Abstract
Birth timing, a relative measure of the timing of births within a season, has been shown to be related to the ways mothers and infant interact as well as to infant behavior and physiology. Although effects of birth timing on the hypothalamic-pituitary-adrenal (HPA) axis have previously been associated with variation in social relationships, these effects could also be related to seasonal variation in climate conditions when the birth season is long. The current study examines the effects of birth timing and ambient temperature on the activity and regulation of the HPA axis in 3-4 month old rhesus monkeys (N=338). Subjects were part of a BioBehavioral Assessment in which infants were separated from their mothers and relocated to a novel testing environment for a period of 25h. Four blood samples were collected and assayed for cortisol concentrations and reflected HPA response to (1) 2h maternal separation and relocation, (2) 7h maternal separation and relocation (sustained challenge), (3) dexamethasone suppression, and (4) ACTH challenge. Nonlinear mixed modeling was used to examine the independent effects of birth timing and temperature on HPA axis activity and regulation over the study period. Results indicated that birth timing and ambient temperature both had significant, but opposing effects on the cortisol response to sustained challenge. Chronic exposure to low ambient temperatures was associated with higher cortisol levels. After controlling for the effect of ambient temperature, birth timing was positively associated with cortisol such that late-born infants exhibited higher cortisol concentrations than did early-born infants. These results highlight the fact that climate conditions, even mild, subtropical conditions, can have potentially important influences on the activity and development of the HPA axis.