Data from multiple-capture live traps were analyzed to obtain indications of social structure within a meadow vole, Microtus pennsylvanicus, population in southern Wisconsin. The population was estimated to be in the first year of the decline after a peak density. More than 750 instances of multiple captures were obtained from August to December 1967. These captures were analyzed for indications of pair formation, social antagonism between sexes or age classes or both, and general level of aggressive behavior within the population. There was no indication of greater antagonism between adult males than between adult females or between males and females. There was no indication of significant antagonism between adult and immature males. Survival of immature males was as great as, or greater than, that of immature females. These observations are consistent with the theory of little antagonism between males during the declining phase of the population cycle. There were significantly more captures of adult females with immature males than any other age-sex combination; the biological significance of this was not determined. There was no indication of lasting pair formation or of siblings remaining together once they had dispersed from their site of birth. There was no indication of formalized social structure within the meadow vole population; movement and association of individuals within the population appeared random.

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