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Crib-biting is the most common stable vice, or stereotypy. It is where a horse grasps a solid object, such as a stable door, and inhales air. It is usually a stress response to management conditions.
What makes crib-biters different?
A small bundle of nerve cells in the horse’s forebrain known as the nucleus accumbens (NA), and an even smaller group of cells in the midbrain known as the ventral tegmental area (VTA), are key regions responsible for controlling behaviour.
In response to events that horses find naturally rewarding, such as eating, the VTA releases a chemical messenger, dopamine, which binds to docking sites within the NA known as dopamine receptors. It is thought that this binding effect may well be what produces the feelings of pleasure.
In humans, repeated activation of this system by illicit drugs such as cocaine causes such receptor sites to multiply and become more sensitive.
Interestingly, laboratory-based experiments indicate that where the dopamine receptors are concerned, brains from crib-biting horses show similar alterations to those of people who have taken cocaine.
Cribbing is therefore likely to have rewarding consequences for the horse and, in this respect, could be used to counter the effects of stress.
Training crib-biters
Young horses learn through trial and error. A stimulus, such as leg pressure, motivates a range of responses, one of which will be correct.
Through the timely delivery of a reward, a good trainer will bring about precise activation of the NA and appropriate brain stimulation, which encourages the responses that we need in a competition horse.
Masters students at the Royal Agricultural College in Cirencester set up an experiment to assess whether there was any difference in the way crib-biting and non-crib-biting horses tackled a simple task and learned from it.
Ten crib-biters and 10 “control” horses had to select a large playing card marked with a circle and ignore one marked with a cross in order to obtain a food reward.
Crib-biters required fewer tries to reliably select the correct card. However, when the cards were switched, cribbers took much longer to cotton on.
So, if subjected to excessive repetition and positive reinforcement, crib-biters seem to become quite fixed in their response and can find it difficult to progress to more complex tasks.
It must be stressed that buying horses that crib-bite should not be discouraged. In fact, aspects of their behavioural repertoire could be favourable.
Translated into an everyday competition scenario, whether this is teaching a new dressage movement or negotiating a technical cross-country fence, a crib-biter’s ability to react quickly to his rider’s instructions could be a help rather than a hindrance.
For this article in full, see the current issue of Horse & Hound (3 September, ’09)
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