Hair cortisol concentration as a marker of long-term stress: sex and body temperature are major determinants in wild-living Alpine marmots

Hair cortisol concentration (HCC) has recently gained popularity as an easy-to-measure biomarker of long-term stress in wild and domestic animals. Hair integrates cortisol over long time periods within a single sample and it can be collected non-invasively, which makes its use particularly interesting for wildlife studies. Interpreting HCC values, however, is challenging, because they are determined by the interplay of multiple factors. Here, were explore potential determinants of HCC in the Alpine marmot Marmota marmota. We tested the relationship of sex, age class, physical condition and body temperature with the hair cortisol concentration of free-ranging marmots. We found marked sex difference in HCC, with higher levels in females. This might be related to sex-specific variation in social stress or resulting from physiological difference, e.g., in baseline and stress-induced levels of cortisol secretion. Interestingly, body temperature was also positively related to HCC, possibly hinting at individual short- and long-term stress reactivity as part of coping styles. Although further work is needed to entangle possible mechanisms underlying the neuro-endocrinological modulation on HCC, our results emphasize that determinants such as sex and body temperature in Alpine marmots should be accounted for, when using HCC as marker of chronic stress.