Physiological correlates associated with cribbing behaviour in horses: changes in thermal threshold, heart rate, plasma beta-endorphin and serotonin.

The research focuses on understanding the physiological mechanisms underlying the cribbing behavior in horses. The study explores the changes in thermal threshold, heart rate, plasma beta-endorphin, and serotonin levels in horses during cribbing behaviors.

Research Methodology

• The researchers developed a device for telemetric measurement of thermal threshold in horses, testing their responsiveness to external stimulus. The thermal threshold was measured using the forelimb withdrawal reflex.
• These measurements were taken in 11 horses known for displaying cribbing behavior both before and during the instances of this behavior. The heart rate of these horses was also monitored continuously.
• Blood samples were collected from the same horses. Beta-endorphin and cortisol were analyzed in the plasma using radioimmunoassay techniques, while serotonin was measured using high-performance liquid chromatography.
• Control equines, matched in age and sex to the cribbing horses, provided additional comparative data.

Key Findings

• The thermal threshold was found to be significantly lower (P = 0.003) during cribbing periods. The mean difference was 4.9 degrees C.
• The heart rate was observed to decrease significantly. There was an average reduction of 2.4 beats per minute during cribbing. As arousal is usually associated with an increase in both heart rate and the threshold for feeling nociceptive (painful) stimulus, these findings suggest that cribbing may act as a stress reduction activity for horses.
• Moreover, horses that displayed the cribbing behavior had three times higher basal beta-endorphin levels compared to the control horses. The averages were 49.5 pmol/l for cribbing horses and 16.2 pmol/l for control horses (P = 0.006).
• There was a trend towards lower basal levels of serotonin in cribbing horses (mean 201.5 nmol/l), compared to the control horses (414.3 nmol/l), though this trend was not statistically significant (P = 0.07).

In short, this research provides evidence of notable differences in the endogenous opioid and serotonin systems of horses who exhibit cribbing behavior, emphasizing the linkage between cribbing and physiological stress relief mechanisms in horses.