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Cortisol is widely thought of as a stress hormone because its levels rise during episodes of acute stress. It has been used as a measure of stress in many horse studies.
However, its relationship to chronic stress is less clear.
Researchers from the Animal and Human Ethics Laboratory at the University of Rennes 1 in France, working with the University of Veterinary Medicine in Vienna, have linked lower cortisol levels to states of chronically poor welfare in adult horses observed under their usual living conditions.
Their findings were published this week in the peer-reviewed open-access journal PLOS ONE.
Cortisol, deemed the quintessential stress hormone, allows us to cope with important events and imminent threats.
A spike in cortisol levels mobilizes necessary resources — such as by tapping into our body’s reserves to produce energy — and then allows us to return to a stable state. But can our bodies cope with prolonged or repeated stress in the same way?
Some studies have reported lower cortisol levels in humans — or other mammals — that are affected by chronic stress, while other studies contradict these findings.
In light of this, is cortisol still a reliable stress indicator?
To answer this question, the researchers studied 59 adult horses, comprising 44 geldings and 15 mares, from three different riding centers, under their usual living conditions.
The horses were kept in individual stalls that are both spatially and socially restrictive. They were ridden by inexperienced equestrians — both potential stressors that, if recurrent, can lead to chronically compromised welfare.
The scientists monitored various behavioral and sanitary indicators of the horses’ welfare and measured cortisol levels using blood and stool samples.
The horses had all been living under the stated conditions for at least a year at the start of the study, and they were observed for several weeks.
Surprisingly, cortisol levels in the horses that showed evidence of compromised welfare (for example, ears pointed back, back problems, and anemia) were lower than in other horses.
These findings were in accord with early observations by the ethology team, which recorded abnormally low cortisol concentrations in horses with depressive-like behavior.
Furthermore, levels of metabolic byproducts of cortisol measured in their droppings correlated with blood cortisol levels in the evening. This suggested that testing of stool samples could be used as an alternative, noninvasive way of gauging horse welfare.
The study team said low cortisol levels may seem counter-intuitive in horses under chronic stress, but they could be explained by a breakdown of the system when horses experienced stress at excessive levels for excessive lengths of time.
So when exactly does duration and intensity of stress become excessive for these horses? This is one of the questions the team of researchers is now seeking to answer.
At any rate, their study has shown that cortisol levels are not always reliable indicators of stress or compromised welfare. On the one hand, high cortisol may be a sign of positive stress, driving higher performance; on the other, low cortisol does not necessarily mean lack of stress. Quite the contrary, under a certain threshold, low cortisol levels may be cause for concern.
Future research aims to determine how living conditions such as living in stable social groups at pasture with minimal human interventions versus living in high density populations with few foraging opportunities, known to negatively affect horses’ welfare, can affect a horse’s wider measures of well-being and cortisol levels.
The study team comprised Jodi Pawluski, Patrick Jego, Séverine Henry, Anaelle Bruchet, Caroline Coste and Martine Hausberger, all with the University of Rennes 1; and Rupert Palme, from the University of Veterinary Medicine in Vienna.
Pawluski J, Jego P, Henry S, Bruchet A, Palme R, Coste C, et al. (2017) Low plasma cortisol and fecal cortisol metabolite measures as indicators of compromised welfare in domestic horses (Equus caballus). PLoS ONE 12(9): e0182257. https://doi.org/10.1371/journal.pone.0182257