Karyotype and intermale aggression in wild house mice: Ecology and speciation

Social aggression shown by male mice from two races from upper Valtellina in Italy with 2n=24 and 26 chromosomes, respectively, was contrasted in dyadic encounters. In encounters with mice of the same karyotype 2n=26 were uniformly more aggressive than their 2n=24 counterparts. When resident mice from both these races were tested against intruders of the other karyotype, 2n=26 mice again were more likely to prove aggressive and to eventually dominate their partners even when they were the intruder. Perhaps those behavioral differences account for the apparent replacing of the 2n=24 chromosome race by the 2n=26 variety in geographical areas where they overlap.