Exposure to early life adversity has been identified in both animal and human models as a risk factor for developing anxiety later in life (Coplan et al., 1996; Huot et al., 2001; Heim and Nemeroff, 2001). In nonhuman primates, these effects have been studied in infant monkeys exposed to different early rearing environments (see review by Stevens et al., 2009). Infants reared with other naïve infants for the first 8 months of life (peer rearing) develop most species-typical patterns of behavior but show more intense reactions to social separation (Higley et al., 1991) and also display heightened anxiety, particularly in response to novel events, compared to mother-peer-reared (MPR) monkeys (Harlow, 1969; Suomi, 1991). As juveniles, peer-reared (PR) monkeys show lower levels of affiliative behavior and are less likely to have their stress levels reduced by the presence of a companion than normally reared monkeys (Winslow et al., 2003). Juvenile PR monkeys also show a heightened acoustic startle response (Parr et al., 2002), enhanced fear potentiated startle (Nelson et al., 2009), and disrupted sleep patterns (Barrett et al., 2009) compared to mother reared controls. Very little information is available on PR adults. However, in one study, adult PR animals showed heightened vulnerability to alcohol consumption compared with normally reared adults (Fahlke et al., 2000), a finding consistent with the view that increased anxiety may persist into adulthood following peer rearing.
Despite the link between peer rearing and anxiety, no consistent pattern has emerged from an examination of hypothalamic-pituitary-adrenocortical (HPA) axis function in these infants. PR monkeys have been reported to show higher basal levels of cortisol (Barrett et al., 2009; Higley et al,. 1992), lower basal levels but no difference in stress responsivity (Shannon et al., 1998), no difference in basal levels but lower stress responsivity (Clarke, 1993), higher stress responsivity (Fahlke et al., 2000), a lower cortisol set point (Capitanio et al., 2005), and lastly no effects in either baseline or stress responsivity (Winslow et al., 2003). Variation in rearing practices or other differences between primate facilities might account for some of these inconsistencies.
HPA function has also been examined in a second kind of early nursery rearing environment: surrogate-peer rearing (SPR). In this condition, infants are reared with inanimate surrogate mothers and given up to 2 hours of daily play experience with other infants. The purpose of this rearing condition is to reduce the profound attachment that occurs between infants, thereby presumably reducing anxiety and facilitating play behavior. Although less is known about the effects of this rearing procedure, infant monkeys appear to acquire all the typical forms of species-normative behavior without showing excessive clinging (Hansen, 1966). As adults, SPR monkeys are indistinguishable from mother reared monkeys in their reproductive and parental behavior (Novak et al., 1992; Sackett et al., 2002). Converging lines of evidence suggest that infant SPR monkeys have significantly lower basal concentrations of circulating cortisol (Capitanio et al., 2005) than MPR and PR counterparts, although in one study (Davenport et al., 2003), this difference was present only in the first month of life. Other studies have revealed that SPR monkeys respond significantly less to the stress of brief social separation (Meyer et al., 1975; Shannon et al., 1998) than both MPR monkeys and PR monkeys, which is perhaps not surprising given the inanimate nature of their mother and their restricted interaction with peers.
One of the limitations of previous research on HPA activity in differently reared monkeys is a reliance on blood plasma as the primary sample matrix, given the well-known lability of circulating cortisol to circadian variation and environmental disturbances. Moreover, studies of HPA function over the course of development necessitate repeated sample collections, each requiring capture and sedation of the animals which not only further stresses the subjects but may also confound the results. A long-term measure of circulating glucocorticoids would greatly reduce the frequency of sampling required and would eliminate the confounds imposed by capture and restraint.
As a potential alternative to measuring cortisol “point samples” in plasma or saliva, hair cortisol has recently gained attention as an index of chronic HPA activity (Davenport et al., 2006; Sauvé et al., 2007; Kirschbaum et al., 2009) and as a possible biomarker for stress-related health disorders in both human and nonhuman primate populations. Hair cortisol has proven to be a reliable indicator of the effects of various stressors. In adult rhesus monkeys, relocation to a different environment resulted in elevated hair cortisol along with marked behavioral changes and sleep disturbances (Davenport et al., 2008). These hair cortisol samples were also strongly correlated with salivary cortisol samples obtained in the same time frame (Davenport et al., 2006). In human studies, increased hair cortisol concentrations have been associated with perceived stress in healthy pregnant women (Kalra et al., 2007), hospitalization of at-term infants (Yamada et al., 2007), periods of elevated cortisol secretion in patients with Cushing’s syndrome (Thomson et al., 2010), and long-term unemployment (Dettenborn et al., 2010).
The present study was designed to determine how differences in early experience affect the response of infant rhesus monkeys to the major stress event of relocation by measuring both anxious behavior and chronic concentrations of cortisol derived from hair. Repeated assessment of both variables over 2 years enabled us to test the hypothesis that heightened HPA activity (as measured by hair cortisol concentrations) early in development predicts later anxious behavior in response to stress. Another novel and important aspect of this study was examining anxious behavior and HPA axis activity in two different nursery-rearing environments (PR vs. SPR) in comparison to MPR controls. In this regard, the rhesus monkey infants used this study were part of a larger project aimed at understanding biobehavioral development at the Laboratory of Comparative Ethology at the National Institute of Child Health and Human Development. Infants were reared in all three rearing conditions described above for the first 8 months of life and then, as is standard practice at this laboratory, were relocated and placed together in one large social group. That this relocation can be considered initially stressful is based not only on previous findings of substantial relocation effects in adult rhesus monkeys (Davenport et al., 2008) but also on the facts that the new environment was substantially different from the monkeys’ previous housing environments, both in size and complexity, and in sheer number of animals with which to interact.
Thus, the aims of the present study were 1) to characterize hair cortisol concentrations in differently-reared infant rhesus monkeys across the first 2 years of life both before and after the stressor of relocation and 2) determine whether hair cortisol was a predictor of anxious behavior after the major social stressor of weaning and relocation in these young monkeys.