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Office of Neuroscience Research > WUSTL Neuroscience News > Which came first: big brains or demanding environments?

Which came first: big brains or demanding environments?

Testing the idea that environmental challenges drive the evolution of bigger brains

From the WashU Newsroom...

Given how proud we are of our big brains, it’s ironic that we haven’t yet figured out why we have them. One idea, called the cognitive buffer hypothesis, is that the evolution of large brains is driven by the adaptive benefits of being able to mount quick, flexible behavioral responses to frequent or unexpected environmental change.

It is difficult to test this idea on people because there is only one living species in the genus Homo. Birds, according to Carlos Botero, assistant professor of biology in Arts & Sciences at Washington University in St. Louis, are another matter. There are many species, they have a range of brain sizes and they live everywhere. In many ways, they are the ideal group for testing this hypothesis.

As a young scholar, Botero was able to show how mockingbirds that live in variable habitats have more elaborate songs. Since song complexity is a proxy for learning ability, this finding seemed to support the cognitive buffer hypothesis.

But, after a while, he began to think about alternative explanations for his results. The hypothesis requires that big brains improve survival, but Botero’s study didn’t show this. And it didn’t settle a crucial timing issue: Did large brains evolve in variable habitats, or did they evolve elsewhere and then make it easier to colonize harsh environments? However,  the mockingbird study didn’t look back in time.

So together with Trevor Fristoe, postdoctoral associate in biology at Washington University and Canadian biologist Andrew Iwaniuk of the University of Lethbridge, Botero decided to tease out the assumptions behind the cognitive buffer hypothesis and test each of them separately.

Their study, published Sept. 25 in Nature Ecology and Evolution, showed that large brains weren’t more likely to evolve in variable compared to stable habitats, so that part of the hypothesis wasn’t supported. But it also showed that brainier birds were better able to colonize seasonal, unpredictable places. So birds with big brains were able to move into a broader range of environments.

“The findings were pretty surprising,” Fristoe said. “In the first part of the study, we showed that a big brain really does give birds a survival advantage in variable environments. So the mechanism works. But that made it all the more puzzling when the second part of the study showed that big brains often evolved in stable — not in variable — habitats.”

What does size have to do with it?

Botero is the first to acknowledge that brain size is an imperfect measure of cognition, a term that itself has many definitions.

What the scientists looked at was not absolute brain size, but the difference between brain size and the statistically predicted brain size for the bird’s body size. “An ostrich seems to have a huge brain, but relative to its body size, it’s really not that impressive,” Botero said. “A raven is not much larger than a chicken, but its brain is proportionally much more massive.

“The correlation between relative brain size and cognitive ability is better for birds than for mammals,” Botero said. “Although relative brain size is a noisy metric, it’s still one of the better ways we have to measure brain-related differences among species at large taxonomic scales.

“This whole field is fraught with caveats.”

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