Equine Stereotypic Behaviour- a neurochemical perspective
SD McBride PhD University of Wales Aberystwyth

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The term 'vice' is often used to describe abnormal equine behaviour which is usually stereotypic in nature. This term can be misleading since it attributes to the horse the ability to make moral decisions as to whether or not to act in an anti-social manner. This affects our perception of these behaviours, normally in a negative context, and has implications about how we subsequently treat horses that perform them. It also results in behaviours that are not stereotypic by definition, being classified as such; common examples being wood-chewing and door kicking. In accordance with the current definition of stereotypies (a behaviour pattern that is repetitive, consistent and has no obvious goal or function), the major equine stereotypic behaviours are crib-biting, weaving and box-walking. It is very important to make this distinction if we are to understand why these behaviours develop and to try and prevent or reduce their performance.

General Causes of Equine Stereotypic Behaviour

Many people have talked in very general terms about the causes of equine stereotypy normally in terms of frustration, boredom or stress of the animal. Only recently have studies started to support this speculation. A study carried out in 1995 indicated that the risk of horses developing weaving decreases when levels of forage are increased (above 6.8kg/day) and that a similar pattern is observed if forages other than hay are given. With regard to the latter, the number of forages given to the horse appears to be important because choice is something that the horse would naturally have in the wild. A more recent study has shown that increasing the number of forages available to the horse (2-3 presented at the same time) reduces the amount of time that it will stand in an inactive state; inactivity being associated with boredom, a considered causal factor of stereotypies, it is apparent why a choice of forages would reduce stereotypy incidence. In actual fact, a crib-biting horse was used in the latter experimental trial no stereotypy was performed when more than one forage was offered.

The risk of abnormal behaviour generally (stereotypies and wood-chewing) has been reported to increase when stable design prevents visual contact with other horses. This observation has been recently supported by another study which tested the effect of different levels of visual stimulation within a stable (number of windows) on weaving behaviour in horses. It was observed that the more windows within the stable design, the less stereotypy the horse performed. A fourth study carried out in Sweden demonstrated that the occurrence of stereotypies was directly related to the amount of cereal-based concentrate and forage given to the horse, as well as the number of horses per trainer. The authors concluded that horses that spent more time eating or were in greater contact with stable staff, were less likely to perform a stereotypy. These studies support the idea that stereotypies arise from frustrated motivation and in fact may be replacement behaviours for those that the animal cannot perform due the environment that it is kept in. For example, oral stereotypies (crib-biting) may be the result of frustrated eating motivation due to restricted or monotonous food supply (insufficient ad-libitum forage or number of forages). Locomotory stereotypies (weaving) may result from frustrated motivation to perform locomotory action to gain food or access other horses. These causal factors are strong indicators of the types of management strategies that could prevent or reduce these types of behaviour.

Neurochemical mechanisms underlying the cause of equine stereotypic behaviour.

Not all individual horses develop stereotypies when placed in the same environment. For example, when the 280 horses held at the Household Cavalry Mounted Regiment all kept in the same husbandry conditions were examined in 1995, only four were reported to crib-bite and three to animals weave. This suggests individual predisposition to performing these behaviours. Inheritance-based studies using stereotypy horses have, however, been unable to come to any meaningful conclusions about genotypic predisposition of equine stereotypy due to difficulties in obtaining data sets of sufficient size. Horses are not, however, the only species to perform stereotypies and much research has been carried out using rodents to try and understand why this predisposition exists. Research to date suggests that the activity level of certain neurotransmitters within the brain (e.g. dopamine) are important in this respect and this is affected by the way the individual animal responds to a stressful environment. The individual animal's genes dictate this latter response.

Recent work has suggested that this same mechanism exists for equine stereotypy where the same differences in neurochemistry as seen in mice have now been reported for the horse. This is preliminary data, however, and further research* needs to be carried out to establish fully the neurochemical profile of stereotypy horses. Such information will help determine whether individual horses are predisposed from the outset to perform stereotypic behaviour.Neurochemical mechanisms underlying the function of equine stereotypic behaviour.

The function of stereotypic behaviour may differ depending on its stage of development in the animal. In the initial stages the behaviour may simply be replacing other behaviours that the animal cannot perform because of the restrictive nature of its environment. However, when the behaviour becomes more established, it is performed during periods which are not normally associated with the replacement of other oral or locomotory behaviours (Figure 1).

Here it is thought that the animal may be using these behaviours to allow it to cope with a stressful or unstimulating environment. There has been a lot of speculation about the mechanism by which this is taking place, much of which has centred on the theory of opioid release (naturally occurring substances equivalent to morphine being released into the brain). Two studies have provided information that may help us understand if this is the case for equine stereotypies. The first study assessed pain thresholds in horses before and after crib-biting. Here it was considered that if opioids are being released during stereotypy, then there should be an increase in pain-threshold after crib-biting compared to before. Results in fact suggested the opposite, that the pain-threshold decreases, suggesting that these behaviours do not seem to function in a coping capacity. Other studies have assessed the behavioural effect of drugs that block the effect of opioids (opioid antagonists). These are pharmaceuticals routinely used to help reduce addiction in humans by blocking neurochemical receptors for opioids. For horses with stereotypies, they cause an 80-90% reduction in stereotypic behaviour. What is important here is that these drugs also reduce other behaviours known to have reward characteristics (i.e. behaviours that produce a sensation of pleasure), such as eating.

It is argued that because these drugs reduce these behaviours and also reduce stereotypy, then stereotypies must also be reward behaviours that could then be allowing the animal to cope with its environment.Interestingly, the majority of crib-biting is performed after eating, and especially after eating foods that are highly palatable (e.g. cereal-based concentrates). In fact the most reliable way to test for a crib-biting horse is to give a small handful of concentrate and observe the immediate behavioural response. Palatable foods also cause opioid release in the brain and there has been some speculation that the crib-biting response is greatest after eating because eating has a 'neurochemical priming effect'. In other words, the neurochemical effect of eating palatable food (through opioid release) primes the brain reward centre such that there is more activity than normal when stereotypy is performed. In this way, equine stereotypies may be similar to other reward-seeking behaviours performed by humans; cigarette smoking for example is rewarding to the participant, however, it is most rewarding when performed after other reward behaviours, such as eating, alcohol and caffeine consumption. All of this evidence points to the fact that stereotypy may be a reward behaviour which allows the animal to cope with an environment that it finds stressful.

Figure 1 Horse with an established stereotypy crib-biting on a field -post. The behaviour has become emancipating from the original causal stimuli.

Equine stereotypy and animal welfare

Crib-biting, weaving and box walking are all equine behavioural conditions that owners have actively sought to avoid in their horses since this issue was first discussed over one hundred and sixty years ago. This is mainly attributable to the clinical effects of these behaviours (e.g. reduced performance, incisor wear, strained ligaments), but also the apparent social stigma associated with the ownership of these supposedly 'vice-ridden' animals.

In general, horse-owners are also under the belief that these behaviours are learned. The result of these two perceptions is that stereotypic animals are currently being subjected to management practices that may be seriously reducing their welfare. For example, in a recent study it was observed that around 32% of riding schools prohibit stereotypic animals from entering onto their premises and on average 39% of racing, riding school and competition yards socially isolate horses which perform stereotypies. This is a practice known to induce a stress response in the horse. In addition, equine stereotypies are physically prevented using devices such as the crib-strap (Figure 2) and anti-weave bar (Figure 3) (77, 67 and 79% of racing, riding school and competition yards respectively), the implications of which are twofold. Firstly, if stereotypies are acting to allow that animal to cope with a stressful environment, then the restriction of such behaviours may prevent activation of the coping response, thereby placing the animal at risk from a welfare perspective. Secondly, even if equine stereotypies are not acting in this capacity, other results have shown that the use of the crib-strap and anti-weave bar are themselves stressful to the animal. Overall, therefore, the current management of animals may be open to criticism if the welfare of the animal is considered important.

Figure 2 Horse wearing a crib-strap preventing contraction of the neck muscles.

Figure 3 Anti-weave bar placed on the stable door to prevent lateral movement of head (weaving).

Practical means of reducing stereotypic behaviour without affecting the horse's welfareOnly some horses are predisposed to performing stereotypic behaviour. Until methods have been devised to identify these individuals, it is difficult to test for husbandry conditions that would prevent the onset of equine stereotypies. However, we can make recommendations to reduce the performance of stereotypies once established, and more importantly, in a way that is not compromising the animal's welfare.· Move away from the discrete meal feeding system (i.e two meals of cereal-based concentrates per day) towards a more ad libitum feeding system. Although this may not be possible for concentrates, it is possible for forage-based substitutes.

· Increase the level of visual contact time with other animals.
· Increase the amount of visual stimulus generally within the stable environment.
· Increase the level of exercise or turn-out

*If you are interested in helping current research into the neurochemistry of equine stereotypy, veterinarians or horse-owners in the unfortunate position of having to put down either stereotypy or non-stereotypy animals can donate brain material by contacting The University of Wales Aberystwyth on 01970 621690 or by e-mailing sdm@aber.ac.uk


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