Dolphins form decade-long social bonds and cooperate among and between cliques, to help each other find mates and fight off competitors, new research shows – a previously unconfirmed behavior in animals.

“These dolphins have stable long-term alliances, and they have intergroup alliances. Alliances of alliances of alliances, really,” said Dr Richard Connor, a behavioral ecologist at the University of Massachusetts at Dartmouth and one of the lead authors of the paper. “But before our study, cooperative alliances between groups were thought to be unique to humans.”

The results, published Monday in the journal Proceedings of the National Academy of Sciences, appear to support the “social brain” hypothesis: that mammalian brains evolved to get bigger for animals that keep track of their social interactions and networks. . Humans and dolphins are the two animals with the largest brains relative to body size. “It’s no coincidence,” Connor said.

Connor’s team of researchers collected data between 2001 and 2006 by conducting intensive boat surveys in Shark Bay, Western Australia. The researchers tracked the dolphins by watching and listening to them, using their unique identification whistles to tell them apart.

They observed 202 Indo-Pacific bottlenose dolphins (Tursiops aduncus), including during the high mating season between September and November.

Back in the lab, they pored over data focusing on 121 of these adult male dolphins to see patterns in their social networks. And for the next decade, they continued to analyze animal alliances.

The social structures of dolphins are fluid and complex. Researchers have found alliances between two or three male dolphins – like best friends. Then the groups grew to 14 members. Together, they helped each other find females to congregate and mate with, and they helped steal females from other dolphins and defend against any attempts at “stealing” by their rivals.

“What happens as a male, you could be in a threesome, breeding a female. And if someone comes to take that female, the other males on your team and your second-rate alliance come in and you help,” said Dr Stephanie King, professor of animal behavior at the University of Bristol and one of the study’s authors. “These men have a very, very clear idea of ​​who is on their team.”

Six male bottlenose dolphins with a female. Alliances with male dolphins can last for decades. Photography: Simon Allen

These teams can last for decades and are formed when the dolphins are still young, although they don’t tend to reap the rewards of parenthood until their teenage years, King said. “It’s a big investment that starts when they’re very young – and those relationships can last a lifetime.”

Sometimes, especially when groups of dolphins feel there is a risk to themselves, two second-rate alliances will also come together to form a larger team. Thus, among the dolphins observed by the scientists, each male was directly linked to between 22 and 50 other dolphins.

The researchers’ observations show that in these groups, the tighter the clique – and the stronger the bonds between the dolphins – the more successful they are in attracting females.

It’s their cooperative relationships, rather than alliance size, that give males more reproductive success, King said.

It is already widely known that dolphins are highly social and cooperative, in addition to being remarkably good at adapting and teaching behavior specific to their environment, said Stephanie Venn-Watsonformer director of translational medicine and research at the National Marine Mammal Foundation in San Diego, Calif., who was not involved in the study.

“One wouldn’t rule out the possibility that other cetaceans could develop similar alliances,” Venn-Watson said. “These complex behaviors will likely be limited to large-brained mammals.”

According to the researchers behind the paper, this is the only non-human example of this type of multilevel strategic alliances to have been observed. But these findings also highlight the cognitive demands these animals face, suggesting that dolphins’ large brains help them keep up with different relationships, Connor said.

“I would say dolphins and humans have converged in the evolution of group alliances — an incredibly complex social system,” Connor said. “And that’s amazing because we’re so different from dolphins.”