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Maternal Programming of Defensive Responses 161functional modification of the genome that does not involve a change of DNAsequence.What is perhaps most surprising is that these developmental effects derive fromvariations in parental care that appear to lie within a normal range for the species.However, as Hinde (1986) suggested, this is likely due to the fact that naturalselection shaped offspring to respond to subtle variations in parental care as aforecast of the environmental conditions they would ultimately face followingindependence from the parent. Evolution should come to favor offspring that areable to accurately “read” variations in parental behavior as forecasts of environmentalconditions, and thus as useful signposts for developmental trajectories. Bydefinition, such responses should occur in reaction to variation within the normalrange: why evolve responses to forms of parental care that are unlikely to occur?Moreover, parents (or parent in some cases) are a logical source of such informationsince they are the one “environmentally-informed,” constant experience ofthe offspring. Moreover, since parents are genetically invested in the developmentof their biological offspring, one would expect that the fidelity of signals emanatingfrom a parent would be greater than that of adult conspecifics. Thus, to theextent that parent and offspring share a common interest in the adaptive value ofsuch phenotypic plasticity, selection may also act on the signaling of the parent.Indeed, phenotypic plasticity in response to parental signals may also be thoughtof as a parental strategy (Wells, 2003). In either case, the sensitivity of the offspringto parental signals during critical phases of development may be thoughtof as a strategy that favors a highly predictable relation between environmentalconditions, parental input and phenotypic variation in defensive responses. Hence,parents matter.Potential Mediating MechanismsMaternal care alters the expression of genes in brain regions that subserve emotional,cognitive, and endocrine responses to stress. These effects are associatedwith tissue-specific alterations in gene expression that are sustained into adulthood.Existing evidence suggests a direct effect of maternal care, at least withrespect to modifications of glucocorticoid receptor and GABA A receptor subunitexpression. Studies on HPA function and performance in the Morris water mazesuggest a direct link between the changes in gene expression and function. Neverthelessthere are substantial gaps in the story. First, although cross-fostering studiessuggest a direct link between variations in mother-pup interactions, the evidencefor a causal role of pup licking/grooming or arched-back nursing remains circumstantial.More recent studies with the artificially reared pups raised under conditionsthat systematically vary the level of tactile stimulation represent a usefulexperimental approach to this problem (e.g., Lovic & Fleming, 2004). Second,