Genotype Versus Experience Effects On Aggression in Wild and Domestic Norway Rats

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Abstract
The agonistic behavior of field-trapped and first-generation laboratory-reared wild male Norway rats was compared in a resident-intruder context in order to examine the hypothesis that aggressive behavior is significantly reduced by laboratory rearing. In addition isolate and group-reared wild and domestic rats were compared in order to determine the effect of early social deprivation on levels of aggression. Experimental subjects were housed in test chambers for at least one month prior to their exposure to anosmic hybrid (wild X domestic) intruders. Agonistic behaviors of both residents and intruders were recorded in two 30-minute trials. Field-trapped males were significantly more aggressive than any of the laboratory-reared groups. The behavior of isolate-reared subjects of both stocks did not differ significantly from their group-reared counterparts. Three hypotheses were advanced to account for the greater aggressiveness of the wild-caught rats. The evidence favors an interpretation based on the greater opportunity for social conditioning in the field environment and the blocking of habitual behavior patterns (i.e. escape) in captivity.