Rival teams of male dolphins form the animal world’s biggest social networks, long-running study finds

Anthropologists have long celebrated and puzzled over humans’ ability to cooperate. Our special talent lies in forming nested cooperative networks that involve unrelated individuals: family, community, city, state, nation, and allied nations. Not even our closest relative, the chimpanzee, does this. But over the past 4 decades, researchers have shown that another animal does: the sea-going Indo-Pacific bottlenose dolphin (Tursiops aduncus) of Shark Bay in Western Australia.

Unrelated male dolphins deploy their social smarts to build complex alliances that boost their chances of reproductive success. A new study concludes these are the largest such complex cooperative societies outside of humans. Moreover, they appear to have evolved in a different way from our own. “It’s an exciting finding that helps bridge the immense, perceived gap between humans and other animals,” says Mauricio Cantor, a behavioral ecologist at Oregon State University who was not involved in the study.

In an exploration of dolphin society launched in 1982, behavioral ecologist Richard Connor, now affiliated with Florida International University, and his team have been following more than 200 male dolphins in the exceptionally clear waters of Shark Bay, recording which males spend the most time together. Over the years, they have found that males form close relationships with one or two other males, and that these partnerships are nested inside a larger alliance, which in turn are nested inside yet another alliance—rather like being a member of “a platoon, a company, and a regiment,” notes Harvard University primatologist Richard Wrangham, who is not part of the team. The male dolphins cooperate in order to capture and defend fertile female dolphins from other groups of males. A lone male cannot corral a female; he needs partners.

In the new study, the team analyzed data collected between 2001 and 2006 on 121 individual males, revealing a super-connected social network with every male connected to one another either directly or indirectly. The males even cultivate relationships with males outside of their three-level alliances, forming the biggest network known in any nonhuman species, and thereby increasing their reproductive success, the researchers report today in the Proceedings of the National Academy of Sciences. Each male had on average 22 allies; some had as many as 50.

Male dolphins form bonds by swimming and diving side-by-side, petting, holding flippers, engaging in sex, whistling to each other when apart, forming “teams,” and coming to one another’s aid should rivals attempt to spirit away a female. Those with the strongest social bonds spend the most time with females, thus increasing their chances of reproducing. “They’re making strategic social decisions,” says Connor, who suspects dolphins use their big brains in part to remember which individuals came to their aid and which ones fled during fights.

Cooperation isn’t exactly rare in the animal kingdom—animals from social insects to lions, wolves and spotted hyenas, and many primates cooperate; some, such as chimpanzees and bonobos, even do so with nonrelatives. (And unrelated female bonobos have recently been reported to form coalitions with outsiders against males). But none of these species form “multilevel alliances to accomplish goals,” says Athena Aktipis, a cooperation theorist at Arizona State University. “It’s interesting and cool that the dolphins do.”

Wrangham adds that Connor’s decadeslong study constitutes some of the most compelling support for the “social brain hypothesis,” the idea that the need to keep track of numerous social relationships drove the evolution of large brains and intelligence. The dolphins provide “a dramatic demonstration of the positive correlation between brain size and social complexity,” he says.

Anthropologists have argued that human intergroup cooperation is unique and tied to the evolution of bonds between males and females and the role of males in taking care of offspring. These long-lasting pair bonds lead to extended social networks because both partners have relatives interested in ensuring the survival of their genes. But in dolphins, as in chimpanzees, males and females don’t form lasting pairs and males don’t help with parenting. “Our results show that intergroup alliances can emerge without these behaviors, and from a social and mating system that is more chimpanzee-like,” Connor says.

In other words, there’s more than one way for these highly complex alliances to evolve, says Frans de Waal, an emeritus primatologist at Emory University. “It’s good to ponder that there may be multiple evolutionary paths to this outcome.”

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