The Causes of Fighting in Mice and Rats

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Abstract
A review of literature bearing on fighting behavior of Mus musculus and Rattus norvegicus. Subject headings include descrs. of behavior patterns; methods; and expts. with naive animals, trained animals, training processes, heredity, and physiol. mechanisms. Two kinds of fighting can arise in naive animals object-centered and opponent-centered fighting, the latter much more severe. Hunger is an effective agent in the 1st, and pain seems involved in starting the 2d. Expts. with trained animals largely involved an established dominance order, with habits of dominance and subordination difficult to modify by altering ecological and physiol. factors. Frustration seems to figure as a cause of aggression only in animals trained to react aggressively after frustration. Expts. with heredity are consistent with theoretical expectations of multiple factors. Sex is a major hereditary factor affecting fighting. Evidently the male hormone operates to lower the threshold of opponent-centered fighting, although fighting may continue after castration in male rodents trained to fight. There is no spontaneous internal cause of opponent-centered fighting, though hunger may arouse attacks in mice trained in object-centered fighting. In general, factors of training affect fighting behavior in a major way, with genetic factors, and particularly the male hormone outstanding under certain situations.