Considerations of context and scale when using fecal glucocorticoids to indicate stress in large mammals: a study of wild American plains bison

Non-invasive measures of the stress response are used to understand the impacts of natural and anthropogenic disturbances on wild animals. They can, however, be challenging to interpret without additional contextual information and specifics of the animals in question. Here, we used fecal samples collected from the Henry Mountains bison herd in Utah to measure the glucocorticoid hormone corticosterone (CORT), which is indicative of stress. We compared site-specific measures of fecal CORT concentration with measures of covariates related to geography (elevation, slope, aspect, distance to roads, distance to water, food quality, habitat type, season), bison physiology (body condition, parasite load, sex), and human activity (traffic volume at multiple time scales, hunting seasons). Our aim was to determine whether an unexpected habitat selection pattern could be a response to human disturbance, and thus whether ecological covariates could explain variations in fecal CORT concentration in free-ranging bison. No meaningful relationships were found for any of the covariates included in the study. At least some of those covariates should be related to the stress state of the herd, but in large and highly mobile species such as bison there is a scale mismatch between the physiological stress response of an animal and the spatiotemporal distribution of fresh feces left on the landscape. We offer our assessment of fecal CORT in bison as a case study demonstrating the utility and complications associated with using fecal indicators of stress in wildlife populations.