Measuring Physiological Stress in Mammals: How Metabolism Influences Fecal Glucocorticoid Levels

ABSTRACT The concept of physiological stress is widely used but poorly understood. In mammals, physiological stress is often assessed noninvasively based on the concentration of glucocorticoid hormone (i.e., cortisol, corticosterone) metabolites in feces (i.e., fecal glucocorticoids—FGCs). Yet, it remains unclear if FGC levels are measuring stress and how such levels vary across species. Previous work has shown that much of the variation in plasma cortisol levels in mammals is explained by differences in species' metabolic rates (i.e., rates of energy use), indicating that differences in plasma cortisol levels may simply be reflecting differences in metabolic rate rather than differences in physiological stress levels. Here we examine whether variation in FGCs is similarly explained by differences in species' metabolic rates among mammals. As predicted, data from 77 mammal species show that FGCs increase in proportion to mass‐specific metabolic rate (MSMR) and similarly scale to the −1/4 power of body mass. Most variation in baseline FGC levels across species was explained by differences in MSMR or body mass. Furthermore, after exposure to stressors, elevated FGC levels increased in proportion to baseline levels. Taken together, these results suggest that increases in FGC levels in response to a perceived threat or perturbation are largely measuring changes in rates of energy use (i.e., metabolic rate).