Rhesus monkeys have an orthologue of the human 5-HTTLPR, making them an excellent model species for G×E studies. Like the human variant, the rhesus S allele is associated with decreased transcriptional efficiency in vitro (84). The modulating influence of the polymorphism on early life stress has been tested by separating infant rhesus monkeys from their mothers and rearing them with other infants (a long-established model of early life adversity in this species). During initial episodes of separation, monkeys carrying the rh5-HTTLPR S allele exhibit less “protest” and self-directed behaviors that are considered active coping responses to this stressor (85). Instead, separated S-allele monkeys display greater anxiety, agitation, stereotypies, and an exaggerated HPA axis response (85, 86).
