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PloS one2025; 20(11); e0328430; doi: 10.1371/journal.pone.0328430

Exploring the association between stress-related hormonal changes, behaviours and facial movements after an interval training exercise in French Standardbred.

Abstract: Physical exercise can act as a physiological and a mental stressor. Monitoring exercise-induced stress is therefore essential to understand racehorses 'responses to effort and to ensure their welfare. Stress perceived by the horse during physical activity can be measured using various indicators, including stress-related hormones such as cortisol and adrenaline, and other neuromodulators such as serotonin, all involved in the stress response and its regulation. Another approach to assess physiological and emotional responses to stimuli such as exercise is through behaviours and facial movements. In this study, we aimed to 1) evaluate the changes in these three hormones following a trotting exercise, 2) determine the changes in behaviour and facial movements in response to the same exercise and 3) investigate potential relationships between hormonal variations and specific behavioural patterns that could serve as indicators of exercise-induced stress in horses. Fourteen French Standardbred horses from two stables were monitored over one day. In the morning, they performed an interval training trotting exercise. Behaviours and facial movements were recorded via video for 2 min 30 both before and just after exercise. Saliva and blood samples were collected at four time points: before exercise, just after exercise, 1 h post-exercise and 24 h post-exercise to assess salivary cortisol, and serum concentrations concentration of adrenaline and serotonin. Results showed significant post-exercise increases in all three hormones with peak concentrations observed immediately after exercise, and elevated cortisol and adrenaline levels persisting one hour later. These variations are consistent with normal physiological responses to physical effort, reflecting activation of regulatory systems rather than necessarily indicating negative stress. However, inter-individual variability in the magnitude of these responses suggests that horses did not all experience the exercise in the same way, highlighting potential differences exercise-induced stress. In terms of behaviour, horses exhibited higher frequencies of facial movements, particularly mouth movements, after exercise than before. Moreover, increases in serotonin and adrenaline concentrations were positively associated with agitation-related behaviours (pawing and head turning) and mouth movements. Overall, our findings suggest that a behavioural profile characterised by increased agitation and mouth movements may reflect a post-exercise arousal response in French Standardbreds. These behaviours, in association with hormonal changes, could provide a useful non-invasive tool to assess to assess horses' response to exercise, and potentially exercise-induced stress. However, further studies are needed to confirm this interpretation, particularly by investigating the potential effects of post-exercise management practices such as cross-tying, which may induce frustration.
Copyright: © 2025 Hennes et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Publication Date: 2025-11-05 PubMed ID: 41191577PubMed Central: PMC12588519DOI: 10.1371/journal.pone.0328430Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study investigated how interval training affects stress-related hormones, behavior, and facial movements in French Standardbred horses.
  • The research aimed to understand exercise-induced stress by linking hormonal changes with behavioral and facial indicators after exercise.

Background and Objectives

  • Physical exercise acts as both a physiological and mental stressor for racehorses.
  • Monitoring stress responses during exercise is crucial for optimizing horse welfare and training effectiveness.
  • Stress responses involve hormones such as cortisol and adrenaline, and neuromodulators like serotonin.
  • Stress can also be assessed behaviorally through observable behaviors and changes in facial movements.
  • The study’s specific goals were to:
    • Measure changes in cortisol, adrenaline, and serotonin following a trotting exercise.
    • Record behavioral and facial movement changes before and after the exercise.
    • Examine correlations between hormonal changes and behavior to identify indicators of exercise-induced stress.

Study Design and Methods

  • Participants: 14 French Standardbred horses from two different stables.
  • Exercise protocol: Horses performed an interval trotting training session in the morning.
  • Data collection:
    • Behavior and facial movements were videotaped for 2 minutes and 30 seconds before and immediately after exercise.
    • Saliva and blood samples were taken at four times: pre-exercise, immediately post-exercise, one hour post-exercise, and 24 hours post-exercise.
    • Hormone assays measured salivary cortisol and serum adrenaline and serotonin concentrations.

Key Findings: Hormonal Changes

  • All three hormones—cortisol, adrenaline, and serotonin—increased significantly immediately after exercise.
  • Cortisol and adrenaline remained elevated one hour post-exercise, indicating prolonged physiological activation.
  • The pattern of hormonal changes reflects normal regulatory system activation in response to physical effort, not necessarily negative stress.
  • Considerable individual variation in hormone levels suggested differences in how horses experienced the exercise stress.

Key Findings: Behavioral and Facial Responses

  • Post-exercise, horses showed increased frequencies of facial movements, especially mouth movements.
  • Agitation-related behaviors, such as pawing and head turning, also increased after exercise.
  • Rises in serotonin and adrenaline positively correlated with these agitation behaviors and mouth movements.
  • The observed behavioral profile—heightened agitation and mouth movements—is interpreted as a post-exercise arousal response.

Interpretations and Implications

  • The combination of hormonal and behavioral data suggests that certain behaviors could serve as practical, non-invasive stress indicators after exercise.
  • Differences among horses indicate that exercise-induced stress varies individually, underscoring the need for personalized welfare monitoring.
  • The results promote using facial and behavioral cues alongside hormone measurements to assess horses’ responses to physical effort.
  • Additional research is necessary to clarify these findings, especially considering management factors such as post-exercise restraint (e.g., cross-tying) that may influence stress or frustration behaviors.

Conclusions

  • Exercise induces marked hormonal and behavioral changes reflective of arousal rather than harmful stress in French Standardbred horses.
  • Monitoring specific facial and behavioral changes could aid trainers and veterinarians in evaluating horses’ post-exercise state and welfare.
  • A multi-faceted approach combining biochemical and behavioral assessments offers a promising avenue for understanding and managing exercise-induced stress in racehorses.

Cite This Article

APA
Hennes N, Tutin L, Foury A, Vancassel S, Bourguignon H, Duluard A, Ruet A, Lansade L. (2025). Exploring the association between stress-related hormonal changes, behaviours and facial movements after an interval training exercise in French Standardbred. PLoS One, 20(11), e0328430. https://doi.org/10.1371/journal.pone.0328430

Publication

PloS one
ISSN: 1932-6203
NlmUniqueID: 101285081
Country: United States
Language: English
Volume: 20
Issue: 11
Pages: e0328430
PII: e0328430

Researcher Affiliations

Hennes, Noémie
  • INRAE, CNRS, Université de Tours, PRC, Nouzilly, France.
  • French Federation for Horse Racing, Paris, France.
Tutin, Léa
  • INRAE, CNRS, Université de Tours, PRC, Nouzilly, France.
Foury, Aline
  • Univ. Bordeaux, INRAE, Bordeaux INP, NutriNeuro, UMR 1286, Bordeaux, France.
Vancassel, Sylvie
  • Univ. Bordeaux, INRAE, Bordeaux INP, NutriNeuro, UMR 1286, Bordeaux, France.
Bourguignon, Hélène
  • French Federation for Horse Racing, Paris, France.
Duluard, Arnaud
  • Société d'Encouragement à l'Elevage du Trotteur Français, Paris, France.
Ruet, Alice
  • INRAE, CNRS, Université de Tours, PRC, Nouzilly, France.
  • IFCE, 4940, Saumur, France.
Lansade, Léa
  • INRAE, CNRS, Université de Tours, PRC, Nouzilly, France.

MeSH Terms

  • Animals
  • Horses / physiology
  • Hydrocortisone / blood
  • Hydrocortisone / metabolism
  • Physical Conditioning, Animal / physiology
  • Epinephrine / blood
  • Saliva / metabolism
  • Saliva / chemistry
  • Behavior, Animal / physiology
  • Serotonin / blood
  • Male
  • Stress, Physiological
  • Female

Conflict of Interest Statement

The authors have declared that no competing interests exist.

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