Stressors that Affect Cribbing Horses - A Literature Review
"Cribbing is a common vice in domestic and captive horses, although there is no official documentation of it in free-roaming wild horses or other species"
Cribbing is a common vice in domestic and captive horses, although there is no official documentation of it in free-roaming wild horses or other species (Waran, Clarke et al. 2008, Wickens and Heleski 2010). Cribbing horses latch on to an object, usually wooden, with their incisors and contract their neck muscles to draw air into their oesophagus (McGreevy and Nicol 1998, Wickens and Heleski 2010). This behaviour has been labelled a stereotypy as it is a repetitive behaviour that appears to serve no benefit to the horse (Hausberger, Gautier et al. 2007, Clegg, Buckley et al. 2008, Waran, Clarke et al. 2008). It has been hypothesised that cribbing, among other stereotypies, develops as a result of unnatural horse-keeping practices (Clegg, Buckley et al. 2008, Wickens and Heleski 2010). It is also important to note that the presence of stereotypic behaviour has often been used as an indicator of poor animal welfare (Mason 1991, Wickens and Heleski 2010). It is vital to understand the factors that contribute to this behaviour so to analyse the welfare implications (Cooper and Albentosa 2005).
This particular stereotypy may be referred to as cribbing, crib biting, windsucking, or aerophagia, often interchangeably although many authors disagree on the definitions of each individual term, as horses may inhale air without grasping on to any objects, and they may grasp on to, chew, or eat wooden objects without inhaling any air. This behaviour may account for 15 - 65% of the horses daily time budget (Wickens and Heleski 2010) and stereotypies occur in up to 15% of horses (Sarrafchi and Blokhuis 2013).
Why is this behaviour an issue?
Cribbing horses are at risk of temperomandibular osteoarthritis, inhaling or swallowing wood, thriftiness or poor weight, colic and abnormal dental wear (Clegg, Buckley et al. 2008, Wickens and Heleski 2010). Horses have difficulty stopping this behaviour even when it causes them problems or pain, similar to humans who engage in repetitive self-biting (Mason 1991). Once established, it is highly addictive (Nagy, Bodo et al. 2010). They may be required to leave boarding facilities due to the damages they do to wooden fixtures such as fences and stable doors and also due to a misconception that they might teach other horses to crib despite no evidence that horses learn this behaviour from conspecifics (Clegg, Buckley et al. 2008, Wickens and Heleski 2010). Surveys of horse owners showed that some owners felt their horse had started cribbing after another cribber had arrived at the barn, however it may simply be that these horses were now being exposed to the same triggering factors (Wickens and Heleski 2010). This tendency for owners to believe it is a learnt behaviour may also contribute to them overlooking the management practices that could be responsible (Normando, Meers et al. 2011).
The perception of cribbing horses can differ greatly among horse owners who have, or who have never, owned one. Owners who have had a cribbing horse are less inclined to view it as a problem (Wickens and Heleski 2010).
Many owners attempt to prevent their horses from cribbing by use of a cribbing collar, or making sure no suitable surfaces are available to their horse or by electrifying any suitable cribbing areas (McGreevy and Nicol 1998). In some instances the horse may undergo radical surgery to the omohyoid and sternothyrohyoid and associated nerves (Wickens and Heleski 2010). Surveys of the owners of cribbing horses indicate that 81% of them attempt to stop the behaviour (Wickens and Heleski 2010). The major issue of attempting to address cribbing in this manner is that it fails to identify and correct the underlying causes of the behaviour, which is important considering stereotypic behaviour may be an indicator of current or past suboptimal conditions (Cooper and Albentosa 2005, Wickens and Heleski 2010, Sarrafchi and Blokhuis 2013). Further, the horse may not respond favourably to physically preventing cribbing. For example, a horse may still attempt to crib even with a cribbing collar attached, to the point where the horse might develop pressure sores (Mason 1991).
Stressors and Other Factors Affecting Cribbing Horses
Horses naturally eat a high fibre diet of roughage, however it is common practice to feed horses more concentrated feeds especially when grazing access is restricted or unavailable (such as small paddocks or stables). Elia et al. (2010) discovered that horses that are fed high concentrated feeds develop “roughage hunger” and will work harder to obtain hay, indicating that there is a high motivation for fibrous feeds. Although the horses needs are met in domestication or captivity there still might be a psychological need to display normal feeding behaviours even if there is no physical need (Cooper and Albentosa 2005), although it appears the requirement for a more natural diet may be more complicated.
Numerous studies demonstrate that feeding high concentrate, and in particular sweet, feed increases cribbing frequencies (Gillham, Dodman et al. 1994). This may be due to a change in gut pH, due to irritation of the gut or abnormal digestion and metabolism in cribbing horses, or due to a release of endogenous opioids (Gillham, Dodman et al. 1994, Whisher, Raum et al. 2011) similar to the effect of chocolate on humans (Gillham, Dodman et al. 1994). The study, above, by Elia et al. (2010) found that faecal pH was more acidic in horses fed concentrated diets compared to hay, and that horses chew more when eating hay. In a study by Gillham et al. (1994) cribbers were fed hay cubes, high protein pellets, or sweet feed and it was found that sweet feed and pellets caused an increase in cribbing behaviour, relative to the control period in which no feed was given. Similarly, Whisher et al. (2011) found that horses cribbed more frequently when fed sweetened grain than when fed plain oats. Gillham et al. (1994) also found that ß-endorphin levels in cribbing horses were half that of control horses. And while they found no interaction between diet and a significant change in ß-endorphin levels, they postulate that perhaps due to having such lower levels of ß-endorphins they may actually be more sensitive to tiny and otherwise seemingly insignificant increases in endorphins from the consumption of palatable feeds. However, McGreevy and Nichol (1998) found that there was no difference in baseline ß-endorphins between control and cribbing horses, but by physically preventing cribbing the horses ß-endorphin levels would actually increase above those of control horses. As such the role of ß-endorphins in cribbing horses and its association with diet is unclear (and differences between studies may stem from the management of the horses during or immediately prior to the trial - Gillham et al. (1994) imply their horses were fed concentrates pre-trial but it was withheld on the morning of the study, McGreevy and Nichol (1998), however, did not feed their horses concentrates prior to the trial and feel that this may have caused their results to be different). The effect of concentrates, however, has been replicated and consistently shown to increase cribbing behaviour.
Clegg et al. (2008) showed that cribbers also took longer to consume their rations than control horses, and that cribbers displayed the most during and immediately preceding concentrate feeding. Cribbers would stop eating to engage in a bout of stereotypic behaviour suggesting that there may be a link to trying to normalise gut function or abdominal discomfort from causes such as gastric ulcers or hindgut acidocis (Clegg, Buckley et al. 2008, Wickens and Heleski 2010). Another study showed that the provision of an antacid diet reduced the frequency of cribbing behaviour (Moeller, McCall et al. 2008, Waran, Clarke et al. 2008).
Stable kept horses are more likely to engage in the behaviour than pasture kept horses. This is a multifaceted issue in itself as stable kept horses do not have access to grass or conspecifics and therefore it may not even be about the stable. The stable environment is also the antithesis of a horse’s natural environment and many do not meet the needs of the animal (Leme, Parsekian et al. 2014). The increase in the behaviour may be due to lack of constant grazing affecting the gut, frustration at inability to express normal grazing behaviour, or increased boredom as the horses daily time budget is altered by having no access to constant roughage and being more likely to be fed concentrated feeds and thus requiring less hours of eating per day to reach their calorie requirements (Elia, Erb et al. 2010, Whisher, Raum et al. 2011, Sarrafchi and Blokhuis 2013) (see figure 1).
Figure 1: The time budgets of horses fed a complete pelleted diet of free choice hay. Arrows mark the feeding times (Elia, Erb et al 2010).
Use of a toy in the stable may help to reduce cribbing frequency, but in an experiment by Whisher et al. (2011), where horses were tested with several types of commercially available toys, only the Tongue Twister® had a significant effect on cribbing frequency. The Lixit®, Snak-a-Ball®, and Boredom Breaker® had no significant effect. In this study some of the toys were not used by the horses at all.
As touched on above, horses are generally stabled individually. However a study has shown that cribbing decreases when horses are stabled with another individual in the same box as them (Sarrafchi and Blokhuis 2013). The behaviour seems to decrease when housed in large group paddocks as opposed to small private paddocks. Increased competition may mean they need to spend more time eating to access the same amount of food, or that the stress of solitary housing affects this herd animal enough to increase stereotypic behaviour. It may also be that the horse feels a need to form and choose its own social groups as it would in a wild scenario (Cooper and Albentosa 2005). Housing horses in a way that they can make physical contact with conspecifics seems to decrease the incidence of cribbing, but a viable alternative may be the provision of mirrors in the stable which shows a similar effect (Cooper, 2005). Regardless of the mechanism, social isolation certainly seems to have an effect on displays of stereotypic behaviour in horses, and also in other species such as primates, parrots and laboratory birds (Sarrafchi and Blokhuis 2013).
Stable weaning of foals also seems to produce a higher incidence of cribbing than weaning of foals in large group pastures (Cooper and Albentosa 2005). A review of weaning by Waran et al. (2008) discusses that horses naturally wean at 8 - 9 months of age, but domestically this typically takes place at 4 - 6 months of age. After weaning the foal and mare usually maintain a close social bond in a wild herd, often until the foal reaches sexual maturity. However, in a domestic setting the mare and foal are often separated suddenly and placed in stables far apart where they can neither hear nor smell each other, and at the same time the foal in introduced to a completely new, and often novel, diet. Weaning prematurely may also mean that the foals gut is not yet mature enough to be able to cope with these new feed substitutes.
It has been suggested that after the first month of weaning the majority of equine stereotypies have already started to develop within the foal (Waran, Clarke et al. 2008). Cribbing behaviour may develop before weaning, but appears to increase in incidence rapidly at this crucial point. Foals that tended to develop cribbing behaviours had spent more time suckling and nuzzling prior to weaning than those that did not. Cribbing foals also tended to show more gastric inflammation than non-cribbing foals (Wickens and Heleski 2010). As touched on in other sections, feeding and social isolation also have a strong link to cribbing behaviour (Hausberger, Roche et al. 2008) and weaning is a notable time when a foal will first experience being boxed on its own and being given provisions of concentrate meals (Waran, Clarke et al. 2008). Wickens and Heleski (2010) note that feeding concentrates to young post-weaning thoroughbreds (and part thoroughbreds) made them four times more likely to develop cribbing behaviours than controls. Conversely, gradual weaning and weaning at grass instead of in stables makes the behaviour less likely to occur. Taking these factors into consideration, these foals might be genetically different to peers considering they have a difference in feeding behaviour pre-weaning and stereotypy development, or it may commence due to the first encounters with other nutritive and social triggers, or a combination. Further studies in this area would be beneficial to understanding its development.
Different work-loads and work types seem to influence cribbing behaviour (Sarrafchi and Blokhuis 2013). The way a horse is ridden may cause physical discomfort or chronic stress to the horse (Normando, Meers et al. 2011). Horses ridden in the English style proportionately show more stereotypic behaviour than horses ridden in other styles. These horses are typically ridden with greater rein and leg contact with the rider asking for poll flexion, but this may be confounded by an increased likelihood to be exposed to other triggers such as stabling and feeding concentrates in that style (Normando, Meers et al. 2011). For example, horses may be ridden in hacking in a Western or English style, but even in the discipline of hacking English-style horses were still more likely to display stereotypic behaviour than Western-style hacking horses. This issue may also be confounded by pain factors, with English style horses more likely to be suffering from locomotion issues such as lameness than non-English style horses. Similarly, within the English disciplines it was found that racehorses and dressage horses displayed more cribbing behaviour than those used for other sports such as pleasure or endurance (Whisher, Raum et al. 2011, Sarrafchi and Blokhuis 2013). Again, this may be confounded by the management differences between these sports rather than the differences in work and workload or, maybe, an interaction effect between them.
A study by Whisher et al (2011) exercised cribbing horses for different lengths of time and found that exercise of 25 minutes significantly increased the rate of cribbing compared to control cribbers who did not receive exercise. However, the exercise did not affect other parameters measured such as lying, standing or eating of hay. Horses may actually have perceived the act of being exercised in the experimental manner of free lunging in a round pen to be a psychological stressor, rather than the physical stress of the exercise itself.
Difference in Brain Anatomy and Function
Some studies indicate that there may be a link between cribbing and altered neuroendocrine physiology and brain function (Wickens and Heleski 2010). There is much speculation that stereotypic horses may have lower learning capabilities. Hausberger et al. (2007) subjected control and stereotypic horses to a novel learning task and found that stereotypic horses took longer to learn the task, and once learned they took longer to execute the task. They note also that stereotypic horses spend less time lying down and sleeping than control horses and speculate that this may be indicative of different attentional processes. Similarly, one study reports that crib biting horses are more susceptible to stress than non-cribbing controls, and that they are physiologically and psychologically less flexible than control horses (Bachmann, Bernasconi et al. 2003). However, Waran et al. (2008) notes that early training practices can affect later life trainability. Some of these differences in training practices may be overlapping with factors that cause a higher incidence of cribbing such as weaning methods. Further studies in this field would be necessary to determine any correlation and causation.
While the areas of brain function involved in cribbing are not well understood, in other species the mesoaccumbens, basal ganglia and nigrostriatal system seem to be involved with stereotypies (Hausberger, Gautier et al. 2007). Cribbing horses were found to take a longer time to reach extinction of a taught behaviour during non-reinforced trials than controls (that is, they would continue to press a button despite no longer receiving a food reward for longer than non-cribbers, see figure 2) (Wickens and Heleski 2010) which may be indicative of basal ganglia dysfunction (Hemmings, McBride et al. 2007, Wickens and Heleski 2010).
Figure 2: Mean number of total button (±S.E.M) presses for total extinction for control and stereotypy animals (Hemmings, McBride et al 2007).
There is recent evidence to suggest that cribbing horses have a different number of dopamine receptors than non-cribbing horses (Hausberger, Gautier et al. 2007). Cribbing horses have a higher density of dopamine receptors in the nucleus accumbens, but lower in the caudate nucleus than non-cribbing horses(McBride and Hemmings 2005). Some studies mention the commonality between equine stereotypic disorders and human obsessive compulsive disorder, or OCD (Gillham, Dodman et al. 1994, Wickens and Heleski 2010). In humans serotonin reuptake inhibitors have shown to help OCD and in horses they have been shown to reduce stereotypic behaviour. However this may be due to a general sedative effect rather than actually influencing behaviour (Wickens and Heleski 2010). As mentioned in the section “Dietary Stress”, several studies have noted a link between endogenous opioids and cribbing behaviour, which may also have a link in human OCD (Gillham, Dodman et al. 1994, McGreevy and Nicol 1998, Wickens and Heleski 2010).
There may also be a complex relationship between pain from other issues such as gastric inflammation and brain physiology through the reprogramming of the basal ganglia (Hemmings, McBride et al. 2007) and changes in dopaminergic activity (Hemmings, McBride et al. 2007, Wickens and Heleski 2010). A study in mice found that stress induces changes in mesoacummbens and nigrostriatal dopamine receptor densities (Cabib, Giardino et al. 1998). If a similar effect occurs in horses, it may mean that stress is affecting brain biology, rather than the other way around, or a combination or feedback of both. So in this manner cribbing may not occur as an attempt to aid gut discomfort, but rather because long-term gut discomfort leads to a change of the basal ganglia which causes the increase in cribbing in these horses (Hemmings, McBride et al. 2007).
A survey in 1986 investigated over 1000 thoroughbred horses and interviewed 80 trainers (Vecchiotti and Galanti 1986). They found that in families where anomalous stereotypical behaviour was identified the incidence was much higher than in the general population, including the incidence of cribbing. This indicates it is not just cribbing but other problems such as stall walking and weaving that may have a genetic component and highlights the need to carefully consider breeding from these animals. This could have further ramifications for the resale and market value of these animals if they are no longer suitable breeding prospects from this perspective. Thoroughbreds in general tend to be 3.1 times more likely to develop cribbing behaviours than other breeds (Wickens and Heleski 2010, Whisher, Raum et al. 2011), however one must consider that these are also the horses most likely to be exposed to other risk factors such as abrupt weaning, stabling and feeding of concentrates (Whisher, Raum et al. 2011).
This behaviour can be seen in mares, geldings and stallions, however it is most common in stallions (Wickens and Heleski 2010). Stallions are, however, the most likely of the three groups to be individually housed and therefore fed concentrated feeds in an effort to prevent accidental breeding and protect valuable animals. Consider, too, that stallions are also more likely to play with toys than mares or geldings (Bulens, Van Beirendonck et al. 2013), so this does indicate that there may be some difference in attention or boredom levels between genders and also that the use of toys might be more successful for reducing cribbing behaviour for stallions and less successful for mares or geldings.
Cribbing as Coping Mechanism for Stress
Many authors describe cribbing as potentially being a way for the horse to take control of its environment and reduce its stress levels. When cribbing is prevented a raise in heart rate and cortisol may be seen, and when cribbing is allowed again horses will crib excessively for a period of time. Cribbing horses will also work hard to obtain suitable cribbing surfaces, and studies suggest that some horses may value cribbing about as much as they value food (Wickens and Heleski 2010). However a counter argument to this is that rather than cribbing decreasing stress, this kind of experimental design may simply be proving that prevention of an already learned behaviour is stressful and may cause frustration and anxiety that the horse cannot perform a behaviour that it’s highly motivated to perform (McGreevy and Nicol 1998, Cooper and Albentosa 2005). Further, a study by McGreevy and Nichol (1998) did not show any rise in cortisol by preventing the horses from cribbing.
Cribbing is a complex and multifactorial issue. Whilst management practices traditionally steer towards physically preventing the behaviour, this mentality ignores the underlying causes of the disorder which may be indicative of present or past poor welfare. Currently “traditional” practices such as early weaning, stabling, concentrate feeding and possibly even the way horses are ridden all contribute to the exacerbation of this disorder. As such, management should aim towards a holistic approach of providing a more “natural” lifestyle for the domestic horse with later, gradual weaning in to groups, increased foraging provisions, access to conspecifics and increased turnout time. Further research in to the mechanisms that cause the behaviour such as neurobiology and husbandry practices are required, particularly a large scale study of foals prior to the onset of the behaviour to determine commonalities. This will enable Equine Scientists to come up with best-practice management systems to hopefully help prevent the stereotypy from starting in the first place, especially when considering the addictiveness of the behaviour and the inability of the horse to stop despite the behaviour potentially being detrimental to its health.
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Copyright Sarah Kuyken, no reproduction in whole or part without written permission. This article is not a substitute for qualified and personalised veterinary advice.