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Genes, Behavior, and the Social Environment: Moving Beyond the Nature/Nurture Debate
allow subordinates to “escape” may be particularly stressful. Personality also is important in determining the influence of the social environment, regardless of rank. Individuals who perceive and react to innocuous or neutral situations as threatening and/or who are not able to muster social support are likely to experience a greater stress burden (Sapolsky, 2005). Regardless of rank, individuals who are chronically stressed as a result of their social environment show higher basal glucocorticoid hormones levels, enlarged adrenal glands, and reduced sensitivity of the HPA axis to negative glucocorticoid feedback (Sapolsky et al., 1997; Sapolsky, 2005). Such stress profiles have been associated with decreased levels of high-density lipoprotein cholesterol (Sapolsky and Mott, 1987) and decreased hippocampal volume (Uno et al., 1989).
Studies also have shown that low social status in the context of an experimental social stress paradigm is associated with a greater susceptibility to experimental viral infection (Cohen et al., 1997). Studies by Capitanio et al. have shown that social stressors that include separations and housing relocation of macaques increase susceptibility to SIV (Capitanio et al., 1998; Capitanio and Lerche, 1998) and that personality characteristics of individual animals are related to stable HPA axis response characteristics (Capitanio et al., 2004). However, other studies have shown that exposure to mild early life stressors strengthens emotional and neuroendocrine stress responses in adulthood (Parker et al., 2005). Winslow and Insel showed that oxytocin, a neurohypophyseal peptide that is thought to modulate many aspects of social behavior, has different effects depending on the social status of pairs of male squirrel monkeys. Central oxytocin administration results in increased sexual behavior and aggression in dominant males and in increased associative and marking behaviors in subordinates (Winslow and Insel, 1991). It also has been shown that individual differences in the promoter for the serotonin transporter gene interact with early rearing conditions to affect behavioral development (Champoux et al., 2002), HPA axis reactivity, and vulnerability to alcoholism (Barr et al., 2004) in nonhuman primates. Tree shrews, which are thought to provide a model for early primate behavior, have been used to study chronic social stress, which is thought to model depression in subordinate animals (Fuchs et al., 2001; Fuchs, 2005). It has been suggested that the rhesus monkey provides a particularly attractive model for aging because of the similarities between human and rhesus aging phenotypes and the close genetic relationship of this species to humans (Roth et al., 2004). Therefore, nonhuman primate models provide a significant resource for examining interactions among social environment, behavior, and health outcomes. These models, although more difficult to study than rodent models, may offer the closest