Abstract
Investigations into cooperative partner choice should consider both potential and realised partners, allowing for the comparison of traits across all those available. Male bottlenose dolphins form persisting multi-level alliances. Second-order alliances of 4–14 males are the core social unit, within which 2–3 males form first-order alliances to sequester females during consortships. We compared social bond strength, relatedness and age similarity of potential and realised partners of individual males in two age periods: (i) adolescence, when second-order alliances are formed from all available associates, and (ii) adulthood, when first-order allies are selected from within second-order alliances. Social bond strength during adolescence predicted second-order alliance membership in adulthood. Moreover, males preferred same-aged or older males as second-order allies. Within second-order alliances, non-mating season social bond strength predicted first-order partner preferences during mating season consortships. Relatedness did not influence partner choice on either alliance level. There is thus a striking resemblance between male dolphins, chimpanzees and humans, where closely bonded non-relatives engage in higher-level, polyadic cooperative acts. To that end, our study extends the scope of taxa in which social bonds rather than kinship explain cooperation, providing the first evidence that such traits might have evolved independently in marine and terrestrial realms.
Highlights
Investigations into cooperative partner choice should consider both potential and realised partners, allowing for the comparison of traits across all those available
Social bond strength during adolescence foreshadowed second-order alliance memberships in adulthood and was of particular importance when future allies were younger than the focal males
species[47], relatedness between males did not affect choices at either alliance level, while age similarity influenced the choice of second-order members only (Table 1)
Summary
Investigations into cooperative partner choice should consider both potential and realised partners, allowing for the comparison of traits across all those available. We compared social bond strength, relatedness and age similarity of potential and realised partners of individual males in two age periods: (i) adolescence, when second-order alliances are formed from all available associates, and (ii) adulthood, when first-order allies are selected from within second-order alliances. Alliance formation in bottlenose dolphins (Tursiops spp.) is intriguing because kin selection appears to explain partner choice in some populations[30,31] but not in o thers[32,33], while the complexity of alliances v ary[34] Cooperation in this species may reveal mechanisms other than kin selection, such as various forms of reciprocity[35,36] or by-product mutualism[37,38]. Allied males associate throughout the year, despite the fact that mating is markedly s easonal[43,44]
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