Multilevel Societies of Female Sperm Whales (Physeter macrocephalus) in the Atlantic and Pacific: Why Are They So Different?

We can examine the evolution of multilevel societies using comparative studies. Intraspecific comparisons are valuable because confounding factors are reduced. Female sperm whales live in multilevel societies. However, studies at several locations have found substantial and consistent differences in social structure between the eastern Pacific and North Atlantic Oceans, even though nuclear DNA shows no significant differentiation between the populations. In the Pacific, female sperm whales live in nearly permanent social units that typically contain about 11 females and immatures of multiple unrelated matrilines. These units form temporary groups with other units for periods of days, apparently exclusively with other units from the same cultural clan. Clans contain thousands of females, are not distinct in nuclear DNA, but are sympatric and have distinctive culturally determined vocalizations and movement patterns. In the North Atlantic social units rarely group with other units, and there is no evidence for sympatric cultural clans. Possible drivers of these contrasts include oceanographic differences, predation, the effects of whaling, and culture. We suggest that protection against predation by killer whales is the primary reason for grouping in the Pacific, and as killer whales do not seem such a threat in the Atlantic, social units there rarely form groups, and have not evolved the clans that primarily function to structure interunit interactions. This analysis highlighted several factors that may influence the evolution of multilevel societies, ranging from the attributes of resources, to predation, anthropogenic effects, culture, and even the cultures of other species.