Social organization in northern bottlenose whales, Hyperoodon ampullatus: Not driven by deep-water foraging?

It is postulated that deep-water foraging in sperm whales, Physeter macrocephalus, has led to communal care of young and long-term female bonds. By studying the social organization of a second, unrelated, deep-diving species, the northern bottlenose whale, we investigated the role that deep diving may play in the evolution of cetacean sociality. Northern bottlenose whales in a deep-water canyon, the Gully off Nova Scotia, Canada, form small groups (X̄ ±SD=3.04 ± 1.86). Associations within age/sex classes (female/immature, subadult male and mature male) were significantly higher than associations between different classes. Females and immature bottlenose whales formed a loose network of associations, showing no preferential associations with particular individuals or those from specific age/sex classes nor any long-term bonds. Mature and subadult males had stronger associations with individuals in their own class, and associations between some males lasted for several years, although males also formed many short-term associations. Overall the social organization of northern bottlenose whales in the Gully appears to resemble that of some bottlenose dolphins, Tursiops truncatus, living in shallow, enclosed bays. Thus deep-water foraging does not appear to necessarily lead to the evolution of long-term bonds between females.