Evidence for seasonal variation in aggressive behaviour by Macaca mulatta

Observations were made on the Macaca mulatta colony on the island of Cayo Santiago over a period that included two reproductive cycles. The frequency of aggressive behaviour was found to fluctuate cyclically as a function of the colony's reproductive state. Males were wounded and died significantly more frequently during the mating season than at any other time of year. Females were wounded about as often during the mating season as during the birth season, and more died during the birth season than during the mating season, although not significantly so. There was no significant difference between sexes in age at death, nor was there a significant departure from expectation of number of deaths per age class. However, mortality among the reproductively mature males was greater than among mature females. It was suggested that differential injury and mortality rates among adult males might be an indirect result of high levels of androgens which could conceivably alter threshold levels for stimuli initiating severe aggressive interactions.