Aggressive behavior between humpback whales (<i>Megaptera novaeangliae</i>) wintering in Hawaiian waters

Abstract

Humpback whales (Megaptera novaeangliae) wintering in Hawaiian waters engage in strenuous aggression toward con-specifics. The social context and sex of individuals involved suggest that aggression is the result of male–male competition for sexually mature females, including cows with newborn calves. Characteristic behaviors associated with aggression occur in a roughly hierarchical scaling of intensity and include broadside displays, underwater exhalations, head lunges (in which the throat is inflated and enlarged), physical displacement, and charge–strikes. Humpback whales do not form stable pair bonds during the winter breeding season; females are seen serially and simultaneously with multiple males and males are seen serially with multiple females. Repeated observations of individually identified whales indicate that escorting and singing are interchangeable reproductive roles of mature males. Incidents of aggression show a seasonal increase and decrease that parallel changes in abundance and average pod size. A seasonal peak in the frequency of aggression is probably related to an increase in population density and to changes in the reproductive physiology of mature males and females. It is suggested that singing may function, in part, to synchronize ovulation in females with the peak abundance of mature males on the wintering grounds.