Abstract
Evolutionary explanations for mammalian sociality typically center on inclusive-fitness benefits of associating and cooperating with close kin, or close maternal kin as in some whale societies, including killer and sperm whales. Their matrilineal structure has strongly influenced the thinking about social structure in less well-studied cetaceans, including beluga whales. In a cross-sectional study of group structure and kinship we found that belugas formed a limited number of distinct group types, consistently observed across populations and habitats. Certain behaviours were associated with group type, but group membership was often dynamic. MtDNA-microsatellite profiling combined with relatedness and network analysis revealed, contrary to predictions, that most social groupings were not predominantly organized around close maternal relatives. They comprised both kin and non-kin, many group members were paternal rather than maternal relatives, and unrelated adult males often traveled together. The evolutionary mechanisms that shape beluga societies are likely complex; fitness benefits may be achieved through reciprocity, mutualism and kin selection. At the largest scales these societies are communities comprising all ages and both sexes where multiple social learning pathways involving kin and non-kin can foster the emergence of cultures. We explore the implications of these findings for species management and the evolution of menopause.
Highlights
Evolutionary explanations for mammalian sociality typically center on inclusive-fitness benefits of associating and cooperating with close kin, or close maternal kin as in some whale societies, including killer and sperm whales
Seven distinct group types were identified in beluga whales, two of which fell under the definition of a herd (i.e., > 50 animals) and five that were defined as social groups (i.e., ≤ 50 animals; Table 1)
Beluga whales formed a variety of group types that were consistently observed in multiple populations across the species range and certain behaviours were associated with grouping type (Table 3)
Summary
Evolutionary explanations for mammalian sociality typically center on inclusive-fitness benefits of associating and cooperating with close kin, or close maternal kin as in some whale societies, including killer and sperm whales. Their matrilineal structure has strongly influenced the thinking about social structure in less well-studied cetaceans, including beluga whales. Individuals can derive benefits from associations with unrelated individuals where cooperation is conditional on the behaviour of the companion (i.e., r eciprocity2,3), always yields the highest benefit (i.e., mutualism4,5) or results in shared fitness advantages from helping increase group size (i.e., group augmentation6,7) which, for example, could lead to more effective group defense. Bottlenose dolphin groups are not strictly matrilineal and the extent of kinship within groups varies dramatically[32,33,34]
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