Levels of male aggressiveness in fluctuating populations of Microtus ochrogaster and M. pennsylvanicus

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Abstract
Levels of male aggressiveness, determined by observation of dyadic encounters, were compared during low, increase, peak, and decline phases of the population cycle in free-living populations of Microtus ochrogaster and M. pennsylvanicus. There were no significant differences in the proportions of nonaggressive, aggressive, and highly aggressive males during the low, increase, and peak phases of M. ochrogaster populations in an alfalfa and in a bluegrass habitat. Large numbers of nonaggressive voles were present during the decline phases. The aggressiveness of individual males appeared greater at peak densities only in the bluegrass field. Phase of cycle explained 18.6% of the variation in behavioral variables for the bluegrass area population and 5.8% for the alfalfa area population. The proportions of aggressive types for M. pennsylvanicus populations in bluegrass and prairie habitats did not vary significantly with phase of the population cycle and there was no indication of an increased level of individual aggressiveness at peak densities. Phase of cycle explained 25.2% of the variation in behavioral data for the bluegrass area population and 12.2% for the prairie population. Seasonal heterogeneity was found in the behavior of M. ochrogaster and season accounted for more of the behavioral variation in each population than had phase of cycle. Although seasonal trends were not entirely consistent, the data suggested that males may be more aggressive in winter and autumn. For M. pennsylvanicus the proportions of nonaggressive males were lower during spring and autumn (which correspond to the breeding seasons) than during summer and winter. The results of the present study were not consistent with the polymorphic behavior hypothesis.