Quantitative evaluation of behavioral and physiological adaptation during retraining of retired Thoroughbred racehorses.

Overview

• This study quantitatively examined how retired Thoroughbred racehorses adapt both behaviorally and physiologically when exposed repeatedly to startling stimuli during retraining.
• The research aimed to determine whether behavioral calmness aligns with physiological regulation, highlighting the importance of using multiple indicators to assess adaptation and safety in retraining.

Background and Rationale

• Retired Thoroughbred racehorses are often retrained for new riding roles after concluding their racing careers.
• Assessing their adaptation to new environments is critical for ensuring safety and welfare.
• Behavioral calmness is commonly used to gauge adaptation, but it may not accurately reflect internal physiological states.
• Understanding both behavioral and physiological responses to stressors, such as sudden startles, is important to comprehensively evaluate how horses adapt during retraining.

Objective

• To quantitatively evaluate changes in behavioral and physiological responses—specifically heart rate (HR), behavioral scoring, and response duration (RD)—to repeated startle exposures over time.
• To assess the concordance between behavioral indicators and physiological indicators during the retraining period.

Methodology

• Subjects: 12 retired Thoroughbred racehorses.
• Experimental Design:
  • Three measurement sessions (T1, T2, T3) spread over the retraining period.
  • Each session had two phases: Before (pre-training) and After (post-training).
  • Within each phase, horses were exposed to three standardized startle trials using plastic bags at 5-minute intervals.
• Behavioral Measurement: Responses scored on a 0 to 4 Behavioral Score (BS) scale reflecting intensity of startle behavior.
• Physiological Measurement:
  • Heart rate (HR) measured continuously during trials as an autonomic indicator.
  • Response duration (RD) measuring length of time the startle response lasted.
• Statistical Analyses:
  • Friedman tests to assess changes across repeated exposures.
  • Nonlinear exponential decay modeling to characterize adaptation dynamics.
  • Linear mixed-effects models to evaluate effects of training and session.
  • Vector-based similarity analyses to compare trajectories of physiological and behavioral indicators.

Key Findings

• Habituation Across Repeated Exposures:
  • All measured variables (BS, HR, RD) significantly decreased with repeated startle exposures, indicating habituation.
  • The largest decreases occurred between the first and second sessions, suggesting rapid early adaptation.
• Differential Adaptation Dynamics:
  • Heart rate decreased more rapidly than behavioral scores, indicating that physiological adaptation precedes behavioral adaptation.
  • Behavioral scores showed a more gradual decline over time.
• Effect of Retraining:
  • Comparisons before and after the three-month retraining showed overall reductions in behavioral scores, HR, and response durations.
  • This suggests the retraining facilitated better physiological and behavioral regulation in response to startling stimuli.
• Correlation and Concordance Analyses:
  • HR and RD trajectories were strongly coupled, reflecting coordinated physiological recovery dynamics.
  • Behavioral scores showed more variability in their relationship with physiological responses, indicating behavioral calmness may not fully reflect physiological state.

Conclusions and Implications

• Behavioral and physiological adaptations to startling stimuli are coordinated but occur on different time scales; physiological adaptation occurs faster.
• Behavioral assessments alone may overlook important physiological recovery processes, potentially leading to incomplete evaluations of retraining progress and horse safety.
• Using both behavioral indicators and physiological measures provides a more comprehensive and accurate picture of adaptation in retired racehorses undergoing retraining.
• This combined approach may improve welfare monitoring and safety assessment during retraining programs, reducing risks associated with under-recognized physiological stress.