Hair Cortisol, Testosterone, Dehydroepiandrosterone Sulfate and Their Ratios in Stallions as a Retrospective Measure of Hypothalamic–Pituitary–Adrenal and Hypothalamic–Pituitary–Gonadal Axes Activity: Exploring the Influence of Seasonality

Steroid-hormone concentrations from non-invasively obtained biomarkers, like hair, can provide a representation of circulating hormones diffused over relatively long time periods (e.g., weeks or months). The hormone cortisol is often associated with physiological or even psychological stress, while testosterone is strongly associated with male development and reproductive success. Increasingly, studies are using hormone levels derived from hair to make inferences among both domestic animals and wildlife. For horses, all previous hair hormone analysis has been done on companion or working animals. We evaluated the levels of hair cortisol (n = 153) and testosterone (n = 48) from 136 feral horses living on Sable Island, Canada that have been part of a long-term individual-based study since 2008. This population has been undisturbed and unmanaged for over 50 years, and exhibits the natural social organization for horses, harem defense polygyny. Hair samples were collected in mid to late summer and the segment analyzed corresponds with hair grown during, and following, the peak of the reproductive season. Social position was determined based on the male’s role as either a dominant breeding Stallion (Stallion), a non-breeding subordinate male (tag), adult Bachelor (5 years old or older), or Immature male (2–4 years of age). While there was no difference in hair-cortisol concentration among any class of adult males (i.e., Stallion, tag, or Bachelor), Immature males had significantly lower hair cortisol concentrations than the other groups (p = 0.001). Hair testosterone levels among the four social positions were significantly higher among Stallions (p = 0.04). Hair testosterone concentration was also significantly related to the probability of a male being either a Bachelor or Stallion and was the only variable remaining in AICc model selection (p = 0.016, AICc = 32.3, Null AICc = 38.8). While not a significant relationship, Stallions had a negative correlation between hair cortisol concentrations and testosterone (R2 = −0.20, p = 0.383), and Bachelors, conversely, had a positive association (R2 = 0.43, p = 0.246). Our observations of hormone concentrations in relation to physiological, social, or reproductive parameters in this population suggest trends that are similar to what has been established using blood or other matrices.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Hair Cortisol and Testosterone Concentrations in Relation to Maturity and Breeding Status of Male Feral Horses

Medill

Janz

McLoughlin

2023

“…The dosage of CORT from foal hair has already been performed in previous studies, but only in healthy subjects born from normal pregnancy [11,12]. In contrast, the dosage of DHEA/DHEA-S has been performed in a few recent studies but only in adult horses [16,17,31].…”

Section: Discussionmentioning

confidence: 99%

“…However, lower hair CORT concentrations have been reported in horses with a severe squamous gastric disease when compared to horses without gastric ulcers [15]. Although the CORT/DHEA ratio in equine hair has been previously described to evaluate the influence of different housing conditions [16] or seasonality [17], there is still a lack of information on its biological relevance in horses, especially in pregnant mares hospitalized for assisted delivery.…”

Section: Introductionmentioning

confidence: 99%

Hair Cortisol and DHEA-S in Foals and Mares as a Retrospective Picture of Feto-Maternal Relationship under Physiological and Pathological Conditions

Lanci

Mariella

Ellero

et al. 2022

Equine fetal hair starts to grow at around 270 days of pregnancy, and hair collected at birth reflects hormones of the last third of pregnancy. The study aimed to evaluate cortisol (CORT) and dehydroepiandrosterone-sulfate (DHEA-S) concentrations and their ratio in the trichological matrix of foals and mares in relation to their clinical parameters; the clinical condition of the neonate (study 1); the housing place at parturition (study 2). In study 1, 107 mare-foal pairs were divided into healthy (group H; n = 56) and sick (group S; n = 51) foals, whereas in study 2, group H was divided into hospital (n = 30) and breeding farm (n = 26) parturition. Steroids from hair were measured using a solid-phase microtiter radioimmunoassay. In study 1, hair CORT concentrations measured in foals did not differ between groups and did not appear to be influenced by clinical parameters. A correlation between foal and mare hair CORT concentrations (p = 0.019; r = 0.312, group H; p = 0.006; r = 0.349, group S) and between CORT and DHEA-S concentrations in foals (p = 0.018; r = 0.282, group H; p < 0.001; r = 0.44, group S) and mares (p = 0.006; r = 0.361, group H; p = 0.027; r = 0.271, group S) exists in both groups. Increased hair DHEA-S concentrations (p = 0.033) and decreased CORT/DHEA-S ratio (p < 0.001) appear to be potential biomarkers of chronic stress in the final third of pregnancy, as well as a potential sign of resilience and allostatic load in sick foals, and deserve further attention in the evaluation of prenatal hypothalamus-pituitary-adrenal (HPA) axis activity in the equine species. In study 2, hormone concentrations in the hair of mares hospitalized for attended parturition did not differ from those that were foaled at the breeding farm. This result could be related to a too brief period of hospitalization to cause significant changes in steroid deposition in the mare’s hair.

“…Hair is an attractive matrix, as it is minimally invasive to collect, robust to various storage conditions and exposure [ 8 , 13 , 14 ], and it offers a prolonged retrospective view of an individual’s HPA axis activity [ 10 , 11 , 15 , 16 , 17 ]. Hair hormone analysis has been used to look at differences between the sexes [ 18 , 19 , 20 ], different ages or development periods [ 8 , 21 , 22 , 23 ], access to resources or energy requirements [ 24 , 25 , 26 , 27 ], the reproductive state [ 24 , 28 ], or physical injury [ 8 , 14 , 16 , 29 ]. Glucocorticoid levels have also been linked to social correlates such as changes in social group membership [ 16 , 30 ] and aggression given or received in relation to a hierarchal rank [ 16 , 31 , 32 , 33 ].…”

Section: Introductionmentioning

confidence: 99%

Hair Cortisol Concentrations in Feral Horses and the Influence of Physiological and Social Factors

Medill

Janz

McLoughlin

2023

Cortisol is a glucocorticoid hormone produced during activation of the hypothalamic–pituitary–adrenal axis (HPA) in response to psychological or physiological demands. High amounts of circulating cortisol can be found in individuals experiencing energetically demanding physiological events, such as pregnancy, lactation, injury, or starvation, but, also, in individuals who may have less obvious HPA activation from social situations. The feral horse population on Sable Island (Nova Scotia, Canada) provides an opportunity to look at hair cortisol concentration (HCC) as a proxy for circulating cortisol concentration to better understand physiological correlates. The horse’s complex social structure also allows us to look at how the population and group structure may influence HPA activation. Hair samples (n = 282) were analyzed from 113 females and 135 males. Females with dependent offspring (foals) had higher HCC than those females without dependent offspring (p = 0.005). Horses in poor body condition were also more likely to have higher HCC (females: p < 0.001, males: p = 0.028); females had greater variation in the body condition index (BCI), which also correlated with foal production. In general, the top-ranked models describing female cortisol levels included age, BCI, presence of a foal, as well as social measures such as harem size and the number of bachelors in the vicinity. The top model describing male cortisol levels included age, BCI, and year of collection only, and the number of bachelors in the home range appeared in subsequent, though still high-ranked, models. Among the variables not of direct interest, we found some significant results relating to hair color and hair texture. Differences in HCC patterns between feral and domestically kept horses (e.g., age and sex) are likely linked to periods of resource limitations, particularly for individuals experiencing energetically demanding processes such as reproduction, illness/parasitism, or related to experiencing the full range of social and reproductive behaviors.