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AB/Gat mice and congenic mice bred for high aggressiveness (CS/ag) were tested for exploratory behavior in novel situations and anxiety-related behavior, using an open-field test and the elevated plus-maze test. Subsequently, the size of hippocampal mossy fiber terminal fields was evaluated. Considerably higher exploratory activity was found in nonaggressive mice, whereas aggressive mice exhibited more anxiety-related behavior. Larger intra- and infrapyramidal mossy fiber terminal fields (IIP-MF) and a larger hilus were found in the highly aggressive strain. Within the nonaggressive AB/Gat strain, larger IIP-MF were correlated with higher exploratory behavior and lower anxiety in the plus-maze test. Within the aggressive strain, no individual correlations between hippocampal morphometry and behavior were found. The results corroborate the "ecotype hypothesis," which suggests that mice of subpopulations with highly aggressive males tend to display reduced exploratory behavior. The findings support the view that genetic factors involved in aggressive behavior also affect hippocampal connectivity. However, our results do not support the hypothesis that a higher level of aggressiveness is necessarily related to smaller IIP-MF.

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