Behavioral heterogeneity in Sprague-Dawley rats

Because of consistent individual differences in animal behavior, it is often difficult to interpret experimental data. We therefore attempted to rank Sprague Dawley rats by a hierarchical ascendant clustering technique using data from avoidance conditioning and open-field testing (scores of exploratory activity before, during, and after an unpredictable noise stress, and calculated stress-related inhibition). This procedure allowed partition of the animal population into distinctive groups and subgroups, extremes being represented by low avoidance-low emotional and high avoidance-high emotional animals. These results are in discrepancy with others showing an opposite pattern of emotional responses in rat lines selectively bred for low or high avoidance. Considering the psychophysiological variability of the animal population in experimental designs may thus be more informative than classical statistical methods, particularly in the evaluation of drug effects or environmental adaptation.