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Neuropeptides and Social Behavior Development 185early manipulations were deliberately reduced, in comparison to normal husbandryprocedures in our laboratory.In our first version of an early experience paradigm in prairie voles, for 21 daysafter birth one group of voles underwent weekly “manipulations” (MAN). Thisinvolved lifting each parent (the father and mother are both present in prairie volefamilies) by the scruff of their necks during the weekly cage changes. Infants wereattached by milk teeth to the mother’s nipples and were usually not directlytouched. Another group was “unmanipulated” (UNMAN), which involved transferringpups and parents in a clear cup rather than touching them. In the immediatepostweaning period, juvenile animals received an alloparental care test asdescribed above. The proportion of adolescent males showing alloparental behavior,over several replications, was approximately 80% of MAN, whereas between6% and 45% of UNMAN males showed spontaneous alloparental behavior. Inaddition, adult UNMAN females failed to form pair bonds after 6 hours of cohabitation;this is 6 times the amount of time needed for a normal female to forma pair bond under our current colony conditions (Bales, Lewis-Reese, & Carter,2003). Subsequent experiments have indicated that the critical period for manipulationoccurs during the first week of life, but also revealed that it is possible tooverdo early handling. Animals picked up on three separate times on the first dayof life, like those left undisturbed, showed a significant reduction in later sociality(Boone et al., 2006). Analysis of levels of OT- and AVP-immunoreactive cellsand levels of receptor binding in the brain as a function of differential amounts ofmanipulation in early life are currently underway. In a separate preliminary studyof the effects of handling on postpartum Day 1 versus Days 1–7, we found indicationsthat picking up animals once a day for 7 consecutive days was associatedwith an increase in the number of OT-immunoreactive cells in the PVN (Carteret al., 2003).The effects seen in these paradigms are particularly interesting because of thesubtle nature of the neonatal manipulation, which does not require active separation,but which nonetheless may create enduring changes in the tendency of individualsto show positive or negative social behaviors. Effects of early differentialmanipulations are presumably mediated, at least in part, by an observed increasein pup-directed behaviors (including sniffing, retrievals, etc.) by the parents inthe MAN group (Tyler, Michel, Bales, & Carter, 2005), similar to those describedin rats (Levine, 2001; Smotherman & Bell, 1980; Meaney, 2001).Possible Epigenetic Mechanisms for Long-LastingChanges in Brain and BehaviorThe effects of early experience may be mediated in part by genetic differences.However, of particular interest are chains of epigenetic events, such as those describeddue to deacetylation/methylation-mediated changes in gene expression(Weaver, Diorio, Seckl, Szyf, & Meaney, 2004). The mechanisms for long-lasting