Social enrichment mitigates facial expressions and physiological indicators of short-term stress in horses.

Overview

• This study investigated whether social noncontact environmental enrichment can reduce stress indicators in horses subjected to restraint by evaluating physiological signs and facial expressions.
• The results showed that horses accompanied by another horse without physical contact displayed lower physiological and facial stress indicators compared to isolated horses, suggesting social buffering effects.

Background and Objective

• Stress in horses during handling and restraint is a significant welfare concern.
• Social enrichment, even without physical contact, might help reduce stress by providing a calming influence known as social buffering.
• The objective was to assess if social noncontact environmental enrichment during restraint lowers stress responses in horses.
• The study aimed to evaluate emotional experiences during restraint by measuring physiological parameters alongside facial expressions that are linked to stress.

Methods

• Subjects: 11 Pantaneiro horses were studied using a crossover design.
• Treatments: Each horse underwent two treatments:
  • Social noncontact enrichment during stock restraint — horses could see and be near another horse but without physical contact.
  • Social isolation during stock restraint — horses restrained alone.
• Measurements were taken throughout a 24-minute restraint period and included:
  • Physiological parameters:
    • Heart rate (HR)
    • Heart rate variability (HRV)
    • Respiratory rate (RR)
    • Ocular temperature via infrared thermography
    • Auricular (ear) temperature using an infrared thermometer
  • Facial expressions assessed via EquiFACS (Equine Facial Action Coding System), which identifies specific muscle movements associated with emotional states.

Key Findings

• Physiological responses:
  • Lower heart rate when horses were accompanied by a conspecific compared to when isolated.
  • Lower respiratory rate in the social enrichment condition.
  • Lower eye temperature with social enrichment, which is associated with reduced stress.
• Facial expression findings:
  • Reduced frequency of stress-related facial actions, including:
    • Nostril dilator (AD38) — often signifying increased arousal.
    • Inner brow raiser (AU101) — linked with discomfort or negative emotions.
    • Upper eyelid raiser (AU5) and eye white increase (AD1) — often indicate vigilance or stress.
    • Ear positions such as ears forward (EAD101) and ears back (EAD104) — reflect different stress or attentive states.
  • All these expressions occurred less frequently under social noncontact enrichment versus isolation.
• The combined assessment supports that horses under isolation experienced higher arousal and a more negative emotional state compared to when social enrichment was present.

Interpretation and Implications

• The study provides evidence that social noncontact enrichment acts as a social buffer, reducing both physiological and behavioral stress markers in horses under restraint.
• This type of enrichment might mitigate negative emotional experiences during potentially stressful handling procedures.
• Employing social noncontact enrichment could improve equine welfare during veterinary procedures, transport, or other necessary restraint situations.
• Integrating facial expression analysis with physiological measurements offers a comprehensive tool for assessing horse welfare and emotional states.

Conclusion

• Social noncontact environmental enrichment reduces short-term stress indicators in horses during restraint.
• This mitigation occurs through social buffering effects that lower heart rate, respiratory rate, eye temperature, and stress-related facial expressions.
• These findings highlight the importance of social context in managing stress and improving welfare during equine restraint procedures.