The ‘expensive-tissue’ hypothesis of Aiello and Wheeler is well-known in anthropology for positing that an increasingly small gut was a key factor in the evolution of the large hominin brain. The insight that organs and tissues in the body compete for energy resources was also central to the ‘thrifty phenotype’ hypothesis of Hales and Barker, which proposed that nutritional stress in fetal life resulted in differential growth of the brain and pancreas. Both hypotheses are consistent with life history theory, which assumes that energy allocation trade-offs occur in energylimited environments. The prediction that somatic traits trade off against one another in the context of the body’s fixed energy budget has, however, yet to be rigorously tested in humans. The current thesis project aimed to fill this gap by recruiting 70 healthy young women and obtaining comprehensive, high-quality data on their brain and body composition. This included, specifically, measures of brain gray and white matter volume, fat mass, skeletal muscle mass, and volumes of the heart, liver, kidneys and spleen. Additional outcomes included resting energy expenditure and two proxies of early-life growth: birth weight, a marker of fetal weight gain, and tibia length, a marker of linear growth indexing postnatal experience. With these data, three principal hypotheses were tested: 1) there is variation in the energy expenditure of tissues and organs; 2) trade-offs are observed between brain and body organs/tissues; and 3) trade-off relationships are mediated by early-life growth. Results suggest the metabolic cost of organs and tissues is variable, and that the brain – in particular its gray matter component – trades off against lean tissues in the body (i.e. skeletal muscle, the liver and kidneys), but not fat mass. However, less support was found for the prediction that trade-offs are mediated by fetal and infant growth
