Assessing Stress in Arctic Lemmings: Fecal Metabolite Levels Reflect Plasma Free Corticosterone Levels

Interest in the ecology of stress in wild populations has triggered the development of noninvasive methods for quantifying stress hormones. Measurement of fecal corticosteroid metabolites (FCMs) is one such method, but it is still unclear whether FCMs can be a reliable proxy of free plasma glucocorticoids. To assess the validity of this assumption, we carried out a robust assessment on brown lemmings (Lemmus trimucronatus) from Bylot Island, Nunavut, Canada, that were hand captured and anesthetized and related plasma glucocorticoid levels to fecal metabolite glucocorticoid levels. We examined endogenous factors that could explain interindividual variability. Blood corticosterone was measured from samples obtained on capture and 30 min later, and FCM levels were measured from animals kept in captivity for 72 h. Plasma free corticosterone increased 135-fold over baseline values 30 min after capture, which confirmed that initial handling was perceived as a stressor. We found that FCM levels were highly related with free (marginal R-m(2) = 0.53) but not with total (R-m(2) = 0.02) corticosterone levels, regardless of age, sex, and reproductive condition. FCM levels started increasing 2 h after capture and reached maximum levels 4 h after capture. No circadian rhythm in FCMs was found. Plasma total corticosterone levels were much higher in adult females compared with adult males, but this difference was much smaller when measuring free corticosterone levels and FCM levels. Our results suggest that FCM levels are good measures of stress by being closely related to plasma free corticosterone levels in brown lemmings.