The Annual Cycle of Aggression in Male Microtus Pennsylvanicus, and Its Relation to Population Parameters

Intraspecific aggression of male Microtus pennsylvanicus drummondii was ob- served in 320 10-minute paired encounters in a neutral arena, during 17 months. Voles were drawn from a population live-trapped biweekly at Pinawa, Manitoba. The 17 months were divided into the following reproductive periods: non-breeding, testes scrotal, and littering. There were two non-breeding, two testes scrotal periods, and a single littering period. Threats, vocalizing, and "mutual uprights" were significantly and positively associated in all but one of the periods. Together with fighting, these three acts formed the aggressive component of the behavior recorded. All aggressive acts increased in frequency as males became reproduc- tively active, and decreased as the breeding season ended. Latencies to aggressive acts were negatively related to the acts' frequencies. Less frequent acts (fighting, "grooming other," and "submissives") also showed seasonal changes associated with breeding: fighting was com- mon throughout the breeding period, submissives most common early in the reproductive period, and grooming other most prevalent when juveniles were present in the population. Incidences of tail-wounding in the field population were significantly more common during the breeding season, and in males. A comprehensive Index of Aggression was devised by means of principal component analysis, based on the total frequencies of aggressive acts. Mean Indices of Aggression for the five periods showed that aggressive levels in the three breeding periods were significantly higher than in the two non-breeding periods. Re-tests of 53 voles showed scorn-. ii)m their two encounters to be positively correlated. Aggression mea,,:10 i n the encounters was related to population parameters derived from live-trapping results. Breeding season home ranges were larger than winter home ranges. Overwintered adults were more aggressive, had larger home ranges and survived longer than young of the year. Aggression and survival were positively related in adults but not in young. Resident voles were more aggressive than non-residents. Dominant voles were usually slightly heavier. Pairs of voles of similar weight exhibited more aggressive acts, but "following" was more frequent in dissimilar weight pairs. Aggressive levels did not vary consistently with population density. These observations demonstrate there is an annual cycle of intraspecific aggression syn- chronized with the breeding season in male M. pennsylvanicus. They support studies on Peromyscus, which also showed that aggression increased with spring breeding and declined as reproduction ceased in fall; thus, behavioral density regulation seems most likely to occur during the breeding period.