Any mother will tell you that children can be a constant source of stress. It transpires that mares, too, have elevated stress levels when raising a foal – at least in the wild.
The study, reported this week in the journal Animals, centered on the wild horses that inhabit Sable Island, 275km off the coast from Halifax, in Novia Scotia, Canada.
Researchers with the University of Saskatchewan measured levels of the stress hormone cortisol in hair samples collected from the horses, who exist under fully natural conditions, including a self-determined social structure.
Studies in horses often use salivary, blood or fecal concentrations of cortisol to assess stress. However, a horse’s hair records a much longer activity profile of an animal’s hypothalamic-pituitary-adrenal axis, and the resulting production of cortisol.
While cortisol is a key hormone involved in mobilizing stored energy reserves to help meet physiological demands, production can also be triggered by psychological stimuli or perceived demands.
Sarah Medill and her fellow researchers noted that most studies investigating hair cortisol concentrations have used domesticated individuals where nutritional requirements are fully met, social structures may not be as dynamic, or reproductive behaviors and physiological demands are inhibited.
Some tail hair samples used in the study were collected after they were observed to be scratched off onto previously cleaned natural and artificial hair snags (velcro strips on known rub structures), but most were collected by approaching and directly pulling the hairs from unrestrained individuals.
Hair collected in the field was handled with nitrile gloves, placed in labeled envelopes, and stored in a dark location. The 282 samples were identified as coming from 113 females and 135 males.
The study team found that younger females with dependent offspring had higher hair cortisol concentrations than females without foals.
Hair cortisol concentrations in young females tended to be lower in those without a foal compared to same-aged counterparts with offspring, while, among the horses aged six or older, the difference in hair cortisol was negligible for those with or without foals.
Females accompanied by yearlings tended to have only slightly higher cortisol concentrations than females without foals, but not significantly so.
Horses in poor body condition were also more likely to have higher concentrations. Females had greater variation in body condition scores, which also correlated with foal production.
Hair cortisol concentrations in males were not found to be tied to their social position as either a non-breeding bachelor or successful band stallion.
“Overall,” they said, “both female and male hair cortisol concentrations were more strongly linked to biological factors such as age and body condition (and reproduction for females), or even to year effects than it was to our measures describing local population demographics.”
However, some modeling pointed to an influence among females from harem size and the number of bachelor males in the vicinity. In contrast, for males, the number of bachelors and the overall horse density were factors.
“In general, the top-ranked models describing female cortisol levels included age, body condition index, 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, body condition index, and year of collection only, and the number of bachelors in the home range appeared in subsequent, though still high-ranked, models.”
The authors said differences in hair cortisol concentration patterns between feral and domestically kept horses (for example, 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.
“One surprising observation was that females, in general, had lower hair cortisol concentrations than males given that females were sampled at a time when many should have been experiencing greater metabolic demands related to lactation, and our sample had a higher number of lactating compared to non-lactating females.
“Males were also experiencing high energy requirements through mating and harem defense activities among dominant band stallions and challenging for mating opportunities by bachelors.”
This observation should not be concluded as males being more “stressed” than females, but more likely, males and females potentially have different baseline levels or respond differently to physiological or psychological circumstances.
The authors said that, since they did the sampling at only one time of the year, they potentially missed out on variations that may be related to seasonal changes in endocrine function between the sexes.
The study team comprised Medill, David Janz and Philip McLoughlin, all with the University of Saskatchewan.
Medill, S.A.; Janz, D.M.; McLoughlin, P.D. Hair Cortisol Concentrations in Feral Horses and the Influence of Physiological and Social Factors. Animals 2023, 13, 2133. https://doi.org/10.3390/ani13132133
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