Dolphins form complex social alliances


Dolphins in Shark Bay, Australia. © Simon Allen

So far, complex social relationships have been considered typically human. A long-term study now shows that dolphins also live in a complex social structure: unrelated males form long-term groups and cooperate both within their group and with other groups. In this way, they increase their reproductive success - and form the largest social networks in the animal world. The findings also open up new perspectives on the social evolution of humans.

If male dolphins want to win over a female alone, they don't have a good chance. They are more successful if they follow and circle the female together with some male comrades. As previous observational studies have shown, close bonds between two to three cooperating males often persist while mating partners change. Pairs or trios of male dolphins swim together, flip-flops on one another, have sex with one another, whistle in turn when separated, and come to one another's aid in combat situations.

Alliances on multiple levels

But even beyond these so-called first-order alliances, male dolphins maintain complex social relationships. A team led by Richard Connor from the University of Massachusetts in Dartmouth in the USA has now shown that male dolphins form a complex network of alliances and in this way increase their reproductive success. The team has been monitoring bottlenose dolphins in Shark Bay, Western Australia, since 1982. For the current study, the researchers analyzed data from 121 males that they observed between 2001 and 2006. "Our analysis reveals the largest known non-human alliance network with highly differentiated relationships between individuals," the researchers write.

As early as the 1990s, the team had shown that the pairs or trios of adult males form so-called second-order alliances, which typically consist of four to 14 unrelated individuals. These animals support each other when it comes to courting mates or fighting other competing alliances. The current study now shows that these second-order alliances not only compete with each other, but can also cooperate - and in this way form third-order alliances.

With allies to more reproductive success

"Cooperation between allies is widespread in human societies and one of the hallmarks of our success," says co-author Stephanie King of the University of Bristol. "Our ability to form strategic, cooperative relationships at multiple social levels, such as trade or military alliances at the national and international levels, was previously thought to be unique to our species." The current research indicates that male bottlenose dolphins make up the largest known network of multi-tiered alliances in the animal kingdom. On average, each male had 22 allies, some had as many as 50.

The observations revealed that the alliances enable the males involved to increase their reproductive success. "We show that the length of time these teams of male dolphins associate with females depends on how well connected they are with third-order allies," explains King's colleague Simon Allen. "This means that social ties between the alliances lead to long-term advantages for these males."

New look at the social evolution of man

Until now, cross-group cooperation was considered uniquely human. Anthropologists hypothesized that this form of cooperation evolved on the basis of two other traits that distinguish us from other primates like chimpanzees: pair bonding between males and females, and parental care by the males. "However, our results show that alliances between groups can also arise without these traits, based on a social and mating system more similar to that of chimpanzees," said Connor.

Co-author Michael Krützen from the Institute of Anthropology at the University of Zurich explains: "It is rare that non-primate-related research is conducted by an anthropology department, but our study shows that important insights into the evolution of traits previously considered considered uniquely human can be gleaned from the study of other highly social, large-brained species". From the authors' point of view, the work shows that dolphin societies can also be a valuable model system for understanding human social and cognitive evolution.

Source: Richard Connor (University of Massachusetts Dartmouth) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2121723119

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