Revisiting the importance of accounting for seasonal and diel rhythms in fecal stress hormone studies

Measurement of fecal glucocorticoid metabolites (FGMs) has been used to quantify stress responses by a wide variety of species; yet, few studies attempt to conduct baseline research prior to investigating effects of disturbances on FGM levels. We evaluated the potential confounding effect of seasonal and diel rhythms on the interpretation of FGM values, by studying long-term patterns of FGMs in white-tailed deer (Odocoileus virginianus) and mourning doves (Zenaida macroura) between 2001 and 2002 in a controlled environment at facilities located at the Charles W. Green Conservation Area, near Ashland, Missouri, USA. For 1 year, we collected fresh fecal samples (<1 hr old) on a weekly basis from 17 captive deer and 8 captive doves. We also conducted hourly sampling over discrete 48-hour periods to evaluate diel rhythms. In deer, we observed that FGMs were nearly 2 times higher during spring months of April and May compared with late winter (i.e., Jan and Feb) and late summer (i.e., Jul and Aug). Dove FGMs were 50% higher during the late spring (i.e., May and Jun) compared with early spring and winter months (Jan-Apr), and twice as high as during late summer, autumn, and early winter months of August through December. In both species, we observed considerable variation in diel rhythms that was inconsistent across individuals. Collectively, our findings emphasize the need for researchers to design sampling schemes that account for high levels of individual and temporal variability in baseline FGMs - and consistently ask: What are "normal" FGM values in the context of the animal's environment and prevailing stressors.