Sperm Whale Reproductive Strategies: Current Knowledge and Future Directions

Abstract

Sperm whales’ reproductive strategies are centered around their extreme sexual dimorphism, both in morphology and behavior. Females are much smaller than males and are highly social. Females live in stable, matrilineally based social units with communal care of calves, including cooperative defense and allonursing. In contrast, male sperm whales are large nearly solitary nomads. Males disperse from their natal social unit and move toward the poles, where they eat and grow almost three times larger than females. Males’ great ranges span across and between ocean basins, allowing global genetic connectivity. As they rove the warm waters where females concentrate, mature males avoid each other; physical aggression on the breeding grounds is rarely observed. Instead, males may rely on powerful acoustic displays to establish dominance over potential competitors and provide females with an honest quality signal. Associations between sexually mature males and groups of females tend to be transitory. Disproportionate mating success of some males is suggested by evidence of paternal relatedness within female social units. Sperm whale mothers provide a substantial investment of time and energy to calves, resulting in the slowest reproductive rate among cetaceans. The peculiar characteristics of sperm whale mating systems reflect the evolutionary interplay between habitat structure, predation risk, sociality, and reproduction. A convergence of reproductive biology between sperm whales and African elephants likely results from similarities in these ecological pressures. Despite sperm whales being one of the most studied cetaceans, much remains unknown about their reproductive strategies. Most of what we know comes from whaling data and long-term observational and modeling studies. The rapid advances in technology for behavioral and physiological studies at sea can refine our understanding of these elusive deep-diving animals’ social, mating, and caring systems and the extent to which these vary across oceans.