The growth and maintenance of the brain require substantial investments of energy, most especially for organisms which have evolved very large and complex brains. One of the most defining characteristics for the human species and the other primates is large brain size relative to body size. Yet, despite having larger brains than most other mammals, human and nonhuman primates do not show an increase in their basal metabolic rate (a measure of energy utilization by the body) compared to other mammals, raising the question of how the high energetic cost of such large brains is met. This trend suggests that there is an energetic trade-off with another energy-demanding tissue in the body when brain size increases; if we are not using more energy overall, then energy that could be invested in another part of our body is instead likely being utilized to fuel our large brains. Preliminary research shows that primates have low muscle mass when compared to other animals, and humans, who have the most notable increase in brain size, show a 50% reduction in overall muscle mass when compared with other mammals. This research therefore tests the hypothesis that skeletal muscle is in direct competition with the brain for glucose and oxygen, such that the high energetic demands of large brain size are met through constraining muscle mass, constituting an energetic tradeoff between skeletal muscle growth and maintenance, and brain growth and maintenance.
If the brain does constrain muscle mass, then 1)larger brains should be associated with decreased skeletal muscle mass; 2)the percentage of type I muscle fibers (a type of muscle cell that uses energy [glucose, a type of sugar] in a similar fashion to brain cells) should show a relative decrease in relation to larger brain size; and 3)muscle mass development should be suppressed until brain growth is complete, and once complete, there should be an increase in muscle mass development. To test these predictions, muscle tissue samples will be collected from a diverse array of primate specimens, comprising a range of brain sizes and representing all developmental stages. The generated muscle energy use profiles for each species will then be analyzed in relation to variation in brain size, with the results applied to understanding the interaction between brain size and evolved metabolic strategies.
Reducing muscle mass may have predisposed primates such as humans to certain metabolic disorders (e.g., type 2 diabetes); thus, understanding if there is such a constraint has important health implications. Ultimately, the data collected can be incorporated into studies of growth and development, as well as biomechanics, and the results may encourage development of biomedical gene therapies. The research also will provide a rich database for scientists in other disciplines focusing on animal anatomy and physiology, facilitating and expanding future research. The collaborative project brings together international researchers, and will support the training of multiple undergraduate and graduate students from three US universities. As two of these universities are in EPSCoR states, and one is a historically minority-serving institution, the project will foster research advancement for underserved and underrepresented populations.
