Yawning, it’s something we all do regularly, sometimes more often than others. But what about our horses? Although horses yawn, it is not something they tend to do as frequently as humans. What does it mean when horses yawn and should we be worried if we notice them yawning more often than before?
Why do humans yawn?
In humans, yawning is typically associated with periods of relaxation (or even boredom), or when there is a change in activity level, for example before sleeping or upon waking. One hypothesis suggests that we yawn to cool the brain to improve concentration and cerebral function. In support of this, it has been shown that people yawn more when the ambient temperature is greater than 37C (body temperature). Furthermore, stress may be involved in triggering a yawn, perhaps due to an increased body temperature associated with anxiety. Rarely, certain illnesses may contribute to an increase in yawning behaviour in people, with migraines and multiple sclerosis being implicated.
Yawning is well known to be contagious in humans, something many of us will have experienced. It is also known to be so in chimps, budgies, pigs, wolves, dogs and lions. One theory to explain this behaviour is that yawning may alert others within a social group of animals that live cooperatively to the fact that an individual is tired. This in turn increases the alertness of the others, as they may need to become more vigilant in watching for predators to compensate for the tired group member. Interestingly one study proved that yawning can be contagious between humans and dogs. Pet dogs respond more frequently to the yawns or their owners than those of strangers. No such link has been shown in horses as yet. Perhaps it is worthy of some investigation next time you are at the stables!
But what about horses?
In horses, yawning has been defined as a “deep long inhalation with mouth widely open and jaws either directly opposed or moved from side to side”. It seems to occur far less frequently in horses in comparison to humans or carnivores. In the vast majority of cases, the occasional yawn will indicate a state of relaxation. However, there may be other reasons behind yawning, particularly if it becomes a more frequently observed action.
Like humans, horses have been shown to demonstrate more frequent yawning behaviour in response to increasing ambient temperatures. One study observed horses that were given free access to a choice of three paddocks, one in full sun, one in shade and one cooled by water. The horses yawned more often while in the sunny field, strengthening the hypothesis that it is linked to a need to cool the brain.
Dominance or aggression
Primates use yawning-like behaviour as a threat to assert dominance or demonstrate aggression, as well as in response to a change in arousal state or stress. In horses, a small research study comparing wild Przewalski horses and their domestic counterparts demonstrated a positive correlation between aggressive behaviours and yawning frequency. This behaviour was more pronounced in entire males in both the wild and domestic groups suggesting a link between testosterone induced aggression and yawning.
Stress or anxiety
Stereotypical behaviour can be defined as a repetitive, unchanging pattern of behaviour with no obvious role or function. Examples in horses include box-walking, crib-biting, weaving or windsucking. Stereotypies tend to be an indicator that the animal is not coping with its day-to-day life, with common stressors usually related to the environment or management such as feeding patterns, social isolation or confinement. Stereotypical behaviour is rare in wild or free roaming horses and is seen as a consequence of domestication. Several studies have shown a link between the frequency of yawning and stereotypical behaviour in domestic horses, with those showing stereotypical behaviour yawning more often.
Horses exhibiting stereotypical behaviours tend to sleep less and one hypothesis suggests that one reason for the increased rate of yawning could be that these horses are more tired.
Another study suggests that social isolation and confinement may contribute to yawning behaviour as it demonstrated an increase in yawning in horses living in restricted stabling environments, when compared to those living in more natural conditions which are less likely to cause stress or frustration.
Just because a horse doesn’t yawn often, does not mean that they are not stressed nor does it rule out poor health. But an increase in yawning behaviour may be a warning sign that all is not well with the environment or management. And these should be considered during investigations. It is also worth noting that some stereotypies become habits over time. So established behaviour may still exist even if welfare is positive. A newly emerging behaviour is often more concerning and may respond better to intervention than stereotypies that have been present for a long time.
Some medical conditions, most notably gastric ulceration, are linked to stereotypical behaviours.
Gastric ulceration tends to be associated with oral behaviours such as crib biting or wind sucking. Yawning is linked not only to these but also to many more stereotypical behaviours, such as box-walking. Stress is certainly an important contributory factor in gastric ulceration. But it is not the only cause, with diet and exercise playing an important role. The clinical signs of gastric ulceration tend to be quite vague with a long list of symptoms such as weight loss, crib biting, poor performance and colic all implicated. Yawing is not a commonly encountered clinical indicator of gastric ulceration. But stressed horses are more prone to both yawning and ulceration; so it is often sensible to rule out gastric ulceration, if possible.
Frequent yawning can also be a clinical sign of advanced liver disease, but is usually associated with Hepatic Encephalopathy, where changes occur in the brain due to high circulating levels of toxins that would normally be removed by the liver. However, this, like gastric ulceration, usually occurs in combination with other clinical signs. Clinical signs associated with Hepatic Encephalopathy include depression, ataxia, excessive walking, drowsiness or excessive yawning. In isolation, an increase in yawning alone is unlikely, but not impossible to indicate the presence of severe liver disease. Hepatic encephalopathy is normally confirmed by blood testing.
If your horse is suffering from dental disease, particularly sharp enamel points on the edges of the teeth, or periodontal disease where food becomes trapped between the teeth, they may start to open their mouth more frequently and move their jaws in an attempt to alleviate some of the associated discomfort and to remove trapped food. Whilst this is not considered to be true yawning behaviour, it can easily be mistaken for it. Your vet may perform a dental examination if you are concerned about yawning like activity in your horse.
If you’re worried…
If you have any concerns about your horse’s behaviour, particularly if you feel that your horse’s behaviour has changed or they seem to be yawning more frequently, always consult your vet. A full clinical examination, with a discussion about your horse’s history, circumstances and behaviour may reveal if problems, behavioural or medical, are present. In some cases, your vet may recommend further investigation such as blood tests, a dental examination or a gastroscopy to rule out any of the causes mentioned above. Your vet will also be able to offer advice on environmental enrichment or housing if they feel that stress may be a contributing factor.