Longitudinal assessment of physiological stress in grizzly bears
Understanding physiological responses of wildlife to their environment is fundamental to evolutionary biology, ecology and conservation. One important physiological response to environmental stimuli is activation of the hypothalamic-pituitary-adrenal (HPA) axis resulting in the release of stress hormones (e.g., cortisol) that modulate diverse physiological and behavioral processes. Increasingly, measures of cortisol are used to index individual and population-level stress burdens in wildlife. Although cortisol can be assayed from several biological matrices, hair cortisol concentration (HCC) provides an integrative measure of past HPA activity over the growth period of the hair. Using grizzly bears (Ursus arctos) as an ecological model and HCC as an integrative measure of past HPA activity, we evaluated the relative influence of individual, environmental and anthropogenic factors on the stress burden experienced by grizzly bears during 2006 ̶ 2011 in south-eastern British Columbia, Canada.
We obtained cortisol values from 227 grizzly bear hair samples, representing 177 individuals (female = 95, males = 82); 36 individuals were recaptured between 2–4 years. Grizzly bear cortisol values ranged from 1.61 to 18.67 pg/mg (median = 6.19 ± 1.12 median absolute deviation) and did not differ between sexes (p = 0.28). We found a weak negative relationship between cortisol and fruit production, with cortisol levels decreasing during years of moderate fruit production compared to years of low and very low fruit production. However, extensive among-individual variation in cortisol levels appeared to mask our ability to detect a population-level response to environmental stimuli in this interior grizzly bear population. Thus, understanding the extent to which individual variation in stress responses contribute to measures of stress at population-levels is critical for applying and interpreting cortisol as a physiological biomarker to enhance long-term population monitoring programs for grizzly bears and other species of conservation concern.