Dolphins have largest alliance network outside of humans

Male bottlenose dolphins have the largest known multi-level alliance network outside of humans, according to experts. These cooperative group ties boost male access to a contested resource.

The researchers studied association and consortship data from 121 adult male Indo-Pacific bottlenose dolphins at Shark Bay in Western Australia, along with colleagues from the University of Zurich and the University of Massachusetts. Their findings were published in the Proceedings of the National Academy of Sciences today (PNAS).

Male dolphins in Shark Bay form first-order alliances of two to three males to pursue consortships with individual females jointly. Second-order coalitions of four to fourteen unrelated males compete with other alliances for female dolphin access, whereas third-order alliances form amongst collaborating second-order alliances.

"Cooperation between friends is prevalent in human cultures and one of the characteristics of our prosperity," said co-lead author Dr Stephanie King, Associate Professor at Bristol's School of Biological Sciences. Our ability to form strategic, cooperative ties at numerous social levels, such as national and international trade or military alliances, was once thought to be unique to our species.

"Not only have we demonstrated that male bottlenose dolphins form the largest known multilevel alliance network outside of humans," the researchers write, "but cooperative relationships between groups, rather than simply alliance size, allow males to spend more time with females, thereby increasing their reproductive success."

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

Intergroup cooperation in humans was assumed to be unique and dependent on two additional characteristics that distinguish humans from our common ancestor with chimps: the evolution of pair bonds and male parental care.

“However, our findings show that intergroup alliances can emerge without these characteristics, from a more chimp-like social and mating system,” said Richard Connor, professor emeritus at the University of Massachusetts and now affiliated with Florida International University, who co-led the study with Dr King.

The importance of third level or intergroup alliances in dolphins will be published in 2022, marking the 40th anniversary of the start of Shark Bay dolphin research in 1982 and the 30th anniversary of the publication in 1992 of their discovery of two levels of male alliance formation, also published in The Proceedings of the National Academy of Sciences.

"It is rare for non-primate research to be conducted from an anthropology department," said Professor Michael Krutzen, an author on the study and Head of the Anthropology Institute at the University of Zurich, "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."

"Our work demonstrates that dolphin societies, like those of nonhuman primates, are excellent model systems for understanding human social and cognitive evolution," King said.