People often complain to their friends when others don’t “get” something they are trying to say “they can’t get it through their thick skulls”. Words like “boneheaded” and “numbskull” are things we all recognize. But it might surprise you to realize that our skulls are, on average…very thin. At least compared to our ancestors. In fact, we have in general skeletons that are less robust and skulls that are thinner. And this seems like a kind of odd adaptation. Shouldn’t it be GOOD to have a thick skull?
Our ancestors had much thicker skulls, and a more robust skeleton, than we do now. But if it’s usually considered a good thing to have a thick skull, what has made our skull so thin? What determines skull thickness? Is it a genetic difference…or is it a matter of how much you use it?
Copes et al. “The effects of manipulating the frequency and magnitude of mastication on system skeletal robusticity in mice” Institute of Human Origins, Arizona State University, Tempe, AZ, presented at Experimental Biology, 2011.
Previous studies with other parts of the skeleton have shown that, while we might have a less robust skeleton than our ancestors, the parts we USE can become more robust. For example, studies of tennis players (Shaw and Stock, 2009) have shown that the arm used for serving has higher robusticity than the opposite arm. Bone robusticity is not the same as bone density in this case. While bone density refers to the composition of the bone (whether it is relatively uniform as in healthy bone, or pitted as in pathological conditions like osteoporosis), the robusticity refers instead to the structure, the relatively thickness of the bone walls and the size of the cavity in the middle containing the marrow.
While studies of tennis players have shown that use of an arm can increase the robusticity of a bone, DISUSE can decrease robusticity. In astronauts, for example, the leg bones will have a decrease in robusticity following space flight (Miyamoto, 1998). But the interesting thing is…this doesn’t happen in the SKULL. There are no decreases in skull robusticity following space flight.
So skull robusticity doesn’t appear to vary all that much. But what determines it? Is it a genetic thing? Or is it due to use? As the author of the study pointed out to me…you don’t walk on your skull. So how do you give a skull a workout? You CHEW. Copes et al took a bunch of mice, and put them on different diets. The idea behind this is that the use of the jaw muscles in chewing (mastication) might be able to increase skull thickness, and show whether or not skull thickness is mammals is due to use, or to genetics determinants. So one group of mice had the normal mouse chow diet. The second group had a diet of super soft chow powder made up into a consistency like cookie dough (though it probably didn’t taste as good). But how to make the chow harder? Unfortunately normal rodent chow is pretty hard, to allow the rodents to chew often and wear down their teeth. So Copes et al decided to get a group that chewed MORE, and put the mice in a room that was much colder than they were used to. In order to keep warm, the mice had to burn more energy, and thus were required to eat more chow, resulting in significantly more chewing than they did in other environments. In contrast, the cookie dough group chewed relatively little.
Copes kept the mice on this diet for 3 months, from the time they were weaned when young until their skeletons had fully matured. She then examined the the jaw muscles and skull thickness of the mice.
Copes found that the mice who had the soft diet had weaker jaw muscles (masseters) than those eating normal chow or chewing more in the cold, but it wasn’t by much, and the skull (cranial vault) thickness did not significantly vary in any of the conditions. While this may seem like negative data, this actually suggests that, rather than the activity varying skull thickness, the thickness of our skulls may be genetically determined. Copes hopes to eventually address this question by looking at the skulls of various modern and ancient human groups. By looking at the thickness of adult skulls compared to those of children, she hopes to determine whether skull thickness is genetically determined, and if so, when, and why, our skulls got so thin.
References:
Shaw, C., & Stock, J. (2009). Habitual throwing and swimming correspond with upper limb diaphyseal strength and shape in modern human athletes American Journal of Physical Anthropology, 140 (1), 160-172 DOI: 10.1002/ajpa.21063
MIYAMOTO, A. (1998). Medical Baseline Data Collection on Bone and Muscle Change with Space Flight Bone, 22 (5), 79-82 DOI: 10.1016/S8756-3282(98)00020-9
PS: I got to meet with Copes for a little while to talk about her research. In truly awesome science geek fashion, she showed me her little stuffed OSTEOBLAST (that’s a cell responsible for bone formation). She was kind enough to send me a picture, and here, you shall behold…BLASTY.
