Physiological welfare indicators in wild cetaceans: Epidermal cortisol and oxytocin concentrations in stranded striped dolphins

Anthropogenic pressures and climate change present growing challenges for cetaceans, as the combined effects of multiple stressors can jeopardize their welfare and survival. In this context, validating reliable individual welfare indicators is crucial for quantifying these impacts. This study aimed to validate a method for measuring cortisol and oxytocin from the epidermis of stranded striped dolphins (Stenella caeruleoalba) using enzyme immunoassays, while accounting for confounding factors such as epidermal layer and body location. The effects of different causes of death-'Peracute Underwater Entrapment' and 'Distress Associated'- along with biological factors, were examined in relation to epidermal hormone levels. Furthermore, the relationship between these hormone levels and markers suggesting an impaired welfare, was explored. Validation tests indicated that the method was effective in quantifying both epidermal cortisol and oxytocin concentrations. Specifically, epidermal cortisol levels showed strong correlations with both serum and blubber levels and were 6 times higher in emaciated individuals and 14 times higher in those with distress-associated deaths, supporting its use in assessing hypothalamic-pituitary-adrenal activity. Interestingly, results supported the validity of epidermal cortisol levels as markers of impaired welfare in dolphins, as they consistently increased across conditions assumed to negatively affect welfare but varying in terms of severity and duration. In contrast, epidermal oxytocin levels could not be validated as an indicator of the general oxytocin system nor as an indicator of welfare in this species. In conclusion, this study successfully validated epidermal cortisol as a reliable physiological indicator of welfare in striped dolphins, providing a promising tool for assessing individual and population-level welfare impacts. However, further research is needed to fully explore the potential role of oxytocin as a welfare biomarker in cetaceans.