Investigation into stereotypic behaviour of the horse

Abstract

Two putative functions of equine stereotypies (crib-biting and weaving) were proposed: 1) `behavioural need', the behaviour substitutes an unobtainable consummatory behaviour (e. g. eating); 2) `reward function', the behaviour counteracts the physiological effects of a stressor.

Several hypotheses were established to test these putative functions within one experimental design. This experiment measured the physiological (heart-rate, plasma cortisol and plasma beta-endorphin) and behavioural responses to performing and preventing equine stereotypies (using the crib-strap and anti-weave bar) in cribbiting (n=4), weaving (n=3) and control horses (n=4). The behavioural effects of administering an opiate antagonist (naloxone) to these horses were also assessed. Results indicated that Hypothalamo-pituitary-adrenal (HPA) activity (plasma cortisol) was significantly higher (p<0.05) immediately prior to the onset of stereotypy followed by a significant reduction and suggested that both crib-biting and weaving function as a `reward function' to reduce stress levels in the animal and not as a `behavioural need'.

The crib-strap significantly elevated (p=0.05) mean plasma cortisol in crib-biting horses and a similar trend (p=0.07) was observed for the weaving group during the anti-weave bar treatment. Both crib-strap and anti-weave bar significantly elevated (p<0.05) plasma cortisol in the control horses. The continued performance of stereotypy during these prevention treatments and the significant physiological changes recorded for the control horses prevented a definite conclusion being drawn with respect to the hypotheses being tested. The results, however, did suggest that the use of the crib-strap and anti-weave bar are stressful to horses and thus may be a welfare concern.

The opiate antagonist naloxone significantly reduced crib-biting by 84% (p=0.07) but not weaving (p=0.37) indicating that the former but not the latter of these stereotypies functions as a reward behaviour. However, resting behaviour was also significantly increased (p=0.02) in crib-biting horses suggesting that the reduction in stereotypy was due to a sedative effect of the opiate antagonist. However, since the effect was not measured in control or weaving animals these results may be interpreted differently.

Finally, plasma beta-endorphin and prolactin (as an indicator of central nervous system [CNS] dopamine activity) levels were not significantly different between stereotypy and non-stereotypy horses. The development of equine stereotypy cannot, therefore, be attributed to substantial differences in CNS opioid or dopamine physiology. However, the low numbers of experimental animals used in this study requires that these results be interpreted with caution.