1Department of Anthropology, University of New Mexico, 2Department of Human Evolutionary Biology, Harvard University
Friday 163, Plaza Level
Creatinine, which is frequently used to standardized urinary hormone levels, is a breakdown product of creatine phosphate, an amino acid concentrated in muscle tissue. Because of its derivation in muscle, the amount of creatinine that an individual excretes in 24 hours is a reliable and frequently-used indicator of relative muscle mass in humans and other lab animals. While it is not feasible to collect 24-hour urine samples from wild primates, we apply here an easily accessible method to approximate muscle mass variation from collections of spot urine samples. Specific gravity (SG), an alternative method for assessing urinary water content, is both highly correlated to creatinine and free of mass-dependent effects. Individuals with greater lean body mass should excrete more creatinine for a given urine concentration. We examine this relationship a dataset of ~10,000 urine samples from wild chimpanzees in the Kibale National Park, Uganda. As expected from known differences in body composition, the slope of the relationship between SG and creatinine is significantly greater in adult males than adult females. Furthermore, changes in the creatinine-SG relationship closely approximate previously-described weight curves in chimpanzees, stabilizing at approximately 10 years of age in females but peaking several years later in males. Beyond these validating relationships, we demonstrate the applicability of this method for empirical examinations of muscle mass variation, including significant positive correlations with testosterone in males, reductions with reproductive effort in females, and age and status related effects.
This research was supported by the National Science Foundation Grant BCS-0849380, the Leakey Foundation, and by an American Association of Physical Anthropologists Professional Development Award.