Selection, evolution of behavior and animal models in behavioral neuroscience

We investigated whether genetic differences in various forms of intraspecific aggression and anxiety in four different genetic lines of mice (i.e. wild, outbred Swiss-CD1, inbred DBA/2 and inbred C57/BL6N) may reflect modifications in behavioral strategy. Experiments 1 and 2 used ethologically based paradigms to analyze aggressive and anxiety responses both in social (i.e. aggression) and non-social (i.e. novel environment exploration) contexts. In Experiment 3, an anxiolytic drug (chlordiazepoxide (CDP)) was used to examine possible differences in proximal mechanisms underlying anxiety-related behaviors. The data show that intrasexual aggression, infanticide and maternal aggressions are related and covarying. Genetic lines with the highest levels of intermale attack (i.e. Wild and Swiss-CD1) also have highest levels of infanticide, interfemale attack and maternal aggression but, interestingly, the lowest levels of anxiety. In fact, exploratory behavior is lower and risk assessment behavior markedly higher in DBA/2 and C57/BL6N mice (i.e. the less aggressive strains) compared to Swiss and Wild genetic lines. Although reproductive status influences anxiety levels in female mice, our findings show that (contrary to previous studies) lactating mice are more anxious than virgin females in terms of risk assessment activities. These data demonstrate the importance of studying behavior in a more ecologically-relevant context which emphasizes the function of behavior in a specific situation. Moreover, differential strain sensitivity to the behavioral effects of CDP suggests that genetic lines of mice may differ in the underlying mechanisms mediating behavior. It is therefore possible that artificial selection of different genotypes has resulted in differences in proximate mechanisms modulating the levels of aggression and anxiety, thereby leading to modification of social behavior. Overall, the results presented here suggest that subtle genetic alterations in specific underlying neural mechanisms are likely to cause profound effects on behavioral responses and their adaptive significance. Implications for behavioral neuroscience research that seeks to understand both the proximal and ultimate mechanisms of behavior are discussed.