Kinship study reveals stable non-kin-based associations in a medium-sized delphinid

Abstract

Delphinids display a wide variety of social structures, in which local food availability and defensibility, sexual size dimorphism and interbirth intervals ultimately influence the role of kin within social units. Earlier studies of the social ecology of Risso’s dolphins (Grampus griseus) off Pico Island, the Azores, revealed a sexually stratified social structure, with long-term stable, strongly associated male clusters and temporally weakly associated female clusters. Here we test the predictions that inclusive fitness plays a role in social cohesion and structure and that both sexes are philopatric in this population. We found no correlation between association and relatedness for either males or females. Our results therefore do not support inclusive fitness as an explanation for the stable clusters of males, who instead associate with partners of a similar age, less likely to be kin due to a long inter-birth interval. Genetic data did not reveal clear sex-biased dispersal. We propose that unlike the pattern seen in some other dolphin species, the socio-genetic structure found in Risso’s dolphins is not associated with inclusive fitness but linked instead to the open oceanic habitat and the species’ life history traits.

Significance statement

Studying societies of wild cetaceans poses additional challenges compared to terrestrial mammals, since we can generally only observe behavior when individuals come (close) to the surface for breathing. Yet such studies can expand our knowledge on the links between ecology and social structure (e.g. the remarkable parallels between societies of sperm whales and elephants). This study makes a meaningful contribution, by establishing that the long-term stable male and temporally stable female associations found in earlier studies of Risso’s dolphins in the Azores are not based on kinship. Accordingly, despite very different ecological contexts, there are striking similarities between the male Risso’s dolphin clusters and the second-order alliances found in male Indo-Pacific bottlenose dolphins in Shark Bay, Australia. This offers great potential to enhance our understanding of drivers of male cooperation by further comparative research on two long-term studied systems.