Dolphins form decade-long social bonds, and cooperate among and between cliques, to help one another find mates and fight off competitors, new research has found – behaviour not previously confirmed among animals.
“These dolphins have long-term stable alliances, and they have intergroup alliances. Alliances of alliances of alliances, really,” said Dr Richard Connor, a behavioural ecologist at the University of Massachusetts Dartmouth and one of the lead authors of the paper. “But before our study, it had been thought that cooperative alliances between groups were unique to humans.”
The findings, published on Monday in the journal Proceedings of the National Academy of Sciences, appear to support the “social brain” hypothesis: that mammals’ brains evolved to be larger in size 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 not a coincidence,” Connor said.
Connor’s team of researchers collected data between 2001 and 2006 by conducting intensive boat-based surveys in Shark Bay, Western Australia. The researchers tracked the dolphins by watching and listening to them, using their unique identifying whistles to tell them apart.
They observed 202 Indo-Pacific bottlenose dolphins (Tursiops aduncus), including during the peak mating season between September and November.
Back in the lab, they pored over data focusing on 121 of these adult male dolphins to observe patterns in their social networks. And for the next decade they continued to analyse the animals’ alliances.
Dolphins’ social structures are fluid and complex. The researchers found alliances among two or three male dolphins – like best friends. Then the groups expanded to up to 14 members. Together, they helped each other find females to herd and mate with, and they help steal females from other dolphins as well as defend against any “theft” attempts from rivals.
“What happens as a male, you might be in a trio, herding a female. And if someone comes to take that female, the other males in your team and your second-order alliance come in and help you,” said Dr Stephanie King, professor in animal behaviour at Bristol University and one of the authors of the study. “These males have a very, very clear idea of who is in their team.”
Six male bottlenose dolphins with a female. Male dolphin alliances can last for decades. Photograph: Simon Allen
These teams can last for decades and are formed when the dolphins are still young, although they do not tend to reap the rewards of paternity until their mid-teens, King said. “It’s a significant investment that starts when they’re very young – and these relationships can last their entire lives.”
Sometimes, especially when dolphin groups feel there is a risk to themselves, two second-order alliances will also come together to form a larger team. As a result, among the dolphins observed by the scientists, every male was directly connected 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 success they have attracting females.
It’s their cooperative relationships, rather than alliance size, which gives males more breeding success, said King.
It is already widely known that dolphins are highly social and cooperative, as well as being remarkably good at adapting to and teaching behaviour specific to their environment, said Stephanie Venn-Watson, former director of Translational Medicine and Research at the National Marine Mammal Foundation in San Diego, California, who was not involved in the study.
“One would not rule out the possibility that other cetaceans could develop similar alliances,” said Venn-Watson. “These complex behaviours will likely be limited to large-brained mammals.”
According to the researchers behind the paper, this is the only non-human example of these kinds of strategic multilevel alliances to have been observed. But these findings also highlight the cognitive demands these animals face, suggesting that dolphins’ large brains help them to keep track of the different relationships, Connor said.
“I would say that dolphins and humans have converged in the evolution of between-group alliances – an incredibly complex social system,” said Connor. “And it’s astonishing because we are so different from dolphins.”