The genetics of intersexual aggression in the laboratory mouse

Intersexual aggression was observed during an early experiment concerning the effects of XX male sex-reversal on aggressive and mating behavior. Subjects were from the FVB/NTacfBR strain. Roughly 50% of the subject males were observed to attack females in the mating behavior tests. In the breeding colony, females exhibited a wounding pattern indicative of receiving multiple flank bites. Flank biting is characteristic of male mouse offensive agonistic behavior. A review of the literature on intersexual aggression in mammalian males revealed two common features to male on female intersexual aggression: (1) estrous cycling should modulate aggressive behavior toward females, and (2) there is a positive relationship between the propensity for intermale and intersexual aggression within individuals. The experiments here describe how genotype (FVB/NTacfBR males vs. C57BL/6J males) interacted with these two common features. First, cycling interacted with genotype. FVB males increased the rate of attacks toward estrous females relative to diestrous females. B6 males were not aggressive toward females. Second, prior intermale aggressive experience had only subtle interactive effects with cycling and genotype in increasing intersexually aggressive behavior. A third experiment tested the hypothesis that hippocampal mossy fibers mediate the effects of genetic factors on intermale and intersexual aggression. It is known that there is a negative correlation between the size of the hippocampal intra- and infrapyramidal mossy fiber fields (IIPMF) and intermale aggression. Some evidence also supports such a negative relationship between IIPMF size and intersexual aggression. To test this, early postnatal transient hyperthyroidism was induced by injections of thyroxine into FVB/N pups from days 1–12. This treatment is known to increase the size of IIPMF. Experimental subjects did not differ from saline and no treatment controls on measures of intermale and intersexual aggression. Hypotheses for why this relationship did not exist were examined. Overall, these experiments identified the importance of cycling effects on intersexual aggression in the male mouse. They also enhanced the understanding of the relationship between genotype, intermale and intersexual aggression.