# Hair cortisol concentrations as marker of chronic stress in wild roe deer (Capreolus capreolus)

## TL;DR

This study validates a method to measure chronic stress in roe deer using cortisol levels in their hair, finding that age and season affect these levels.

## Contribution

The study validates an ELISA test for measuring cortisol in roe deer hair and identifies factors influencing cortisol concentrations.

## Key findings

- Hair cortisol concentrations in roe deer ranged from 0.7 pg/mg to 27.25 pg/mg with a bimodal distribution.
- Age class and season of collection significantly influenced hair cortisol levels, especially during the rutting season in adults.

## Abstract

The stress response to different stimuli involves the release of mediators, with cortisol being the principal, aimed to induce physiological, metabolic, and behavioral changes for restoring allostatic balance. However, prolonged or excessive cortisol secretion may impair welfare by altering immunity, growth, and reproduction. Cortisol concentrations or its metabolites can be measured in several biological matrices, and may accumulate in the hair shaft over periods ranging from weeks to months, making hair a valid matrix for assessing chronic stress. Nevertheless, a validation of this method in roe deer is currently lacking. The objective of this study is to validate an ELISA test for the measurement of cortisol concentration in roe deer hair. Additionally, the effects of age, sex, environmental conditions, and seasonal variations on hair cortisol concentrations are evaluated. Materials and Methods. Hair samples were collected from 45 European roe deer housed in a wildlife rescue center. Cortisol was extracted from hair using methanol, and concentrations were quantified using a commercially available competitive ELISA kit originally developed for salivary cortisol. Results. Hair cortisol concentrations in roe deer ranged from 0.7 pg/mg to 27.25 pg/mg, exhibiting a distinctive bimodal distribution. Factors influencing hair cortisol levels included the age class and the season of collection, with particular emphasis on the rutting season in adults. No significant differences were observed with regard to the type of environment or the cause of rescue. Conclusion. The ELISA method provides a rapid, cost-effective, and reliable approach for assessing long- term alterations in glucocorticoid concentrations in roe deer, yielding results comparable to other more sensitive techniques.

The online version contains supplementary material available at 10.1007/s11259-026-11106-6.