Plasma cortisol responses to stress in lactating and nonlactating female rhesus macaques

Abstract

Lactating female rats without their pups exhibit lower HPA responsiveness to stress than nonlactating females. However, responsiveness to stress is similar when lactating females are tested with their pups and the stressor involves a potential threat to the offspring. This study constitutes the first comparison of stress responsiveness in lactating and nonlactating female nonhuman primates. Subjects were 53 multiparous female free-ranging rhesus macaques. Approximately half of the females were lactating and half were nonpregnant/nonlactating. Blood samples were obtained after capture and after overnight housing in an individual cage. Lactating females were tested with their infants. Lactating females had significantly higher plasma cortisol levels than nonlactating females on both days. Having or not having an infant was also a better predictor of plasma cortisol levels among all females than their age, dominance rank, group of origin, time of day at which the sample was obtained, and time elapsed since beginning of the sampling procedure or since anesthesia. Plasma cortisol levels of lactating females were not significantly correlated with post-partum stage or with the cortisol levels of their infants. Capture, handling, and individual housing in a cage are powerful psychological stressors for free-ranging primates. We suggest that the higher plasma cortisol levels exhibited by lactating females reflect greater responsiveness to stress associated with perception of risks to infants. Hyporesponsiveness to stress may not be a general characteristic of lactation in all mammalian species, but a short-term effect of infant suckling that is most apparent with stressors unrelated to the offspring.

Figure 1

Mean (+SEM) concentrations of plasma cortisol on Day 1 and Day 2 in females who were neither pregnant nor lactating (n = 23), pregnant females (n = 2), lactating females (n = 25), and females who had given birth but their infant had died within 6 months prior to this study (n = 3). Pregnant females and females who lost their infants were not included in the statistical analysis. Cortisol levels were significantly higher on Day 2 than on Day 1, and in lactating than in nonlactating/nonpregnant females. * P ≤ 0.0001.

Figure 2

(a). Correlation between female age and plasma cortisol levels on Day 2 across all adult females. The correlation is not statistically significant (see also Downs et al. 2007). (b). Mean (+SEM) concentrations of plasma cortisol on Day 2 in high-ranking, middle-ranking, and low-ranking adult females regardless of reproductive condition. The difference among the three groups is not significant.