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Male dolphins form alliances to access females

Social interactions between animals of the same species are often characterized by cooperation within groups and conflict or competition between groups. However, in certain circumstances, cooperation can arise between different social groups, such as is commonly apparent between groups of humans. Cooperation between groups is sometimes driven by the need for protection against common threats, such as predators, competitors or environmental hardships, but it can also be beneficial in securing access to scarce resources, such as females. 

For a long time it was thought that intergroup cooperation was a behavior unique to humans as it has not been recorded among other social animals. However, a new study on male bottlenose dolphins in Australia has shown that they too benefit from maintaining multiple-level alliance networks between groups of associated individuals. 

The research team, led by scientists from the University of Bristol, analyzed association and consortship data to model the structure of alliances between 121 adult male Indo-Pacific bottlenose dolphins (Tursiops aduncus) in Shark Bay in Western Australia. The findings have been published in The Proceedings of the National Academy of Sciences (PNAS).

“Cooperation 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,” explained study co-lead author Dr. Stephanie King, an associate professor in Bristol’s School of Biological Sciences.

The Bristol University scientists, along with colleagues from the University of Zurich and University of Massachusetts, found that male dolphins in Shark Bay form first-order alliances of two–three males and that these pairs or trios of unrelated males cooperate to herd individual females. Multiple first-order alliances (numbering 4-14 unrelated males) cooperate in teams to pursue and defend females, representing a second-order of alliances. In addition, multiple teams also work together, forming third-order alliances out of cooperating second-order alliances. 

These findings indicate that dolphins make use of a complex, hierarchical social network system with cooperation between non-related individuals. Indeed, the researchers say that these dolphins exhibit the largest known multi-level alliance network of any non-human animal.  

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

“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.

Intergroup cooperation in humans was thought to be unique and dependent upon two other features that distinguish humans from our common ancestor with chimpanzees – the evolution of pair bonds and parental care by males. “However, our results show that intergroup alliances can emerge without these features, from a social and mating system that is more chimpanzee-like,” noted Professor Richard Connor of the University of Massachusetts, who co-led the study with Dr. King.

The publication of the importance of intergroup alliances in dolphins in 2022 holds special significance as the team celebrates 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, 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,” explained study co-author Dr. Michael Krützen, head of the Anthropology Institute at the University of Zurich.

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

By Alison Bosman, Staff Writer

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