In this article, Dr Rachel O’Higgins examines the current understanding of stereotypies – a group of behaviours which are commonly referred to as ‘stable vices’. As Dr O’Higgins explains, horse owners must change the way they think about stereotypies; instead of being offensive, they need to be recognised as a horse’s coping mechanism induced by frustration and brain dysfunction.
Instead of attempting to stop horses from displaying these stereotypic behaviours, we must recognise the management practices that cause the horse stress and leads to their development, so we can be sure to avoid them.
The term ‘vice’ should not be used to describe these behaviours as it implies there is something morally wrong with the horse. Stereotypic behaviours include weaving, crib biting, windsucking and box walking, amongst others, and are defined as a repeated movement with no obvious function, either induced by frustration or brain dysfunction. These repetitive behaviours can be seen commonly in zoo animals, which pace up and down the fence, or caged birds, which pluck out their own feathers.
What are they?
Stereotypies are abnormal behaviours and rarely occur in animals living in their natural environment. Lack of companions, unnatural feeding, and inability to make normal movements and exercise all increase the likelihood that stereotypies will be shown.
Stereotypies must, therefore, be considered as behavioural problems, which are a result of trying to cope with a problem that is causing the animal frustration. One study found that around 15% of Thoroughbred racehorses have stereotypies. In another study of 1,750 horses, stereotypies were more common in dressage and eventing horses, and were also more common in horses that spent more time stabled.
Wind-sucking – or ‘aero-phagia’ – involves the horse opening its mouth, contracting the pharyngeal musculature (muscles of the throat), flexing the neck muscles and sucking air into the proximal oesophagus, usually with a characteristic ‘grunting’ sound. When a horse is crib-biting, this action is accompanied by gripping with, or resting, the upper incisor teeth on a solid object, such as a stable door or fence post. Once acquired, the behaviour is extremely persistent and can occur in environments very different to where it first arose (e.g. a horse that started wind-sucking in the stable may also start to do it in the paddock).
Do they affect the horse’s health?
All stereotypies have an energy cost, which may be high enough to result in weight loss in some circumstances. Some horses may choose to spend a reduced amount of time eating in order to crib-bite or wind-suck, and this can also result in weight loss.
Reports suggest a link between wind-sucking and spasmodic colic. This hasn’t been proven, although an association has been made with a type of colic where the small intestine becomes trapped in a space in the abdomen called the epiploic foramen. The majority of the time, however, there are no perceived adverse effects of stereotypic behaviours.
Contrary to popular belief, crib-biting and wind-sucking horses don’t actually swallow much air. Radiography of horses as they were crib-biting showed there was no movement of the tongue as one would expect in true swallowing. Instead, each horse showed an explosive distension of the proximal esophagus that prompted no peristalsis (the wave-like muscular contractions that go with swallowing). Much of the air exited the proximal esophagus between crib-bites by returning through the cranial esophageal sphincter into the pharynx (the air exited out of the open mouth). This may explain why tympanitic colic (abdominal pain associated with wind or flatulence) is not seen in all cases of crib-biting horses.
A man-made problem
Evidence suggests horses have been crib-biting and wind-sucking since their domestication 15-30,000 years ago and the first written record of stereotypies appeared in a French text in 1609.
Stereotypies are, therefore, thought to be a result of domestication and changes in the way the horse is kept. The horse is designed to spend over 50% of its day grazing and, in the wild, will live in large, open spaces in stable social groups.
The consequences of stabling brings about a lack of social contact, restriction from normal movement and ‘idleness’ because of the use of cereal-based concentrates instead of forage, thereby reducing the horse’s natural feeding time and feeding behaviour.
Studies have shown each of these factors bring about an increase in the likelihood of a horse performing a stereotypy. If animals that show stereotypies are put into improved conditions, they usually show a reduction in occurrence of the stereotypic behaviour.
Studies have also identified weaning as a critical point in the development of oral stereotypies. Two thirds of wind-suckers and crib-biters start within one month of weaning; at the same time as diet and environment are substantially altered.
Causes and motivation
Owners of horses that are established wind-suckers or crib-biters often notice the behaviour is displayed immediately after eating. Palatable, high-cereal feeds can cause an increase in the acidity of the stomach and, it has been speculated, that increased acidity and abdominal discomfort are a trigger for crib-biting and wind-sucking behaviours. Production of alkaline saliva when wind-sucking may help the horse to neutralise stomach acid and, therefore, reduce discomfort.
In addition, some studies have found antacids, as well as acting to decrease gastrointestinal inflammation and ulceration, reduce the wind-sucking behaviour in some horses.
An alternative explanation for stereotypic behaviour in horses relates to stress-induced alterations in central nervous system (brain) dopamine physiology. Dopamine is a neurotransmitter; a type of chemical released during stereotypic behaviour that helps control the pleasure and reward centres in the brain.
Horses that wind-suck have been found to have significantly higher numbers of dopamine receptors in the area of the brain that controls goal-directed behaviours associated with attaining rewards and avoiding negative stimulus.
If the horse’s environment doesn’t allow it to attain goals (movement, socialising, feeding, etc.), the animal is maintained in a heightened state of goal-attainment. It is this ‘appetite phase’ of goal attainment that is the basis for stereotypy development in some horses.
As with other species (in particular humans), some forms of brain damage could potentially be a cause of stereotypies in the horse. However, this is rarely, if ever, the cause of the stereotypies that we commonly see.
Stereotypies are not ‘contagious’
Owners often regard wind-sucking as ‘contagious’, believing other horses will copy the behaviour. As such, these horses are often confined away from other animals on the property because their monetary value is usually less than a similar animal free of any stereotypies.
There is no evidence to support that stereotypies are contagious. Development of stereotypies does not normally occur at pasture and it is unlikely that wind-sucking behaviour would be copied in these circumstances. When stabled horses show stereotypies, they must have frustrations over their environment, whether or not they have initiated that behaviour by copying others. Equally, the idea there is a genetic component to stereotypies is unproven. It is likely this idea stems from related horses being kept in similar housing and management conditions.
Can horses be cured?
Various methods to prevent wind-sucking and crib-biting have been tested and, so far, a cure remains elusive.
A crib-strap or cribbing collar consisting of a strap around the ears and neck, with either protruding leather or two pieces of jointed steel hinged together at the throat.
The collar works by tightening around the pharynx as the horse attempts to arch its neck. These collars don’t work for every horse and, in one study, horses prevented from crib-biting with a collar for 24 hours resumed the behaviour at a higher rate than before, when the collars were removed.
The horses also seem to adapt to the collars, which then require continued tightening, and can result in skin trauma and potentially serious injury.
Aversion therapy has also been attempted with electric shock collars. Despite promising early reports, only nine horses out of 60 were ‘cured’ and three of these required reinforcement therapy after just nine months.
Three surgical methods to prevent wind-sucking have been described.
The first is a neurectomy, which involves removal of the surrounding nerve tissue of the sternomandibularis muscle of the neck. The second is a myectomy, which involves the removal of omohyoid and sternohyoideus neck muscles.
These surgeries work by preventing sufficient arching of the neck, but are not always successful and can cause severe frustration to the horse as they are then unable to carry out certain movements.
The last surgery is a buccostomy and involves making a hole through the horses cheek into its mouth. The idea is that the horse requires a ‘seal’ to suck in air. This surgery often doesn’t work, causes disfigurement and regularly heals over anyway.
Both surgery and collars, in an attempt to reduce the occurrence of wind-sucking and crib-biting, result directly in poorer horse welfare and do not lead to permanent improvement, even if welfare is ignored, so they should be discouraged.
Hanging stable-toys, balls and licks to reduce ‘boredom’ in the horse are often sold to owners to reduce stereotypies. However, there is very little evidence to suggest they reduce ‘boredom’ and the stereotypic behaviour that supposedly results from it.
However, devices that stimulate natural foraging behaviours, like the ones that release small amounts of food as the animal spends time pushing them around the stable, can help to reduce the time spent performing stereotypic behaviours.
Transient elimination of crib-biting has been reported in subjects treated with certain drugs also used to treat alcohol and drug addictions in humans. These work in the brain to reduce the heightened state of ‘goal-attainment’ discussed earlier. The study used nalmefene in a continuous infusion for one week and, for that week, no cribbing was observed. However, daily injections would be expensive and are not practical, and the behaviour returns when the drug is stopped.
It is the causes (motivation) of wind-sucking/crib-biting, rather than the action, that should be treated.
1. Let them do it
Remember that stereotypies are a coping mechanism, so unless the horse is injuring himself or is at high risk of colic, he is unlikely to benefit from attempts to block the stereotypy. Both, the horse’s physical and psychological welfare should be the primary considerations.
If you have a crib-biter, provide him with a non-abrasive surface they can bite (e.g. cover a rail with rubber) and discourage him from biting other surfaces (e.g. with electric tape or a taste deterrent).
2. Improved housing
Long-term, the goal should be to improve housing and management. Housing should provide opportunity for movement and a variety of stimulation. The horse should have adequate space to exercise, ideally in the area they are kept or in an area they can access daily for intervals.
3. Social contact
Full contact with other horses should be maximised, preferably by being kept in groups or with a companion, and being able to touch and see other horses when they are required to be stabled.
4. Constant access to forage
Nutrition should be adequate, and quality forage that takes a long time to collect and eat should be provided. Restrictor haynets (slow-feeding haynets) are a great way to extend foraging time. However, having to work harder for food may frustrate some individuals, and become an additional and counter-productive stressor.
5. Other strategies
Other strategies for reducing the behaviour involve altering dopamine transmission. This can be achieved with drugs, like nalmefene, or without drugs through ad libitum feeding, increasing periods of turn-out and socialisation, and also with acupuncture. Interestingly, acupuncture, which reduces dopaminergic activity in the brain, has been shown to reduce crib-biting and wind-sucking in some horses.
Take home message
So far, a cure for stereotypic behaviours remains elusive. Instead of concentrating on prevention, we should be working to avoid the development of stereotypies by improving the welfare, housing and management of our horses from weaning to retirement.
This case study is reprinted with permission from ‘Equine Behaviour, A Guide for Veterinarians and Equine Scientists’ by Prof. Paul McGreevy.
A four-month-old Warmblood colt was spotted repeatedly grasping at the top of a gatepost and occasionally licking it while the rest of the group, including its dam, idled in the same corner of the paddock. On closer scrutiny, the owners found no evidence of wood-chewing and did not detect a grunting sound with the behaviour. The foal was not receiving creep feed, but the mare, the highest-ranking adult in the group, was receiving concentrated feed every evening. None of the four adult horses in the paddock showed similar behaviours.
Video surveillance was used for this case. Because of its repetitive nature, the invariantly-arched neck posture and the appetitive licking associated with it, the behavior was diagnosed as an early form of stereotypic crib-biting. The grasping behaviour was regarded with concern, because of the likelihood of its becoming stylised into crib-biting after weaning, so the following measures were taken to reduce management factors associated with its appearance.
The action plan
All gateposts in the paddock were coated very generously with a taste deterrent. The fence line was protected with a single line of electrified wire. The pasture in the paddock was short, so hay was fed to the group on a daily basis.
The hay was delivered to the centre of the enclosure, rather than being placed near the gateway where concentrates had previously been offered. This was intended to break down any association between eating and reaching for the gatepost. The hay was placed on the ground in the short-term to avoid there being any uprights in close proximity to the feeding horses and to normalise their posture.
Hay from three farms was sourced and samples of each were offered in seven separate piles, which allowed the five horses a pile each and two forage sources to choose from. Alternating the source of the hay in adjacent piles meant the mare and foal had a choice of forages. When the pair were brought in for supervised supplementary feeding of the dam, it became clear she was dropping a considerable amount of grain while chewing. Great care was taken to ensure that no dropped grain was left lying around for the foal to consume. Meanwhile, the mare received dental treatment that resolved the quidding.
Before weaning was attempted, a mare-and-filly-foal dyad was located by means of an advertisement in a local newspaper and brought to the paddock. The foals bonded extremely well and, one month later, the visiting mare was removed. Two weeks later, the resident mare was removed and the colt was observed carefully for signs of the grasping response. None were seen. The pair of weanlings were kept at pasture throughout the Winter.
Ad libitum hay feeding was maintained and small amounts of concentrated food with a dietary antacid supplement were introduced very gradually. Although the amount fed to the youngsters was small, it was nonetheless divided into three feeds to reduce its effect on gastric pH. Four years later, the colt’s behaviour remains normal. The owners have not challenged him with traditional feeding regimens and intend to keep the time he spends in the stable to a minimum. None of the dam’s subsequent foals have shown signs of similar behaviours.