Hair Cortisol and Testosterone Concentrations in Relation to Maturity and Breeding Status of Male Feral Horses.
Authors: Medill Sarah A, Janz David M, McLoughlin Philip D
Journal: Animals : an open access journal from MDPI
Summary
# Editorial Summary: Hair Cortisol and Testosterone in Feral Male Horses Understanding chronic stress and reproductive status in horses requires reliable biomarkers that reflect hormone levels over extended periods rather than capturing transient fluctuations. This study analysed hair cortisol and testosterone from 136 feral stallions living on Sable Island to determine whether these non-invasive markers could differentiate social rank and maturity in an unmanaged population exhibiting natural harem defence polygyny. Researchers collected hair samples in mid to late summer—a period capturing post-reproductive season growth—and categorised males by social status: dominant Stallions, subordinate tagged males (tags), adult Bachelors (≥5 years), and Immature males (2–4 years). Hair cortisol concentrations remained consistent across adult social classes but were significantly lower in Immature males (p = 0.001), suggesting that developmental stage rather than social stress influences cortisol deposition. Testosterone proved the strongest predictor of whether a male would achieve Stallion or Bachelor status (p = 0.016), with Stallions showing substantially elevated concentrations compared to other groups (p = 0.04), though a curious inverse trend emerged between cortisol and testosterone in Stallions that warrants further investigation. For practitioners, these findings validate hair sampling as a practical field tool for assessing reproductive capacity and developmental maturity in horses, particularly useful in situations where blood sampling proves impractical, though the cortisol–testosterone relationship may be more nuanced than previously understood, especially among high-ranking individuals.
Read the full abstract on PubMed
Practical Takeaways
- •Hair-based hormone analysis can be used to non-invasively assess stress and reproductive status in feral horses, providing insights over weeks-to-months timeframes without capture stress
- •Social position and breeding status are associated with testosterone levels; stallions show distinct endocrine profiles compared to non-breeding males
- •This methodology may have applications for assessing welfare and reproductive fitness in unmanaged or wild horse populations
Key Findings
- •Immature males (2-4 years) had significantly lower hair cortisol concentrations than adult males (p = 0.001)
- •Hair testosterone levels were significantly higher in Stallions compared to other social positions (p = 0.04)
- •Hair testosterone concentration was the only variable significantly predicting Bachelor or Stallion status (p = 0.016, AICc = 32.3)
- •Stallions showed negative correlation between cortisol and testosterone (R² = -0.20) while Bachelors showed positive association (R² = 0.43)