Posted by: Adam Kay | December 3, 2011

I am not a fathead

I am not a fathead

I remember an episode one summer day when I was a teenager. I was daydreaming while I was riding my bike and I rode through a stop sign into an intersection. There was a car at the stop sign on the cross street and a tough-looking guy was watching me while leaning out his open window. He had started pulling out into the intersection and I almost hit him. As I clumsily tried to stop, the only thing he said was “fathead”. And then he drove off.

Well, I do have a big head. But all humans do. Brain mass:body mass ratio is higher in humans than in most other mammals (although it’s certainly not highest in humans – see comment below) ; it’s about 3 times larger than is the ratio for our closest living relative, chimpanzees. So, instead of “fathead”, “encephalized” is a more appropriate description of my appearance.

Why are we humans “encephalized”? Brains are costly to maintain, so animals with larger brains must either have higher overall metabolic costs or have reduced maintenance for other metabolically active tissue. This latter idea has been called the “expensive tissue hypothesis”. It proposes that mammals decrease investment in expensive tissues (like the gut) to fund relatively large brains. This is a classic example of a tradeoff – using more resources for cologne means less money for dinner on your date.

In the most recent issue of the journal Nature, Navarette and colleagues tested the expensive tissue hypothesis using data from 100 mammal species. They found no evidence that mammals with larger brains had relatively smaller guts or that any other major organ was smaller. So much for the expensive tissue hypothesis.

Navarette and colleagues did find evidence for one tradeoff: mammals with larger brains have less body fat. Fat stores aren’t expensive to maintain, but carrying them around might make it harder to avoid getting eaten – luckily for us there aren’t tigers roaming the mall. The authors suggested this fat-brain tradeoff reflected alternative starvation resistance strategies: fat stores offer a “physiological buffer” against starvation, and big brains offer a “cognitive buffer”. In other words, you don’t need to pack on the fat if you’re smart enough to figure out how to get food when times are tough. This tradeoff doesn’t seem to exist in primates. For example, we big-brained humans also carry a lot of fat (14-26%) compared to chimpanzees (3-10% body mass). Navarette and colleagues suggest our efficient form of locomotion – bipedalism – may allow us to be both fat and “encephalized”. Go humans.

So, I’m not “fatheaded”. I’m encephalized and this provides me with a cognitive buffer against starvation resistance. I wish I could have said that before that guy drove off.



  1. Most of us feel that human brain mass:body mass ratio must be the highest (after all, we are the most intelligent animals, right?). In humans, this ratio is about 2%. But in some mammals, the ratio approaches a whopping 10%. How can this be?

    Comparative neurobiologists have tried many methods to account for this discrepancy (and to keep humans on top). One way is by using an “encephalization factor,” or the difference between actual brain size and expected brain size, given a species’ average body weight. Here, humans fare well, having a brain approximately four times larger than would be expected for our body mass. However, this calculation changes based on what animals are included in the analysis of “expected” brain size. If you compare all mammals, human brain size is well beyond what is expected. But if you include just primates in the equation, the encephalization factor shrinks.

    That the relationship between brain size and body mass (and whatever we mean by “intelligence”) is complicated perhaps should not be surprising. I would argue that each species’ brain has evolved under multiple pressures to suit the requirements for the particular environment that species inhabits, and that no single pressure (metabolic, cognitive, energy storage or other) pushes brain development independent of the rest.

    So, what animals have the highest brain mass:body mass ratio? Why? Students in my J-term course, “Human and Comparative Neurology” will find out. Until then, submit your guesses below.

  2. This is an interesting paper and would make a good paper for discussing how to dissect methodology. For example, given how many species they used and how many total individuals they looked at across those species, was there an adequate sample size per species to “average” fat stores when it can vary quite a bit within a species (or individual depending on season). Very interesting – thanks for sharing!

    Kurt, I’ll guess the highest ratio is some kind of small mammal (maybe an insectivore, but I won’t cheat and google it).

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