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Neuropeptides and Social Behavior Development 177contrast to pair bonding, male parental care does not appear to be selective (i.e.,males will care for any pup, not just their own offspring; Lonstein & De Vries,2000; Roberts, Miller, Taymans, & Carter, 1998).Relevant data supporting the notion that OT is not essential for (nonselective)social behavior comes from mice with a mutation in the gene for OT (OT knockoutmice: OTKO). OTKO mice continued to mate and to express maternal behavior(Nishimori, Young, Guo, Wang, & Insel, 1996) and were as likely to showsocial contact as nonmutant control mice. However, the OTKO mice did fail toshow selective social behaviors, as measured in a task for social recognition(Ferguson et al., 2000). A similar pattern of social behavior is also seen in micewith a null mutation in the gene for the vasopressin (V1a) receptor (AVP V1aKOmouse; Bielsky, Hu, Szegda, Westphal, & Young, 2004). These data are consistentwith the hypothesis that nonselective social behavior can be achieved throughthe activation of either OT or AVP receptor systems in either sex, whereas selectivesocial behavior requires access to both OT and AVP receptors (Cho et al.,1999).Sociality is highly interwoven with behavioral homeostasis, and many of OTand AVP’s actions on social behavior may be affected by their interactions withhormones that have been associated with stress or other forms of challenge. Forexample, OT or related hormones may suppress hypothalamic-pituitary-adrenal(HPA) axis activity (Altemus, Deuster, Galliven, Carter, & Gold, 1995; Amico,Johnston, & Vagnucci, 1994; Carter et al., 1997; Heinrichs, Baumgartner, Kirschbaum,& Ehlert, 2003; Legros, 2001; Thoman, Conner, & Levine, 1970). In addition,treatment with exogenous OT is associated with reductions in anxiety and asense of safety and calm (Carter and Keverne, 2002; Uvnas-Moberg, 1998). Aswith other components of the HPA axis, this is not a simple story, because veryrecent studies suggest that chronic stress, such as that associated with social isolation,is capable of increasing the synthesis of OT (Grippo et al., 2006).The effects of AVP also are commonly associated with periods of stress andarousal. However, in contrast to OT, which probably serves to down-regulate stress,AVP may potentiate the secretion of adrenocorticotrophin-releasing-hormone(ACTH; Levine, 2001), especially during chronic stress. AVP is associated withvigilance and defensive aggression, including mate-guarding in male prairie voles(Ferris, 2000; Ferris & Delville, 1994; Stribley & Carter, 1999; Winslow et al.,1993).A particularly interesting feature of OT and AVP synthesis is the fact that theseneuropeptides and at least some of their effects are sexually dimorphic. Exposureto stress also has sexually dimorphic effects, especially in the vole model. Forexample, activation of the HPA axis affects social behavior differently in maleand female voles, with males forming a pair bond more quickly under stressfulconditions, and females forming a heterosexual pair bond less quickly whenstressed (DeVries, DeVries, Taymans, & Carter, 1996). Support for related but