Physiological correlates associated with cribbing behaviour in horses: changes in thermal threshold, heart rate, plasma β‐endorphin and serotonin

Abstract

Summary The aim of the present study was to obtain information on the possible mechanisms underlying cribbing behaviour in horses. To investigate the horse's responsiveness to an external stimulus, a device for telemetric measurement of thermal threshold, using the forelimb withdrawal reflex, was developed and validated. Measurements of thermal threshold took place in cribbing horses (n = 11) before and during cribbing periods. Heart rate was monitored continuously in the same horses. Blood samples were collected before and during cribbing periods as well and in age‐ and sex‐matched control horses (n = 11). β‐endorphin and Cortisol were determined in plasma using radioimmunoassay techniques, serotonin was analysed by high performance liquid chromatography. Compared with basal values, thermal threshold was significantly (P = 0.003) lower during cribbing periods. The mean difference was 4.9°C. Heart rate decreased significantly (P = 0.026) and showed a mean reduction of 2.4 beats/min during cribbing. Given the fact that arousal usually is associated with an increase in nociceptive threshold and in heart rate, the decrease in both during cribbing provide evidence that cribbing may reduce stress. Cribbers showed 3 times higher basal β‐endorphin levels than controls (mean 49.5 vs. 16.2 pmol/l, P = 0.006) and there was a trend for lower basal serotonin levels (mean 201.5 vs. 414.3 nmol/l, P = 0.07). These data indicate differences in cribber's endogenous opioid and serotonergic systems.

Circulating β-Endorphin, Adrenocorticotropin, and Cortisol Concentrations of Horses Before and After Competitive Show Jumping with Different Fence Heights

“…The pre-competition mean b-endorphin concentrations recorded in this study were low, although they were consistent with resting values reported in horses, including jumpers [39][40][41]43], and with the minimum plasma b-endorphin concentrations identified at 8:00 AM during a 24-hour sampling period [26].…”

The physiological consequences of crib-biting in horses in response to an ACTH challenge test

“…Studies investigating the effect of crib-biting on the SAM axis reactivity found similar results for HR as we found for the HPA axis. For instance, HR was shown to decrease during bouts of crib-biting [17,19]. Differences between cribbiters and controls in heart rate variability have also been found in other studies [7].…”

“…Previous studies that have investigated whether stereotypies lowered arousal and anxiety as well as corticosteroid levels, did not find consistent results [7,[17][18][19][20]. If crib-biting is indeed a coping mechanism, animals should show signs of stress in situations in which they are prevented from crib-biting.…”

“…For example, stereotypies may function to cope with high levels of frustration, but the fact that stereotypic behavior often persists after the cause of frustration has been removed contradicts this hypothesis [16]. Another line of argument is that stereotypic behavior functions as a coping mechanism to reduce chronic stress or to provide animals with some form of control over their environments [7,11,[17][18][19]. The main prediction of this argument is that the physiological response of animals should increase if they are being prevented from displaying the stereotypic behavior in response to a frustrating situation [16,20].…”

Comparative analysis of serotonin in equine plasma with liquid chromatography–tandem mass spectrometry and enzyme-linked immunosorbent assay

“…Reported PPP serotonin values in healthy horses range from 2.5 ng/ml to 90 ng/ml with a majority varying between 3 ng/ml and 30 ng/ml. 1,2,4,6,7,13,[17][18][19]21 One possible explanation for this inconsistency is the use of different analytical techniques. Therefore, it is essential to develop fast and reliable detection methods, which are well validated for use on equine plasma samples.…”