The Science Behind 'Licking and Chewing' in Horses

The Science Behind 'Licking and Chewing' in Horses

Licking and chewing behavior reflects an autonomic nervous tone change, such as from pain, fear, or confusion to an ordinary maintenance activity.

Photo: iStock

Q. What does “licking and chewing” really mean? Submission? Processing? Relaxing?

Lisa, California


A. Licking and chewing behavior is probably one of the most misunderstood horse behaviors. It simply reflects a change in autonomic nervous system tone that results in salivation that stimulates licking, chewing, and sometimes a big swallow. And that can happen in a number of situations following a threat or disturbance of some sort. To better explain, when an animal or a person is relatively relaxed and engaged in ordinary maintenance activities, such as feeding and resting, the parasympathetic nervous system (the part of the nervous system responsible for the “rest and restore” response) is more or less in control. When an animal or a person is threatened or acutely stressed, the nervous system switches into alert or fight or flight mode with the sympathetic nervous system. Pain, fear, or confusion can all turn on the sympathetic system. When that which turned on the sympathetic state resolves, nervous system control switches back to the more relaxed parasympathetic state.

Horses show some observable behavioral signs of this back-and-forth switching. This cluster of licking, chewing, and sometimes swallowing that you have asked about occurs right when switching back to parasympathetic after a period of sympathetic. That’s because when sympathetic control switches on, salivation ceases and the mouth and lips quickly dry. When the disturbance resolves and relaxation returns, salivation also returns. So the licking and chewing is just that simple reflexive response to deal with the salivation resuming after a period of dry mouth and lips. So, in a sense, licking and chewing do reflect relaxation, but specifically as a result of returning from a spell of acute stress or pain. People often refer to this moment as “relief.” Another medical term for it is sympathetic attenuation.

You can often identify horses in pain that might otherwise go unnoticed by watching for (a big swallow and sigh).

Dr. Sue McDonnell

The first scientific description I encountered of these behavioral indicators of emotional states in animals and people was in neurophysiology and pharmacology animal labs in graduate school. This licking and chewing in response to resuming salivation, along with a cluster of other responses, such as itchiness, sighing, and sometimes yawning and stretching, are used to monitor what is going on in the nervous system. 

The textbook example of this “relief” behavior that humans can relate to is that moment a police car with flashing lights and sirens whizzes by without pulling you over. You might have a little itch on the scalp or neck, swallow, or take a big deep breath, or sigh. A more dramatic driving scenario would be spinning out on ice, where your heart rate jumps, you break into a cold sweat, and have to pull over and hold your head before regaining focus. But as you get over the scare, you experience many of these outward signs of “relief” and likely go through this stage of resumed salivation, often with a big swallow and sigh. This example has always helped me empathize with a horse or a person when I see it. If it is not obvious, it makes me think about what could have led to the sympathetic state. For example, you can often identify horses in pain that might otherwise go unnoticed by watching for this behavior. The pattern of its occurrence over time can help pinpoint the location or system of the discomfort.

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.” Whether scared or confused or excited from the running around or the trailer loading, the horse is in sympathetic mode. A break in the pressure often allows the horse to return to parasympathetic, so you see the licking and chewing response as that occurs. Drugs that affect the brain’s neurochemistry can also induce these sympathetic attenuation responses, so in horses I tend to think of them as simple neurochemically mediated responses that do not necessarily reflect any thought processes.

Finally, you mentioned submission. These sympathetic attenuation responses are not a submission gesture per se, but they can occur within the context of an interaction where an animal shows submission. In a threatening situation, the submissive animal might lick and chew after its submissive posture or behavior successfully leads to the threatening animal backing off. As the threat subsides, the submissive one goes through this state of relief. A different mouth behavior in horses that does signal submission is the behavior that looks like a foal nursing—with the head held low, making sucking movements. Because the primary behavior of submission in horses is to move away, if the horse is trapped and can’t move away, it might do this “I’m a baby, I give up” gesture.

About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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