACTH). Maternal separation in nonhuman primates during infancy elicits behaviors characteristic of anxiety, such as withdrawal and nervous behavior and a substantial increase in ACTH (Higley and Linnoila 1997; Barr, Newman et al. 2004). Individual differences in anxiety in turn influence ACTH output, similar to what is seen in human children (Gunnar and Quevedo 2007). Infants response to separation physiologically and behaviorally is substantial and represents a significant stressor in the life of a young primate. Nonhuman primates are a good research subjects to model these changes in stress response because they allow researchers to control environmental variables in ways that we could not in humans.
Typically, in humans and in other primates, infants raised by their biological mothers and infants raised by an adoptive mother are treated similarly in research and thought of as having more or less identical developmental outcomes (Champoux, Boyce et al. 1995). In a recent study by Flygare and colleagues, presented last year, they showed that early in development adoptive mother-infant dyads experience greater conflict during day-to-day interactions. Because, infants of adopted mothers are more responsible for maintaining the relationship and are subjected to more rejection and withdraw than infants reared by their biological mother they interact more with other group members (Flygare, K. N. Graham et al. 2008). These experiences as an infant most likely influence the later behavioral and physiological stress response of the adopted infants.
In this study we assessed how the adopted infants differed from the infants reared by their biological mother in their response to a major stressor. It is proposed that adopted infants will have an increased HPA axis response to stress because of the increased conflict and rejection during the first six months of life and increased aggression with other cage members. We propose that infants reared by an adoptive mother will experience higher levels of stress and exhibit more stress behavior than those reared by their biological mothers. We further hypothesize that ACTH levels will be acutely and chronically higher in adopted infants than in mother reared animals.
Rhesus macaque infants (N=195) were grouped into two categories, those reared by their biological mother (BM, n=154) or those reared by an adoptive mother (AM, n=41). Infants raised by an adoptive mother were pseudo-randomly assigned to an unrelated mother within 60 hours after both females had given birth (to assure acceptance, the two females had to give birth within hours of each other). At six months of age infants were subject to four sequential, 4-day long separation periods each followed by a three-day reunion period. Infants were separated from their mothers by the removal of the mother from the home cage, leaving the infant behind. Day-1 of each separation was designated as the acute phase followed by the chronic phase, days 2 through 4.
During each of the four separations physiological and behavioral data were taken from the infants. Blood samples were taken on hour 1 and hour 2 of day-1 (acute stress) and on day-4 (chronic stress) of each separation. They were also taken before (baseline) and after all the separations were completed (recovery). ACTH levels were assayed from the blood samples. The behaviors that are applicable to this analysis, include environmental exploration, locomotion, and stereotypic.
Our findings indicate that under conditions of stress, adopted infants (AM) differ behaviorally and physiologically from infants raised by their biological mothers (BM). We found that AM infants were more likely to explore their environment and were less likely to exhibit locomotion (an index of agitation in this paradigm), or perform stereotypic behaviors, seeming to indicate that AM infants were under less stress or exhibiting less anxiety during separation. However, the AM infant’s ACTH levels were significantly higher than BM infants during the four separations. In essence, AM reared infants show fewer outward signs of stress but exhibited elevated ACTH levels.
Given that all other treatments were identical, it is reasonable to assume that this difference in responses between BM and AM infants is a result of the infant’s relationship with their mother in the first six months of life. One interpretation of our data is that because AM infants are rejected more often by their adopted mother and left to interact with other group members, outside the immediate reach of their mother, they may be forced to develop more adult-like coping behaviors. But, in the absence of immediately available maternal support and contact comfort, they do not learn to regulate their neuroendocrine stress responses to the extent of BM infants. Under this interpretation, AM infants do not have the maternal contact necessary to reduce their arousal during times of stress, leaving those pathways underdeveloped.
While these findings cannot be directly correlated with humans; past research has shown that non-human primate responses, behaviorally and physiologically, are similar to that of humans. Conclusive evidence of the differences between those raised by an adoptive mother or a biological mother and their responses during times of stress is impossible without a study using human subjects being conducted. Further research is needed to understand these quantitatively different responses in AM and BM infants
This research was presented at the Mary Lou Fulton conference ay BYU in 2009 and at the American Society of Primatologist conference in 2009. A publication is in process.
References
- Champoux, M., W. T. Boyce, et al. (1995). “Biobehavioral comparisons between adopted and nonadopted rhesus monkey infants.” Journal of Developmental and Behavioral Pediatrics 16: 6-13.
- Flygare, R. A., K. N. K. N. Graham, et al. (2008). Biological and adoptive mother-infant relationships across the first six months of life: More dyadic conflict, infant withdrawal, and anxiety in adoptive dyads. American Society of Primatology. Ft. Lauderdale, FL.