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.