The Relation of Social Position and Wounding to Exploratory Behavior and Organ Weights in House Mice

Abstract

ABSTRATr: Two experiments suggested a relation between social status, organ variation and general activity. In the first experiment, 80 male PBR/St mice (Mus musculus) were observed for 3 minutes of activity in an open-field (arena), then paired and observed for dominant-subordinate formation, and 2 weeks later observed again for arena activity. The mice were sacrificed and the spleen, adrenals, testes, seminal vesicles and submaxillary glands were removed and weighed. Social position was difficult to determine and fighting and wounding were slight. Neither pre-pairing nor post-pairing arena activity showed a relation with social position, nor did organ weights vary significantly. In a second experiment conducted in a similar manner but with random-bred mice, social hierarchies were clearly evident. Subordinate animals showed significantly more body wounds and greater arena activity 2 weeks after hierarchy formation and also had significantly heavier spleens. Several measures suggest a strong genetic influence on activity. It was concluded that low status animals with body wounds are physiologically aroused and likely to venture into novel areas. This effect might in part account for emigration in populations and also influence the constitution of genotypes at different levels of density. The importance of social status in the regulation of population behavior and survival is not clearly understood. Examples have been cited to indicate that subordinate animals are more susceptible to physiological stress (Thiessen, 1964), but it is not clear if behavioral differences as well play a part in the regulation of population events. Recent evidence obtained with random-bred house mice (Mus musculus) suggests that fighting and wounding may lead to behavioral changes that have implications for emigration (Thiessen, 1966). When population size among males caged in the laboratory was varied from one through 16 it was noted that changes in behavior and organ morphology were closely related to fighting and wounding. In grouped conditions a high number of body wounds was related to an increase in open-field (arena) activity, splenic hypertrophy and decreases in the weights of the submaxillary and seminal vesicle glands. Correlations indicated significant relations between the number of body wounds and both open-field activity and spleen weight. Moreover, those animals judged to be subordinate had significantly more body wounds and were significantly higher in activity than animals judged to be dominant. Adrenal weights and histologies did not indicate that these changes were due to the hypersecretion of adrenocorticoids. These data are compatible with the notion that wounded and subordinate animals are more likely to show organ variation and venture from the focus of the population into novel areas. Differential responses of this sort based on social status suggest implications for emigration, population survival and the natural selection of aggressive animals. Two experiments with paired mice give some support to these possibilities. The first was conducted with a strain