Posted by Dave on August 26, 2011 | 1 Comment
How do you measure a non-human animal’s VO2 max? Put it on a treadmill, just like you would with a human. What do you do with that information? If you’re David Raichlen and Adam Gordon, you use it to figure out if there’s any relationship between an animal’s ability as an “endurance athlete” and its brain size.
Perhaps a better question is why you would even imagine there was a relationship.
In fact, there is some reason to believe that brains might be linked to distance-running ability. One of the arguments put forth in Christopher McDougall’s book Born to Run is that humans have superior running ability because they used it to hunt other animals, simply by chasing them until they became exhausted. Humans are the smartest animals with the largest brains (relative to body size), so it’s possible that human intelligence and human running ability are related.
There’s a somewhat less far-fetched rationale for other animals: Aerobic exercise actually releases compounds that stimulate brain cell growth. So perhaps animals that are better at aerobic exercise naturally grow bigger brains. So Raichlen and Gordon found as much data as possible about as many different animals as possible—both VO2 max data and brain size data. In the end, they managed to find data for 29 different mammals including humans. So, is there any correlation between brain size and aerobic fitness? The graph below shows the results:
“Residual MMR” is a measure of VO2 max adjusted for the animal’s size. Similarly, “residual brain mass” adjusts the brain mass for the size of the animal. So smaller-brained, less-aerobically-fit animals should be down and to the left on the graph, while larger-brained, more-aerobically fit animals should be up and to the right. It’s not a dramatic effect, but the points are indeed mostly aligned long that diagonal from the lower-left to the upper right. Aerobic fitness is associated with about 20 percent of the difference in brain size.
What messes this effect up is adding humans to the sample: Because their brains are so dramatically bigger and their VO2 max isn’t significantly different from the average, when humans are included, the effect goes away. Removing domesticated animals also reduces the strength of the effect, but the authors believe this is only because there aren’t enough non-domesticated animals to have a significant effect.
This brings up one more question: If humans aren’t actually more aerobically fit than the average mammal, why would they have engaged in persistence hunting like McDougall describes in his book? The answer is that they don’t need to be more fit than all animals, just the ones they are hunting!