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Metal in the Mouth


By MSHR Staff

In photographing a variety of equestrian sports, we often unintentionally get photos of horses in distress from bit pain. We capture them opening their mouths, raising their heads, with thick muscle build-up on the underside of the neck (rather than along the topline of the neck), and sometimes even rearing up to try to escape bit pain. You can also see a look of fear in their eyes. Their riders are often oblivious to the pain their horse is suffering as they continue to make attempts to maneuver their horse where they want it to go. But because of bit pain, there is no “communication” between rider and horse, and so performance is greatly diminished because the horse is solely concentrating on escaping the pain in his mouth and head.

Dr. Robert Cook has written many articles on the effects of bits on a horse’s mouth and the nerves throughout the horse’s head. Probably his most famous book is Metal in the Mouth: the abusive effects of bitted bridles (2003, Sabine Kells), by W. Robert Cook and Hiltrud Strasser. This book describes and scientifically documents the multitude of effects of a bit in the horse's mouth. These effects are still widely unknown among horsemen and owners, and range from pain to respiratory and digestive problems, reduced performance and longevity.

Dr. Robert Cook, FRCVS, Ph.D. is Professor of Surgery Emeritus of Tufts University and has been a veterinarian since 1952. He has published nearly a hundred articles in scientific journals and many articles in horsemen’s journals. He believes that the work he has done since 1997 to investigate the bit method of communication in the horse and his validation of the Bitless Bridle is now doing more for the welfare of both horse and rider than anything he has done previously. He believes the Bitless Bridle is helping horsemen in all equestrian disciplines to achieve improved performance. Because the horse’s mouth is one of the most sensitive parts of its anatomy, his research has shown that the bit is responsible for over a hundred behavioral problems. On Dr. Cook’s website, you can find hundreds of his research articles.

Hiltrud Stasser is a German veterinarian who has studied for many years the anatomy, physiology, pathology, and rehabilitation of horses’ feet. She has published papers and books on this topic since the 1980s.

In 2018 Dr. Cook and M. Kibler published in the results of a longitudinal study comparing the behavior of horses ridden with and without a bit. Sixty-nine behaviors in 66 bitted horses were identified as induced by bit-related pain. From this template of 69 behavioral signs, the number of pain signals shown by each horse – first when bitted and then bit-free – were counted and compared. The number of pain signals exhibited by each horse when bitted ranged from 5 to 51 (median 23); when bit-free from 0 to 16 (median 2). Signs of pain included behaviours such as a resentment of bridling, evading capture in paddock, head-shaking, stiffness of the neck, head shyness, tail swishing, moving in an inverted frame, tongue over the bit, bucking, rearing and more. Bit pain had a negative effect on proprioception, i.e. balance, posture, coordination and movement.

Reference: “Behavioural assessment of pain in 66 horses, with and without a bit” W.R Cook and M. Kibler, 2018. Full paper at:
Other research on bit pain
The main effect of bits is pain, but bits can also impair the horse’s breathing, interfere with striding, and impede performance. Carley Sparks’ 2012 article in Horse-Sport explains these bit effects by detailing some of Dr. Cook’s research findings:

“The repeated pressure of bit on bone causes the sensory nerve to the face to become super-sensitive, i.e., to develop trigeminal neuralgia. This is the most common cause of head shaking (tossing). Horses experience pain in the mouth, but also in their face, eyes, and ears. A head-tosser may also be difficult to bridle, a persistent head-rubber, unable to stand bright light, wind or rain, and impossible to handle around the ears.”

Sparks relates how Cook debunks a myth of horsemanship: that a bit controls the horse. “It doesn’t. A bit doesn’t act like the brakes on a car. On the contrary, it often acts like an accelerator. Horses run from pain. If you hurt your horse, it speeds up,” he explains.

“A bit is a foreign body in the horse’s mouth and stimulates salivation, chewing, movement of the jaw, and swallowing. These are not the responses needed for exercising; they are ‘eating’ responses. Eating and exercising have mutually opposed priorities.” In fact, a bit programs the throat for swallowing. “It breaks the lip seal, opens the mouth, admits air, moves tongue and jaw, and triggers salivation. At the level of the throat, all these raise the soft palate and enlarge the food channel at the expense of the air channel, interfering with breathing,” explains Dr. Cook.

One consequence of impaired breathing is restricted movement, specifically stride shortening, according to Dr. Joyce Harman, DVM. Bits can also be an impediment to performance because “bit pain often causes the horse to throw up its head.” This distracts the horse at the very moment it needs to focus on what is ahead.

Read Sparks full article at: “Are Bits Bronze Age Technology?” Horse-Sport. July 3.

David J. Mellor published an article in 2020 on “Mouth Pain in Horses.” Here are some of his findings.

“Mouth pain in horses, specifically that caused by bits, is evaluated as a significant welfare issue. The conscious experiences of pain generated within the body generally, its roles, and its assessment using behaviour, as well as the sensory functionality of the horse’s mouth, are outlined as background to a more detailed evaluation of mouth pain. Bit-induced mouth pain elicited by compression, laceration, inflammation, impeded blood flow, and the stretching of tissues is considered. Observable signs of mouth pain are behaviours that are present in bitted horses and absent or much less prevalent when they are bit-free. It is noted that many equestrians do not recognise that these behaviours indicate mouth pain, so that the magnitude of the problem is often underestimated. The negative experiences that are most responsible for welfare compromise include the pain itself, but also, related to this pain, potentially intense breathlessness, anxiety, and fear.”

Find an illustrative video on “The effects of the bit part 1” by Arno Hendriks on YouTube. It is based on Dr. Cook’s book: Metal in the Mouth.

The seven and a half minute video addresses the questions: “Why does a horse open his mouth and what happens if the horse does not open his mouth? The bit can hit the roof of the horse’s mouth. By opening his mouth, the horse tries to prevent the bit from hitting the roof of his mouth. There are other ways the horse tries to escape the pain. The tongue is one of the most sensitive organs in the horse’s body. One way a horse tries to escape pain on the tongue is to put the tongue over the bit. The bars of the horse’s mouth, on which the bit presses, are as sharp as a knife edge. When the rider pulls the reins, the bit is pressed against the gum, over this knife edge. The main reason the horse opens his mouth is pain.”


Sparks, Carley. 2012. “Are Bits Bronze Age Technology?” Horse-Sport. July 3.

Robert Cook, FRCVS, Ph.D.  “On the bit: a misleading and mischievous phrase.”

The Bitless bridle by Dr. Robert Cook

Mellor, David H. 2020. “Mouth Pain in Horses: Physiological Foundations, Behavioural Indices, welfare Implications, and a Suggested Solution. Animals (Basel). April 10(4).

Readers might also be interested in related research, “Bone damage in horses at site of nosebands identified in study,” posted on HorseTalk:

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