A stereotypic behaviour is an aberrant, repeated, relatively invariate sequence of movements, which serves no obvious purpose. These behaviours are common in over 15% of domesticated horses and include cribbing, wood chewing, pawing, head bobbing and weaving, as well as equine self-mutilation syndrome and others. Often, stereotypic behavioural sequences are incomplete forms of functional behaviours, which suggests that they may originate from specific motivational states. Stereotypies usually occur in situations where the individual lacks control over its environment.
Equine self-mutilation syndrome has commonly been confused with behavioural reactions to chronic or psychogenic colic, dermatologic conditions, viral neuritis, epilepsy and hormonal imbalances, which all share similar symptoms. This stereotypy, also coined as "flank biting", has been described as self-biting and kicking out with the hind limbs, and may be associated with bucking, rubbing, spinning, rolling, vocalization, and hypersensitivity to touch. Affected horses may bite at their flanks or limbs as well as at their lateral thoracic wall, pectroral area and tail. These behaviours are usually displayed daily and some horses engage in several episodes per day.
Equine self-mutilation syndrome is most common in Arabian, Quarterhorse and American Standardbred stallions, although it has been reported in both sexes and in a variety of breeds. This stereotypy usually manifests itself by the time the horse is two years old and almost always by the age of four years.
There is good evidence to suggest that equine self-mutilation syndrome, or at least an element of sensitivity to stressors, may be hereditary. The behaviours themselves may have developed in direct respone to some combination of sexual frustration, inactivity and stall confinement, and may simply be a re-directed expression of male aggressive tendencies. Male dominance ritual aggression, for example, is expressed as biting at other horses' necks and limbs; however, in the absence of other horses, this behaviour may be self-directed by biting at one's own neck instead. Hormones may play a role in triggering or in sustaining stereotypic behaviours. For example, pleasure centres in the brain may be stimulated, which could reinforce the behaviour. Also, hormones which deaden the horse's perception of pain may serve to extend periods of repetitive self-mutilation. The onset of this stereotypy has been associated with a variety of factors, which include: elevated levels of excitement or stress, anticipation of feeding, the presence of mares (for males), the smelling of manure or urine, and weather changes.
Since equine self-mutilation syndrome is a stereotypy, which is often precipitated by environmental stressors, the best prevention is to maintain a stress-free environment. This may include spacious turnout, social contact with other horses, and sufficient levels of activity. Diet requirements should be considered on an individual basis and feed should be altered to maintain minimum levels of sweetfeed and pellets and maximum levels of roughage, in response to exercise schedules. Also, prior knowledge of the presence or absence of stereotypic behaviour problems in a horses close relatives may be useful in determing the tendency of an individual to be prone to similar behaviours.
The most important corrective measure is to identify possible environmental stressors and to remove them wherever possible. This may involve changes in the immediate physical environment, such as stable re-arrangement, as well as changes in daily exercise routines. In cases where confinement to a small areas triggers the onset of self-mutilation behaviours, intervention may be as simple as sheltering the horse from these sorts of situations. In addition, the horse's diet should be examined to ensure minimal intake of sweetfeed and grain while providing an adequate amount of roughage. Companion animals, such as a goats, cats and dogs, have often been reported to reduce the expression of stereotypies in horses. When necessary, most affected horses can also be distracted from these behaviours by human intervention, such as by talking in raised voices.
Drugs are not considered practical for long-term therapy since they are rapidly excreted by horses and so have to be administered regularly and are costly. In the future, however, sustained-release drugs may be developed for the ongoing treatment of this problem. For stallions, castrations can significantly reduce sexual and aggressive behaviours and the age at time of castration does not seem to affect these changes.
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Line S.W., Hart B.L., and Sanders L. Effect of prepubertal versus postpubertal castration on sexual and aggressive behaviour in male horses. J. Am. Med. Assoc. 1985; 186: 249-251.
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