Behavioural studies in different strains of mice and the problem of heterosis

Significant differences between strains were found for open field activity and emotionality, exploratory activity, initial reaction to shock, conditioned avoidance learning, and weight for three inbred strains (Balb/c, C57Bl and C3H), such that the heaviest strain (Balb/c) was the least active and most emotional in the open field, gave the lowest score for exploratory activity and longest reaction to shock times, and was the poorest at conditioned avoidance learning.The hybrids from crosses with C57Bl tended more towards complete dominance and overdominance than did the Balb/c × C3H hybrid, probably because the Balb/c and C3H strains have some common ancestry.Heterosis was most marked for traits involving exploration or learning. Variability of the hybrids for these traits was often lower than the parents, i.e. they showed behavioural homeostasis. For other traits variability of the hybrids tended towards the parent whose genotype was dominant for the trait.Of the non-genetic factors studied, age and litter size were found to have the greatest effect on the behavioural traits. Parity was found only to affect weight. Sex differences were found for weight and emotionality.