Non-invasive techniques for stress assessment in white-tailed deer

Measuring stress hormones (i.e., glucocorticoids) in large free-ranging vertebrates such as white-tailed deer (Odocoileus virginianus) is difficult, and invasive procedures (e.g., animal capture and blood collection) have been the only tools available. Non-invasive techniques, including fecal and saliva glucocorticoid measurements, are currently being developed that offer advantages over traditional techniques. Using adrenocorticotropic hormone (ACTH) challenges, we determined that fecal glucocorticoid metabolite and salivary cortisol assays measured adrenal activity (i.e., stress hormones) in white-tailed deer. For each of four ACTH trials, we housed captive deer singly for 72 hr in an outdoor pen to which they had been exposed previously. After 24 hours, we injected a total of 50 I.U. of ACTH (Cortrosyn™) intramuscularly (2 25-I.U. injections in experiments 1 and 2; 1 50-I.U. injection in experiments 3 and 4). All available fecal samples were collected every hour, homogenized, and frozen. During the last 2 experiments, we collected saliva samples at 10-min intervals for 2 hr prior to and 6 hr after the ACTH injection. Fecal glucocorticoid metabolites were extracted from fecal samples and assayed using a commercially available I¹²⁵ corticosterone radioimmunoassay. We used an enzyme immunoassay to measure salivary cortisol. A peak in fecal glucocorticoids occurred at 20-24 hr (experiments 1 and 2) and 10-13 hr (experiments 3 and 4) post-ACTH injection, physiologically validating the assay's ability to detect biologically important changes in adrenocortical activity. Salivary cortisol levels increased 30-60 min after ACTH injection, confirming the assay's ability to detect changes in cortisol concentrations. These findings demonstrated that fecal glucocorticoid and salivary cortisol assays provide an index of physiological stress in white-tailed deer. These techniques may prove useful in addressing conservation issues surrounding white-tailed deer, provided additional sampling questions are addressed.