Why do bigger mammals have bigger brains?

[changizi]

I've spent some time studying why the brain changes as it gets bigger, from mouse to whale. But one big mystery no one in neuroscience has an answer to is why bigger animals have bigger brains in the first place. Brain mass tends to increase as the 3/4 power of body mass. And this seems to be unrelated to behavioral complexity; that is, a "mere" increase in body mass seems to be enough to lead to the brain mass increase. Why all this extra brain for no extra smarts? That's the enigma.

[iNow]

I've spent some time studying why the brain changes as it gets bigger, from mouse to whale. But one big mystery no one in neuroscience has an answer to is why bigger animals have bigger brains in the first place. Brain mass tends to increase as the 3/4 power of body mass. And this seems to be unrelated to behavioral complexity; that is, a "mere" increase in body mass seems to be enough to lead to the brain mass increase. Why all this extra brain for no extra smarts? That's the enigma.

You should read this study:

http://www.sas.upenn.edu/~ptschoen/papers/schoenemann.BBE.04.pdf

 

In sum: It seems metabolic rate is most likely.

 

Also, a few of your initial assumptions are wrong. For instance, body weight does not predict brain weight in mammals as well as you suggest, and there is a good degree of variation. Also, you suggest that it is body mass increase which causes brain mass increase, when it could very well be the other way around. This is discussed extensively in the link I shared.

 

 

Finally, your closing suggestion is also wrong. Larger brains generally do mean larger intelligence (ala, "extra smarts"):

 

http://www.sciencedaily.com/releases/2007/05/070518172103.htm

When it comes to estimating the intelligence of various animal species, it may be as simple measuring overall brain size. In fact, making corrections for a species' body size may be a mistake. The findings were reported by researchers at Grand Valley State University and the Anthropological Institute and Museum at the University of Zürich, Switzerland. "It's long been known that species with larger body sizes generally have larger brains," said Robert Deaner, assistant professor of psychology at Grand Valley and the first author on the study. "Scientists have generally assumed that this pattern occurs because larger animals require larger nervous systems to coordinate their larger bodies. But our results suggest a simpler reason: larger species are typically smarter."

Edited June 9, 2009 by iNow

[changizi]

Body mass correlates very well with brain mass. Sure, there is variation. But why the strong correlation?

 

The Deaner paper concerns primates only. Deaner would not argue that there is not a strong body-mass trend across mammals generally.

 

And the Schoenemann paper doesn't help the quandary. It concludes that, in a sense, brains actually increase even *more* quickly with body mass, once one rids of the fat out of the mass. The question would be, why do larger (non-fat) body mass animals have brains that scale up nearly as fast as the (non-fat) body mass?

 

To make the enigma more concrete, a cow might have a brain of 450g, whereas a cat has a brain of 30g and a beagle-sized dog a brain mass of 70g. By any measure, cows are not "smarter" than cats and dogs. And certainly not ten times smarter!

 

What is a cow *doing* with a brain that big? (This is also a nice problem, because it goes up against our modern-day intuition of thinking about the brain as a computer. Computers that are bigger can usually do more stuff. But not necessarily so for brains...)

[Mokele]

Bigger bodies mean more surface area for sensation, both internally and externally (the GI tract has a LOT of nerves). Also, larger bodies have more muscle cells, thus more motor units, and thus need more motor control.

[changizi]

Right. That's the best "going theory". But it leaves one unsatisfied, because if it were true, one *might* expect that only the somatosensory and motor cortex should get larger in larger bodies. ...or at least expect that those two regions should disproportionately enlarge in larger bodies. But everything gets bigger, and somato-motor regions don't disproportionately enlarge. Even visual cortex keeps enlarging, which seems weird: the visual "surface area" around us would appear to not depend on body size.

[Mokele]

Even visual cortex keeps enlarging, which seems weird: the visual "surface area" around us would appear to not depend on body size.

 

No, but rod and cone cells remain the same size. Bigger eyes = more cells. The same is true for noses - bigger nose = more surface area and more receptors, therefore more input.

 

Bigger bodies simply have more cells, which means more stuff to control and more input to process.

[changizi]

No, but rod and cone cells remain the same size. Bigger eyes = more cells. The same is true for noses - bigger nose = more surface area and more receptors, therefore more input.

 

Bigger bodies simply have more cells, which means more stuff to control and more input to process.

 

Maybe. But eye mass scales up very slowly with body mass, and so retinal surface area even more slowly. One wouldn't expect a 3/4 power on this basis. For noses, I don't know how to think about its scaling relationship.

 

Generally, these kinds of control and sensing arguments must have some validity, but it would *seem* one would then expect the sensorimotor parts of the brain to disproportionately increase with body mass, whereas the frontal parts of the brain need not. Instead, one finds the entire brain increasing in size.

 

My guess is that these sensorimotor issues do end up, for some complicated reasons no one has yet quite pinned down, "forcing" the entire brain to get larger as it does. Some kind of computational principle might demand it. But we're largely in the dark as the nature of such a principle.

[Mokele]

What we need is more limited, intensive scaling studies across groups with a wide range of body sizes but similar life history traits. Felines would be a good choice (excluding lions and cheetahs due to odd lifestyle), as would monitor lizards, ranid frogs, charadriform birds (gulls & their allies), and Cervids (deer etc.). With these species, body size changes, but little else does, and the limited range of species within a group gives you time to do more measurements per animal, thus a larger data set.

 

Because I'm VERY biased, I think pythons would be an excellent choice. Not only does a tiny brain control a huge body with hundreds of serially repeating units, but the number of repeats varies tremendously within the genus (from less than 200 to over 450). With a big enough sample size and enough variation, you might even be able to pull a correlation from a large sample of a single species (since the vertebral number varies between individuals quite substantially).

 

I'm not aware of anyone doing such studies, but they could test the whole "Bigger body = more control = more brain" hypothesis very well.