Body Condition and Temperature-Humidity Index Are Associated With Cortisol Levels as Indicators of Physiological...

The Sumatran elephant (Elephas maximus sumatranus), a critically endangered subspecies endemic to Indonesia, is increasingly exposed to physiological stress associated with habitat disturbance and environmental conditions. Reliable assessment of stress physiology is therefore essential to support evidence-based welfare monitoring and conservation management, particularly for elephants maintained in conservation rescue units (CRUs). This study aimed to (1) validate a cortisol enzyme-linked immunosorbent assay (ELISA) kit for use in Sumatran elephants and (2) apply the validated assay to examine the effects of body condition score (BCS), ambient temperature, humidity, and temperature-humidity index (THI) on cortisol levels across CRUs in Aceh Province, Indonesia. Twenty elephants (8 males and 12 females; 10-49 years old) housed at six CRUs (CRU A-F) were included. Analytical validation assessed parallelism, accuracy, and assay precision, while biological validation compared cortisol levels during medical treatment and postrecovery. Factors influencing cortisol levels were evaluated using linear mixed-effects models incorporating BCS, location, ambient temperature, humidity, and THI as fixed effects, with elephant identity as random effects. The assay demonstrated acceptable analytical performance, with strong parallelism between serially diluted samples and the cortisol standard curve. Biological validation showed significantly higher cortisol levels during medical treatment than during recovery (p < 0.01). Mixed-effects analyses showed that BCS, location, ambient temperature, humidity, and THI were significantly associated with cortisol levels (p < 0.01). Elephants with BCS 7 (scale 1-9) tended to exhibit lower cortisol levels. Variation in cortisol levels was also observed among locations. Cortisol levels tended to increase with higher ambient temperature and THI, whereas higher humidity was associated with lower cortisol levels. In conclusion, the validated cortisol assay demonstrated suitability for use in Sumatran elephants under the conditions of this study. The findings suggest that body condition and environmental factors (e.g., ambient temperature, humidity, and THI) may be associated with variation in cortisol levels in elephants managed in CRUs, suggesting the importance of considering both physiological and environmental contexts in welfare monitoring.