Sex, Diet, and the Social Environment: Factors Influencing Hair Cortisol Concentration in Free-Ranging Black Bears (Ursus americanus)

Abstract

Increasingly, measures of glucocorticoid levels (e.g., cortisol), key components of the neuroendocrine stress axis, are being used to measure past hypothalamic-pituitary-adrenal (HPA) activity to index psychological and physiological stress exhibited by wildlife for assessing individual and population-level well-being. However, many intrinsic and extrinsic factors affect HPA activity in animals. Using American black bears (Ursus americanus; n = 116) as an ecological model and hair cortisol concentration (HCC) as an integrative measure of past HPA activity, we evaluated the influence of diet, sex and the social environment on black bear HCC in a free-ranging population that spanned adjoining ecoregions with differing densities of potential conspecific and heterospecific competitors. HCC varied by sex, with female HCC ranging from 0.6 to 10.7 pg/mg (median = 4.5 ± 1.2 mean absolute deviation [MAD]) and male HCC ranging from 0.5 to 35.1 pg/mg (median = 6.2 ± 2.6 MAD). We also observed a three-way interaction among sex, δ14C and ecoregion, which may indicate that some differences in HCC between female and male black bears results from variability in the nutritional needs of larger-bodied males relative to smaller-bodied females, slight differences in food resources use between ecoregions as well as sex-based differences regarding the social environment. Once we understand what drives sex-specific differences in HCC, HCC may aid our understanding of the physiological responses by bears and other wildlife to diverse environmental challenges.

Fig 1. Distribution of black bears (Ursus americanus) in isotopic space (δ¹³C and δ¹⁵N) relative to mean food source values.

Trophic discrimination factors were applied to each food category, which are represented by the mean δ¹³C and δ¹⁵N (± SD) of each food category. Black bears sampled from Parsnip Plateau and Hart Ranges of the Rocky Mountains, British Columbia, Canada, 1999.

Fig 2. Black bear (Ursus americanus) isotopic niches and corresponding isotopic niche density plots.

(a) Standard ellipse areas corrected for small sample size (SEAc), representing core (40%) dietary niches of black bear females (MF) and males (MM) from the mountain ecoregion and females (PF) and males (PM) from the plateau ecoregion. (b) Density plot representing the posterior probability distribution of SEAc sizes. Black dots correspond to means and decreasing bar widths represent 50%, 75% and 95% Bayesian credible intervals. Black bears sampled from Parsnip Plateau and Hart Ranges of the Rocky Mountains, British Columbia, Canada, 1999.

Fig 3. Black bear (Ursus americanus) hair cortisol concentration by sex and ecoregion.

Black bears sampled from Parsnip Plateau and Hart Ranges of the Rocky Mountains, British Columbia, Canada, 1999.