A prey animal such as the horse learns better when relaxed, so we want to be able to recognise signs of relaxation. For example, when working with the horse in hand, Warwick will watch for these signs. He encourages you just to be with your horse as a relaxed presence and wait until he shows licking and chewing. So, is this science or pseudoscience?
When I looked for scientific papers supporting that chewing behaviour was associated with the parasympathetic nervous system, I found nothing. In the book Evidence Based Horsemanship, Dr Steve Peters, neuropsychologist, writes that “a response to stress will release adrenaline and lead to a relatively dry mouth. The horse starts licking with the return of saliva secretion when the balance of the sympathetic and parasympathetic chemical reactions return to their set point.”
The International Society for Equitation Science (ISES) facilitates and promotes research into horse training that may “enhance horse welfare, and improve the horse and rider relationship”. At the ISES conference in Rome in 2018, trained scientists Margrete Lie and Professor Ruth Newbery presented their findings, having asked the questions: “Is non-nutritive chewing performed to signal submission to another horse?” and “Do horses chew in between stressed and calm situations?” Non-nutritive chewing that they examined is, of course, when a horse chews with his mouth but there is no food.
HORSES IN THE WILD
They observed wild horses living in natural herds, where about 200 horses were living in a 334 square kilometre park in Ecuador. They observed for 80 hours and collected data on 202 interactions where licking and chewing was observed. When they looked at aggressive interactions, such as when one horse was threatening another, they found that chewing was more often performed by the aggressive horse. There is no comment in the paper whether there was a difference in the type of chewing behaviour, e.g., were the incisors exposed with the lips drawn back?
They also looked at what was happening before and after the chewing and studied whether chewing (non-nutritive) occurred between tense and relaxed situations. They found that “the majority of behaviours before chewing were tense, and the majority of behaviours after chewing were relaxed. The chewing behaviour occurred when the horses transitioned from a tense to a relaxed state”. The researchers wondered if rather than chewing representing a communication between the horses, it may be that a horse has been in a tense situation, (with the sympathetic nervous system switched on), and consequently will have a dry mouth. When the tense situation resolves, the horse licks and chews in order to lubricate his dry mouth. I think that this is the observation that has led to the interpretation that the horse has moved from sympathetic to parasympathetic tone.
All horses showed the chewing, not just the submissive horses. The researchers cannot tell from these observations whether the chewing comes because the horse is relaxed, or if the horse chews in order to relax. Further research to tease this out would need to measure stress objectively (for example, the heart rate variability we visited last month).
This research is especially interesting to me in the face of the current predilection of dressage judges to penalise horses who chew the bit. This has led to the awful practice of over-tight nosebands as riders, incentivised to maximise their marks, clamp the mouth shut so tightly that the horse sometimes has difficulty swallowing his saliva, resulting in the foamy mouth the judges prefer! Top judges such as Steven Clarke do not support this approach but it continues nonetheless.