Why are humans so smart? We must have evolved in an environment that made more intelligent individuals likely to survive. But this just raises new questions: what factors in the environment could have created an evolutionary pressure for intelligence? And how have other species that faced similar pressures ended up evolving?

One prominent hypothesis about our brains is that the human lineage became especially social, which required a suite of advanced cognitive skills to manage the relationships that were intertwined with survival. There’s evidence that social behavior is associated with bigger brains across the primate family, and the same correlation has been found in birds.

There's now evidence that whale and dolphin brains show the same relationship. The finding offers new support for the “social brain hypothesis," and it's an exciting discovery. But not all researchers agree on how this evidence should be interpreted.

Hyper-social whales

Neuroscientist Kieran Fox and his colleagues searched hundreds of scientific papers on cetaceans, the group that includes whales and dolphins. They gathered information on brain size in 46 different species from this group. And because bigger bodies mean bigger brains without anything else going on, they also included information on body size so that they could factor that into their analyses.

The team then collected information on the social behavior of those species. That included the average social group size—ranging from species that tend to be solitary to those that live in giant “mega-pods.” But it also considered the incredible array of complex social behaviors that whales and dolphins are famous for: group hunting, play, social learning, and complex calls.

They found that, as in primates and birds, cetaceans that had more complex social and cultural behaviors had larger brains. “The apparent coevolution of brains, social structure, and behavioral richness of marine mammals provides a unique and striking parallel to the large brains and hyper-sociality of humans and other primates,” the authors write.

Figuring out a causal story

“It’s an interesting study that is consistent with the idea that social behaviour and brain size are related,” says Robert Barton, who studies brain evolution. “But a lot more work needs to be done on a variety of factors that may be involved.”

Until recently, says Barton, he was one of the proponents of the social brain hypothesis. “I’ve become less of an advocate in recent years,” he said. Part of his reason for this was the finding that the relationship between brain and sociality in primates didn’t look so clear-cut in bigger sample sizes with more controls.

Another relevant finding is that brain size is also related to ecological factors, like the variety of habitats a species can live in and the variety of foods it eats. Fox and his colleagues actually found these relationships in their sample of cetaceans, too: larger brain sizes were associated with a variety of food types, as well as a larger range of habitats.

Different researchers have varying opinions on how to interpret all these correlations. Fox’s co-author, Susanne Shultz, points to the fact that larger brains need more calories. She argues that social pressures were the driving force pushing toward the evolution of bigger brains, but getting enough calories was a barrier to be overcome on the way, forcing species to adapt their eating habits accordingly. The data on cetaceans suggests that social behavior is the best explanation for their large brains, she and her co-authors argue.

Barton is more inclined to think that social behavior is just one of many factors pushing toward bigger brains. “I’d be surprised if sociality had nothing to do with brain evolution,” he says. “But we just don’t know what the direction of causality is.”

It could be the other way around, he says: greater intelligence may have allowed for the emergence of cooperative behavior, which in turn proved useful for survival. Alternatively, “it could be other things driving brain size and group size together.” And, Barton adds, brain size is not as simple as it sounds. Different parts of the brain grow at different rates, so two species with large brains might have very different things going on under the surface.

But the ongoing debate is about the interpretation of the finding, rather than the finding itself. The new evidence “supports the idea that the richness of a species’ social world is predicted by their brain size,” writes researcher Susanne Shultz—a claim that Barton agrees with. “The fact that we’ve found it in an independent group so different from primates makes it all the more important,” she writes.

Nature Ecology & Evolution, 2017. DOI: 10.1038/s41559-017-0336-y (About DOIs).