Common dolphins form unexpected strong social bonds: insights into social plasticity of delphinids

When a social species inhabits disparate environments with different requirements, it presents an ideal study framework for investigating plasticity in social structure. Common dolphins (Delphinus delphis) are wide-ranging offshore delphinids that generally form societies with fission-fusion dynamics within large schools and exhibit weak social bonds. In Port Phillip, southeastern Australia, common dolphins of the same species are, against expectations, resident to an embayment. Residency in this species provides a unique opportunity to investigate whether their social structure resembles that of their offshore conspecifics with weak social bonds, or whether bay living leads to stronger social bonds. We investigated the social structure of 12 resident adult common dolphins, between 2007 and 2014, in Port Phillip. Network analyses revealed nonrandom associations and several strong bonds, a social structure unusual for this species. The study shows that the social structure of a wide-ranging gregarious species in Port Phillip reflects the requirements of a confined environment with limited but predictable resources. Their social structure in the bay resembles that of inshore delphinids, rather than of its own species. Our study highlights the extreme plasticity in social structure that common dolphins are capable of and the importance of the environment for social bonds.