Licking and Chewing; What Does It Really Mean?

I was on a horse forum recently and somebody had asked about the licking-and-chewing motion that horses often display after they learn something or when they are relaxed.  There were lots of opinions, some thinking it was a sign that the was horse 'reassuring himself' while others called it the sign of information processing. I love looking at the science behind a thing, so I did some research. Some of the following is still theoretical, but here is a summary of the most logical scientific explanation I could find. 

The licking-and-chewing response in horses is closely linked to the parasympathetic nervous system, controlled by the hypothalamus. This is the part of the autonomic nervous system which deals with automatic body functions during a relaxed state, as opposed to the sympathetic nervous system which triggers functions seen in the 'fight or flight' state (as seen below). For example: You shouldn't exercise your horse just after he has eaten because the body cannot efficiently supply nutrients to the muscles for exercising at the same time as fuelling the digestive system, HOWEVER, when the horse's SNS kicks in, it inhibits digestive activity so that his body WILL be able to fuel the muscles (enabling him to flee from danger if necessary). 

http://medicalterms.info/anatomy/Autonomic-Nervous-System/

So both the SNS and the PNS inhibit and stimulate different organ processes to either relax or excite them respectively. When the horse is in sympathetic state, salivation is inhibited, but during the transition back to parasympathetic state, salivation is re-stimulated, resulting in the licking and chewing response. This is not to say that the horse constantly licks and chews when he is relaxed, but when he is coming 'down off adrenaline' or out of sympathetic state, the stimulation of salivation is more acute. 

The reason that licking and chewing is often thought to be a response of thought processing during learning (which is not entirely untrue) may be that the learning process has a particular relationship to licking and chewing. Here is an example of a learning process in which the pressure-and-release technique is used: 

1. Pressure is applied to the horse until he responds correctly 
2. The horse undergoes 'concentration tension' 
3. Horse responds to pressure in a certain way 
4. The pressure is released; he has found the solution! 
5. What he has just learned is processed in the hippocampus 
6. 'Stimulation of the hippocampus promotes the release of endorphins' - (www.community.equisearch.com/blogs/floridahorseman/default.aspx)
7. The endorphins stimulate parasympathetic state; he can relax! 
8. PNS then induces salivation 
9. Salivation stimulates licking and chewing response 

So does it mean a horse has learned something new when he displays the licking and chewing response? Not necessarily. He may have learned something new, that being the reason for his relaxing, which is then the reason for his licking and chewing, but ultimately it is a sign of relaxation. 

Horses don't just relax when they have learned something, they also learn faster when they are in parasympathetic state. So as a horse trainer it is important to make sure your horse is as relaxed as possible when trying to teach him something new. This should also discourage the use of force when training horses, as forceful methods are likely to cause stress and so inhibit the learning process rather than promote it. 

The following excerpt explains more about the connection between brain and mouth and how stimulating salivation can work conversely, helping in turn to create the parasympathetic state: 

The mouth is linked to the limbic system (including the hippocampus), the area of the brain that is considered to be the control centre for the emotions and the gateway to learning (Daniel Goleman, 1997). This apparent connection is consistent with observations made by Linda Tellington-Jones in her training system. Many horses improved in their behaviour and the ability to operate in a calm and focused mode once tension in the mouth was reduced. Working around the mouth, both inside and out, stimulates the salivary glands, which, in turn, triggers the PNS and quietens the SNS. (Dental imbalances can cause horses to be more dry mouthed and therefore less relaxed and less able to learn.) 

                                                                                                                  - www.lindatellingtonjones.com