It reflects a change in their autonomic nervous system.
… AND it CAN happen following a threat or disturbance of some sort, not just when the horse is in a more relaxed state.
Pain, fear or confusion can turn on the sympathetic system, and when these feelings are lowered the nervous system reverts back to the parasympathetic state. When the horse is in this fearful state, saliva is not produced and the mouth and lips become dry. So, when the disturbance resolves, the licking and chewing commences (a reflexive response) “as a result of returning from a spell of acute stress or pain “
It is ‘relief ‘behaviour
“. . .You asked whether this licking or chewing might mean processing. I have heard trainers comment at this moment that the horse is “chewing on a thought.” It is usually in the context of working a horse by running it around in a round pen or pestering a horse to load into a trailer, then stopping to take a break and saying, “He’s thinkin’ about it”. A break in the pressure often allows the horse to return to parasympathetic, so you see the licking and chewing response as that occurs. It is simply neurochemically mediated responses that do not necessarily reflect any thought processes.”
Once again, it is a ‘state of relief’.
I think with equine therapy, there is a genuine relief of any discomfort the horse may have, if the horse is relaxed during the treatment, and they produce the licking, chewing and yawning etc. Kerryn
Ref: Sue McDonnell, PhD, Certified AAB
Sue M. McDonnell, PhD, is a certified applied animal behaviourist and the founding head of the equine behaviour program at the University of Pennsylvania’s School of Veterinary Medicine. She is also the author of numerous books and articles about horse behaviour and management.
Excerpts from : http://www.thehorse.com