Brain Evolution: Environment May Not Be An Important Factor In Size

Humans, specifically Homo sapiens, have entrenched themselves at the top of the food chain, something most people take for granted and ascribe to the powers of the human brain. But for all that, scientists don’t really understand why our brains evolved to be as large and powerful as they are.

A popular theory, which has been around for a long time, is called the cognitive buffer hypothesis, and it says large brains evolved to deal with fast or unexpected changes to the environment, thereby helping with survival. In other words, a variable environment would lead to larger brains among species that live there. But it is difficult to verify this theory using humans, since us modern humans are the only survivors of the Homo genus, and we have nothing to compare with.

On the other hand, birds — the only extant dinosaurs — have been around since long before the Homo genus itself came into being. And given how diverse and plentiful they are the world over, and with a wide range of brain sizes, they are also great candidates for testing the cognitive buffer hypothesis.

Carlos Botero, an assistant professor at Washington University in St. Louis, had done precisely that in an earlier study which seemed to support the hypothesis. In that study, Botero found mockingbirds who lived in variable habitats had more complex songs, which could be linked to learning ability and hence brain size. But his study showed no link between more elaborate birdsong and survival, leading him to conduct a new study.

Published Monday in the journal Nature Ecology and Evolution, the study — conducted by Botero, a postgraduate associate at the university Trevor Fristoe, and biologist Andrew Iwaniuk of University of Lethbridge, Canada — found large brains were not more likely to evolve in variable habitats, compared to stable habitats. But it did show that birds with larger brains could successfully inhabit a broader range of environments.

“The findings were pretty surprising. 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,” Fristoe said in a statement Monday.

Brain size itself cannot be taken in absolute terms as a measure of intelligence, of course. An ostrich has a far bigger brain than a raven’s but the latter outsmarts the former. So the researchers looked at the difference between birds’ actual brain size and the size predicted on the basis of overall body size.

“The correlation between relative brain size and cognitive ability is better for birds than for mammals. 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,” Botero said in the statement.

For the study, the researchers used data on birds from all over North America, collected as part of the Breeding Bird Survey. The data for environmental conditions was taken from NASA Earth Observations and ecoClimate, an open-access database of climate simulations.

“We found that big brains are equally likely to evolve in places that had variable conditions and places that had stable conditions. We don't see any difference between the two. But we found that variable environments are more likely to be colonized by species that already had big brains. That explains why, when we go out today, we find an association between big brains and variable environments,” Botero explained.