The effects of colony differences on muricidal behavior in rats within one strain

Two experiments were performed to investigate the origin of colony differences in muricidal behavior within the same strain of rat. In the first experiment (N = 52 male Long-Evans rats), the first generation offspring of parents from two different colonies were reared under identical conditions and tested for muricide. No differences were found, indicating that an environmental effect accounted for previously observed differences. The second experiment (N = 36 male Long-Evans rats) examined the effects of the presence or absence of mice in the rearing quarters. Again, no differences in muricidal rates were found. Overall, the offspring of killer mothers killed significantly more often than the offspring of nonkiller mothers.