Behaviour 2019
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Frienemies: The sharing of friends and avoidances
Alexis L Levengood1, Kasha Strickland1, Janet Mann2, Celine H Frere1. 1University of the Sunshine Coast, Sippy Downs, QLD, Australia; 2Georgetown University, Washington, DC, United States

Structural balance theory (where the friend of my friend is my friend), originally designed to understand human social evolution, has been empirically supported in both humans and other animals. Though this theory implies the social transmission of relationships, the biological significance of relationship sharing across human and non-human animals is extremely sparse. By examining the social lives of 160 dolphins over 30 years, we found evidence for structural balance, where dolphins not only share friends but also avoidances. Sharing also varies among individuals and is distinctly different from the social network measure transitivity. We further reveal that these shared relationships are socially transmitted and that sharing is established through three distinct pathways: direct maternal, indirect maternal, and non-maternal social transmission. Finally, we show that relationship sharing has an effect on the structure of the social network, where shared friends bridge sub-communities, while shared avoidances drive sub-community division. We conclude that relationship sharing though previously overlooked is fundamental to our understanding of how animal societies have evolved.