Researchers have found that male bottlenose dolphins form the largest known multi-level alliance network outside humans. These co-operative relationships between groups increase male access to a contested resource.

The international team of researchers from the University of Bristol (lead, UK), the University of Zurich (Switzerland) and the University of Massachusetts (USA) analysed association and consortship data to model the structure of alliances between 121 adult male Indo-Pacific bottlenose dolphins at Shark Bay in Western Australia.

Male dolphins in Shark Bay form first-order alliances of two to three males to co-operatively pursue consortships with individual females. Second-order alliances of four to 14 unrelated males compete with other alliances over access to female dolphins and third-order alliances occur between co-operating second-order alliances.

Co-lead author Dr Stephanie King, Associate Professor from Bristol’s School of Biological Sciences explained: “Co-operation between allies is widespread in human societies and one of the hallmarks of our success. Our capacity to build strategic, cooperative relationships at multiple social levels, such as trade or military alliances both nationally and internationally, was once thought unique to our species.

“Not only have we shown that male bottlenose dolphins form the largest known multi-level alliance network outside humans, but that co-operative relationships between groups, rather than simply alliance size, allows males to spend more time with females, thereby increasing their reproductive success.”

Dr Simon Allen, Senior Lecturer at Bristol’s School of Biological Sciences, who contributed to the study, said: “We show that the duration over which these teams of male dolphins consort females is dependent upon being well-connected with third-order allies, that is, social ties between alliances leads to long-term benefits for these males.”

Professor Dr Michael Krützen, an author on the study and Head of the Anthropology Institute at the University of Zurich, added; “It is rare for non-primate research to be conducted from an anthropology department, but our study shows that important insights about the evolution of characteristics previously thought to be uniquely human can be gained by examining other highly social, large-brained taxa”.

Dr King concluded: “Our work highlights that dolphin societies, as well as those of non-human primates, are valuable model systems for understanding human social and cognitive evolution.”

The findings have been published in The Proceedings of the National Academy of Sciences (PNAS).

Image: Four male allies and a female, copyright Simon Allen.