Effects of Exposure, Diet, and Thermoregulation on Fecal Glucocorticoid Measures in Wild Bears

Abstract

We examined fecal glucocorticoid (fGC) measures of nutrition and thermoregulatory demands on wild bears in Glacier National Park, Montana, and assessed how these measures changed in samples left in the field. Both ambient temperature and exposure can impact thermoregulation and sample degradation. Bear diets vary markedly with season, affecting body condition and thus fGC. We collected fecal samples during September and October, 2001, when ambient temperatures ranged from 30°C to −5°C. We collected half of each sample immediately and left the other half in its original location for 1–28 days. We used generalized linear models (GLM) to first predict fGC concentrations in fresh samples based on proxies of nutrition, ambient temperature, thermal exposure, and precipitation. These same covariates were then used to predict degradation-based differences in fGC concentrations between the paired sample halves. Variation in fGC was predicted by diet, Julian date, aspect, and the interaction between Julian date and aspect in both fresh and exposed samples. Cumulative precipitation was also a significant predictor of fGC concentrations in the exposed samples, independent of time, indicating that precipitation contributes to sample degradation but not enough to mask effects of other environmental factors on fGC concentrations. Differences between sample halves were only predicted by cumulative precipitation and exposure time; cumulative precipitation decreased, whereas exposure time increased, fGC concentrations in the exposed sample halves. Results indicate that fGC can provide reliable indices of nutrition and thermoregulatory demands in bears and that sample degradation impacts on these relations are minimal and can be virtually eliminated by controlling for cumulative precipitation over the estimated exposure times.

When can we measure stress noninvasively? Postdeposition effects on a fecal stress metric confound a multiregional assessment

“…However, a study of grizzly bear ( Ursus arctos horribilis ) and American black bear ( Ursus americanus ) scat reported decreased GCM in response to precipitation (Stetz et al. ). This effect was attributed to steroid metabolite degradation in response to increased water on fecal samples, which could also account for our observed pattern.…”

“…A more recent study on bears found that higher quality diets were associated with lower GCMs, identifying a nutritional influence on GCM concentration measured in fecal samples (Stetz et al. ). However, as we have shown, multiregional comparisons of GCM concentration between populations with access to different types or quality of food and different climatic conditions can be difficult to interpret, as changes in both dietary composition and climate may influence hormone metabolite concentration.…”

Methodological Considerations for Using Fecal Glucocorticoid Metabolite Concentrations as an Indicator of Physiological Stress in the Brown Bear (Ursus arctos)

“…Although we did observe initial declines in some but not all subsamples during the first 24 h after defecation, we also noted marked and consistent increases at exposure times of 3 d to 2 wk. Using an RIA assay, Stetz et al (2013) Crossey et al 2018). We interpret these inconsistencies as strong support for both assay-and species-dependent effects on metabolite stability (Palme 2019), as well as possibly for additional external factors such as gut and environmental bacteria.…”

“…It also showed the lowest tendency to detect unusually high fGCM concentrations among the baseline samples. While we are aware of multiple validations for RIA systems for measuring fGCM concentrations in brown bears (Hunt and Wasser 2003;Stetz et al 2013;White et al 2015), we are not aware of any published validations for EIA systems. EIAs may be beneficial in comparison with RIAs because they do not require the handling of radioactive markers (Sheriff et al 2011).…”

Concentrations of faecal glucocorticoid metabolites in Asian elephant's dung are stable for up to 8 h in a tropical environment

“…Moreover, exposure to different temperatures and humidity treatments had complex effects on the fGCM concentration in faeces of white-tailed deer ( Odocoileus virginianus ; Washburn and Millspaugh, 2002), wild bears ( Ursus spp. ; Stetz et al ., 2013) and jaguars ( Panthera onca ; Mesa-Cruz et al ., 2014). These results highlight the importance of assessing fGCM stability in field conditions in studies using fGCM to monitor wildlife populations.…”