I went to an interesting talk today, one of Indiana University’s themester activities. Steven Gaulin of UC Santa Barbara spoke about work he has done with William Lassek on the relationship between human brain size and sex differences in fat distribution. It’s a fascinating story. We women are fattier overall than men, and we tend to carry our fat in different places. One thing the sexes have in common, however, is that fat is vital to the brain, the dry weight of which is mostly fat. Could the differences and the similarity be linked?
Gaulin presented several lines of evidence suggesting that fat deposited on women’s hips and thighs provides the material needed to build the unusually big brains (as primate brains go) of their offspring. He also suggested that a link between lower-body fat and cognitive ability in one’s offspring might have driven male preference and thus sexual selection for a low waist-hip ratio (WHR, i.e., a smaller waist and larger hips).
Among the things he discussed is something called maternal depletion, evident in hunter-gatherers but also more subtly observable in American women, in which the amount of hip/thigh fat decreases as women bear more children. Another factor is a link between WHR in women and cognitive ability (the data they used showed a link between lower WHR—i.e., more lower-body fat—and higher cognitive abilities in their offspring). Menarche (the onset of menstruation in young girls) appears to be related not to the amount of body fat but to its distribution (specifically, a greater amount of it on the hips).
Other lines of evidence include the fact that hip/thigh fat is very hard to get rid of; the body seriously taps into it only during the last trimester of pregnancy and during lactation. Also, hip/thigh fat and abdominal fat (the kind men are more prone to carry around) have opposite effects on the body’s supply of two long-chain polyunsaturated fatty acids that are crucial to brain development, docosahexaenoic acid (DHA) and arachidonic acid (AA). As best I could understand it—I’m not a biochemist, so I’m speaking fairly broadly here—the former promotes and the latter hinders the synthesis of these two fats.
I can’t possibly do justice to the entire talk here. In a nutshell, the shape of women’s bodies could be intimately connected, in interesting ways, with the need to nurture the development in babies of these unusually large brains we have. You can check out Gaulin’s web page for more information, including links to papers about the maternal depletion and menarche research.
