Conventional wisdom presumes that the larger the brain, the more intelligent the animal must be. Though it seems logical that a creature with more grey and white matter crammed inside its head would have greater mental acumen than an animal with lesser anatomy, the evidence surrounding this claim is mixed.

These days, many scientists believe that it’s the underlying organization of the brain that matters most when it comes to smarts. But according to a new study published in the journal PLoS ONE, bigger noggins may have trickled through evolution’s passageway not because of reason and intellect, but due to exercise.

Endurance exercise has been shown to boost the levels of chemical messengers that promote growth and reorganization in the brain. It’s therefore possible that animals with an increased penchant for endurance exercise may have beefed-up brains. Though a definite cause-and-effect relationship has not been proven, researchers have shown that within a species, the animals with the bigger brains are the ones that run longer than the rest.

“We became interested in whether there was some evolutionary connection between exercise and neurobiology across a wider range of animals than people had been able to examine in the lab,” David Raichlen, an anthropologist from the University of Arizona, told Wired.com.

To that end, he and Adam Gordon, an assistant professor of anthropology at the University at Albany, decided to dig through published scientific papers to see how brain size was linked to athletic prowess across the animal kingdom, from tiny mice to massive cattle.

Brain size is easy enough to get at. Those types of measurements are made frequently enough in scientific studies, but to get a solid measure of endurance ability, Raichlen and Gordon had to decide on a usable metric. They settled on the maximal metabolic rate (MMR), or VO2 max as it’s often called, which is a measurement of an animal’s upper limit for aerobic exercise and a proxy for its level of physical fitness.

Analyzing data from 29 different animal species, Raichlen and Gordon found exactly what they had expected: As the size of a species’ brain increased, so did its MMRs. One explanation could be that bigger animals simply have bulkier brains. With more muscle mass, the larger animals may be able to run farther than their more diminutive counterparts.

But even after adjusting the data to account for the effect of body size, Raichlen and Gordon found that a larger brain size independently predicted how well the animal would rate on the athletic endurance scale.

While their data seemed to hold true for many mammals, it seemed to break down once humans were thrown into the mix. “It’s hard to take our data and apply it to humans,” Raichlen said. “Our sample [didn't] include any other close relatives of ours — animals from the primate group.”

In other words, comparing humans to other non-primate species may be skipping too many evolutionary steps, so Raichlen said his team are changing their strategy for the future, to see whether these evolutionary connections are still at work within humans.

“Our paper makes some suggestions about how this might work on an evolutionary time scale,” Raichlen said, “but I think there’s still a ton of work to do to figure out the mechanisms.”

Citation: Raichlen DA, Gordon AD, 2011 Relationship between Exercise Capacity and Brain Size in Mammals. PLoS ONE 6(6): e20601.doi:10.1371/journal.pone.0020601

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