What is stress?
Stress is the body’s response to a potentially threatening situation (AKA a stressor)1,2. Horses can be exposed to short term acute stress such as a plastic bag flapping in the wind, a companion walking out of sight, or travelling3,4. Exposure to an acute stressor may cause a variety of physiological stress responses (for example changes in heart rate, temperature, breathing rate and hormonal responses) and behavioural stress responses (for example fleeing, biting or kicking).
Acute stress isn’t always bad and responses are generally beneficial in aiding the horse to learn and adapt to its environment which increases survival. However, if the problem persists, particularly if it involves situations that the horse is unable to predict or control, then long term chronic stress can occur5. Signs of chronic stress include gastric (stomach) ulcers, a reduced immune response (immunosuppression), weight loss and the development of stereotypical behaviour6 (see figure one).
Figure one: a horse crib-biting
Some domesticated horses are faced with different challenges compared to horses living within a free-range environment7. Challenges include reduced foraging opportunities, social isolation and early or abrupt weaning8 which can all lead to chronic stress. This stress may trigger the development of stereotypic behaviour which enables the horse to cope (coping mechanism) in an environment that doesn’t meet their needs. This is known as a suboptimal environment9.
Stereotypic behaviours are repetitive10 and are consistently the same each time. They can be caused by a combination of factors including frustration, repeated attempts to cope with a suboptimal environment, genetics11 or due to dysfunction of the nervous system12. The most common stereotypies in horses are:
1) Crib-biting - the horse grasps on a hard surface, arches his neck, pulls back and sucks in air.
2) Wind sucking - similar to crib-biting but the horse sucks in air without grasping onto a surface.
3) Box walking - the horse continuously paces or walks in circles around their stable.
4) Weaving - the horse will most often stand at the stable door and shift his weight from one front leg to the other quickly swinging his head and neck from side to side.
But why do certain horses develop stereotypies when others do not- even if they are kept in the same way?
Individual differences - coping styles
Horses not only show individual differences in relation to breed, age and gender but also in terms of behaviour patterns. Whether a horse perceives a stressor to be threatening and how the horse copes and responds to a suboptimal environment depends on the horse’s temperament13. The response to stress can be described in two ways;
- How much stress the horse can cope with (stress sensitivity/threshold)
- How the individual horse tries to minimise stress once their stress threshold has been reached; this response is determined by their coping style14.
Coping styles exist in the horse to try and minimise stress and can be defined as a set of behavioural and physiological stress responses which are consistent over time15,16. There are two coping styles; proactive copers who use active responses such as the “fight or flight” response to escape or remove the stressor and reactive copers who tend to freeze, show no obvious signs of being affected or possibly become withdrawn or depressed17.
Proactive copers form habits in a similar way and show common behavioural and physiological traits to stereotypic horses. This could suggest that proactive horses are predisposed to developing stereotypical behaviour during times of chronic stress. However, whether a proactive individual develops stereotypical behaviour depends on their environment and stress threshold (see Ijichi et al, 2013 for a review).
Reactive copers that have become withdrawn or ‘switched off’ show behaviour indicators such as ears pointing backwards, neck at the same level as their back, decreased eye contact and a ‘glazed’ expression (see figure two). They may also show reduced responsiveness to touch, less exploratory behaviour, inactivity and show an increased fear reaction when exposed to a ‘scary’ object18,19.
Figure two: a horse showing possible signs of reactive coping
Learned helplessness is where an animal learns that they have no control over unpleasant conditions and that their actions do not stop the problem; they essentially learn that they are helpless20. Learned helplessness can be caused by training techniques that are used inappropriately over a period of time which results in the horse not being able to obtain release from the pressure. An example of this would be when ‘go’ and ‘stop’ aids are used at the same time which creates confusion, asking the horse to stop but not releasing the pressure when they do so or constant use of leg and bit aids21.This may cause them to ‘give up’ trying to escape the pressure and essentially shut down behaviourally22. Horses in learned helplessness seem to show the same sorts of behavioural indicators to clinical depression and those coping in a reactive way. These include showing a decreased response to pressure, lowered levels of aggression, dullness and loss of appetite23.
Take home message
Not all stress is bad for the horse. In small measures, acute, short term stress can help the horse learn and adapt to its environment. However, if the problem persists into chronic stress it could impact welfare in a negative way. Sources of chronic stress are mainly management based and due to the horse not coping with its environment. For the horse’s welfare it is important to create an enriched environment to help satisfy their innate behavioural needs with ample foraging opportunities, turnout and social interaction and companionship (see ‘Environmental Enrichment for the Horse- The Need for Companionship’ and ‘Environmental enrichment for the horse - The Need to Forage’).
Be mindful that a proactive coper is likely to show clear behavioural signs of both acute and chronic stress. Whereas a reactive coper may appear calm and relaxed when they are actually withdrawn, unresponsive and emotionally blunt they are equally or even suggested to be more stressed than a proactive coper who is physically trying to control the situation24. By trying to reduce the effect of acute stressors on the horse it could help to prevent the development of chronic stress and therefore help to improve welfare and prevent stereotypical behaviour25.
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