Hair cortisol as an indicator of chronic stress in Eurasian otters (Lutra lutra): Evaluating influencing variations

Glucocorticoids such as cortisol are secreted via the HPA axis to maintain homeostasis and respond to stress, but chronic over-activation can impair health and survival. Cortisol accumulation in the hair is particularly useful because it reflects long-term stress, making it a valuable physiological indicator for species conservation and management. However, as variables such as housing conditions, age class, sex, hair color, and disease can affect hair cortisol concentration (HCC), ignoring these variables may lead to inaccuracies in chronic stress evaluation. This study measured HCCs in 33 Eurasian otters (Lutra lutra), an endangered species, to examine whether housing type (wild vs. captive), age, and sex affect HCCs. The results showed that juveniles had significantly higher HCCs than adults, suggesting that age may be a factor influencing HCC. However, no significant differences in HCC were found with respect to sex or housing type. Additionally, two of the individuals with exceptionally high HCC were confirmed to have been affected by disease. We propose that standardizing age class in HCC analysis is necessary, and that other important factors potentially influencing HCC variability should also be considered. In particular, disease has been suggested as a possible factor affecting HCC, and such factors may have an even greater impact on HCC variability. Although no significant differences were found regarding housing type and sex, these factors may be more clearly identified through future studies involving a broader range of housing environments and a larger sample size. However, despite the potential of HCC as a promising indicator of long-term stress, a limitation of this study is that its physiological validity has not yet been sufficiently verified in Eurasian otters. Therefore, the results of this study should be interpreted with caution. Nevertheless, by analyzing key factors influencing HCC, this study provides important foundational data for understanding the effects of housing type, sex, and age on long-term stress assessment.