Dolphins form largest alliance network outside humans, study finds



Male bottlenose dolphins have been found by an international team of researchers lead by scientists at the University of Bristol to create the largest known multi-level alliance network outside of humans. These cross-group alliances make it easier for men to get a scarce resource.

In order to reconstruct the structure of alliances between 121 adult male Indo-Pacific bottlenose dolphins at Shark Bay in Western Australia, the researchers, along with collaborators from the Universities of Zurich and Massachusetts, examined association and consortship data. Today, The Proceedings of the National Academy of Sciences published their findings (PNAS).

In Shark Bay, male dolphins group together in first-order alliances of two to three males to cooperate pursue mating relationships with specific females. For access to female dolphins, second-order coalitions of four to fourteen unrelated males fight with one another, while third-order alliances form amongst collaborating second-order alliances.

Dr. Stephanie King, co-lead author and Associate Professor at the School of Biological Sciences at the University of Bristol, said: "Cooperation amongst allies is common in human societies and one of the defining characteristics of our success. It was originally believed that only our species was capable of developing strategic, cooperative ties at different social levels, such as trade or military alliances on a global scale.

We've demonstrated that male bottlenose dolphins form the largest multilevel alliance network outside of humans, and we've also demonstrated that cooperative relationships between groups, rather than just alliance size, allow males to spend more time with females, increasing their chances of having successful offspring.

"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," said Dr. Simon Allen, Senior Lecturer at Bristol's School of Biological Sciences and an author of the study.

The emergence of pair bonds and male parental care were considered to be two further characteristics that set humans apart from our common ancestor with chimpanzees and were necessary for intergroup cooperation in humans. In contrast, intergroup alliances can form from a social and mating system that is more chimpanzee-like, according to the study's co-author Richard Connor, a professor emeritus at the University of Massachusetts who is now associated with Florida International University. Dr. King led the study team.

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

The study's lead author and Director of the Anthropology Institute at the University of Zurich, Professor Dr. Michael Krützen, continued, "Our study demonstrates that significant insights about the evolution of traits formerly believed to be exclusively human can be gained by examining other highly social, large-brained taxa."

According to Dr. King's research, dolphin communities and nonhuman monkey societies serve as useful model systems for studying the social and cognitive evolution of humans.

University of Bristol

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