Rearing is a hyperreactive or agonistic behaviour in horses, characterized by stimulation of the hypothalamic-pituitary-adrenal axis (McLean and McGreevy, 2005). Behaviours controlled by this axis are fast, dangerous, and difficult to extinguish after only one display (McGreevy and McLean, 2010; McLean and McGreevy, 2005; McGreevy, 2002). A horse is said to rear when it stands on its hind legs (McLean and McGreevy, 2005). Rearing is a natural equine behaviour, observed in feral horses as an attempt to dislodge a predator (McLean and McLean, 2008), and in feral and domestic horses during play or agonistic interactions (McGreevy, 2012). Stallions and colts rear more often than mares, fillies, or geldings do in this context (McLean and McLean, 2008). Domestic horses may also rear during interactions with humans in hand and while being ridden. In the horse not intentionally trained to rear, this behaviour is undesirable and dangerous to handlers. It is estimated that 3 to 7% of domestic horses rear during handling (Hockenhull and Creighton, 2012).
Causes of rearing in an inappropriate or undesirable context, such as in the presence of human handlers, vary widely and include: avoidance or defensive behaviour, such as an unwillingness to move towards an aversive stimulus (McGreevy, 2012; McLean and McLean, 2008); aggression (McGreevy, 2012); habituation to pressure (McGreevy, 2012); pain (Jonckheer-Sheehy et al, 2012; McGreevy, 2012); and an inability to perform the task required of it, for physical or mental reasons, which leads to conflict (McGreevy, 2012). Conflict behaviours are expressed when the horse is not permitted to show normal behaviour, or when it is placed in a situation where it must choose between opposing stimuli (McLean and McLean, 2008) such as a rider applying ‘stop’ and ‘go’ signals simultaneously (McLean and McLean, 2008; McLean and McGreevy, 2005). Inability to resolve the conflict or pain can lead to rearing as an expression of the conflicted mental state of the horse (Hall et al, 2013).
Horses are accomplished trial and error learners that attempt to evade pressure, pain, danger, or novel stimuli (Hothersall and Casey, 2012; McLean and McGreevy, 2005). Trainers harness evasion of pressure to teach horses the consequences of behaviours they trial; however, pain, danger, and novelty can unintentionally have the same effects. This is operant conditioning (Hothersall and Casey, 2012), and is essential in traditional training methods that employ negative reinforcement, which is the removal of an unpleasant stimulus as a reward for correct behaviour. However, while attempting to remove pressure, pain, or danger, horses may trial behaviours such as shying, baulking, striking, or jerking the head. These may develop into rearing, if they are progressively reinforced by unintentional removal of pressure (Hothersall and Casey, 2012; McLean and McGreevy, 2005). This concept is called ‘shaping,’ and is used in horse training when the horse is rewarded for offering a behaviour resembling the final outcome. It is then progressively required to offer more effort more correctly to elicit a release of pressure (McLean and McLean, 2008). Therefore, the onset of rearing may appear sudden, but is in reality a result of inadvertently rewarded behaviours unintentionally shaped into rearing through incorrect application of Learning Theory (McGreevy, 2012; McLean and McGreevy, 2005).
Rearing due to aggression is also a shaped behaviour. If a horse trials an agonistic behaviour towards a human and the human moves away, the behaviour is reinforced. Next time, if the human remains during the behaviour, the horse may escalate until the desired response from the handler is attained, thus potentially achieving a rear. The same principle reinforces rearing caused by habituation to pressure. The handler escalates pressure until the horse trials a response, but the response to unbearable pressure is likely to be extreme (McLean, 2012). Pain may also cause rearing for similar reasons. If a trainer presents light pressure to the horse and its reaction is extreme, it is likely in pain (Jonckheer-Sheehy et al, 2012). The horse’s extreme reaction is an attempt to remove the pain, if only for a moment. If it works, the horse is likely to try it again.
Misunderstanding equine cognition may result in rearing being perceived as naughtiness, done deliberately to annoy or scare the handler (Hothersall and Casey, 2012). This is untrue, and may lead to encouragement of the behaviour through withdrawal or an increase in intensity of the stimulus that initiated the rear. These reactions may exacerbate the problem, and even if the underlying cause of the behaviour is removed, it is likely to persist (Hothersall and Casey, 2012).
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