Last revised April 20, 2017.
From the horse’s mouth: Vibrissae.1
In the previous section on the sense of smell, I suggested that your horse’s nose in your armpit was an amorous position that helped in bonding. Such a position doesn’t merely improve his olfactory experience. It improves his tactile experience. His nose is sensitive. Your armpit is warm and soft. The connection must be pleasurable to him.
Touch is very important to your horse. It is the main way that you communicate with him when you are on his back. Horses bond through touch, and relax when they are touched by a loving partner. And because his vision up close is very poor, touch becomes very important to him when you are up close.
The importance of touch is made clear when we learn that his side, where you might have once kicked him, is more sensitive even than a human fingertip or the calf of your leg, and that he can react to a touch that would be too light for you to feel at all.2
As it is in humans, the sensitivity of a horse’s skin to touch diminishes with age. Beyond 20 years, there is a steep decline in such sensitivity. Since touch is critical in detecting a biting fly, I recommend that flysheets, fly masks, and fly spray be used with older horses when flies are a problem.
Touch would seem to be detected by a single set of nerves that can register pressure. But there is some evidence that there are two kinds of nerves that can do this. One of these kinds of nerves may specialize in detecting pleasurable touch. We begin with Athena.
Athena was a Giant Pacific Octopus who befriended Sy Montgomery, a nature writer who frequently visited her at the New England Aquarium. In her wonderful book,3 Sy explains that octopuses get red when they are angry, and turn white when feeling safe and contented. In an interview, she writes “… I plunged my arms into the 57-degree water and immediately we were just embracing each other. Her suckers were all over me. I was petting her beautiful head, and I would notice that her skin would turn light colored right underneath my touch…. That’s what she’s knowing when she touches you. And look how white she’s going now – right under my touch. So, she feels very calm. I feel calm too.4” The localized change in color for Athena — white under Sy’s touch — is a strong indicator that the part being touched was feeling particularly good, and that nerves which specialized in detecting the pleasure from touch were at work.
In horses and other mammals, a variety of different nerves in or near the skin are able to register different sensations. Here I am only interested in mechanoreceptors — those that can sense touch. Each involves a corpuscle (encapsulated nerve ending) and connects to nerve fibers that carry signals on to the brain. There are others, such as thermoreceptors (which detect heat).
Merkel corpuscles. Disk-shaped receptor formed by a Merkel cell and a free nerve ending. Located in the basal epidermis (deepest layer of skin) and hair follicles. Not encapsulated in connective tissue. In humans, found most often in sensitive finger tips, and handy for reading Braille. Respond slowly, but may fire for more than half an hour if stimulation continues. Up to 90 such nerve endings are handled by a single afferent nerve fiber. Store neuropeptides such as dopamine and serotonin which they release to associated nerve endings in response to pressure. Used for tactile discrimination, responding to gentle localized pressure, to feel a detailed surface. Massaging your horse will stimulate these nerves in your finger tips and on his skin where you are rubbing, releasing dopamine, serotonin and other wonders.
Meissner (tactile) corpuscles. Stack of flattened disks in the dermis just below the epidermis. Encapsulated in an envelope of connective tissue. Respond to light touch. Respond only at the onset and offset of stimulation. In humans, they are concentrated in thick hairless skin, such as the finger pads and lips. Do not detect pain. Put on a hat, and your Meissner corpuscles will notice. Wear it a few minutes and they’ll stop firing, so you’ll forget it is on. The density of these corpuscles diminishes with age, until we reach the state of no sense, no feeling.
Pacini (Lamellar or Pacinian) corpuscles. Encapsulated in an envelope of connective tissue. Respond rapidly, but only at the onset and offset of stimulation. Respond only to sudden disturbances. Used to detect vibration and sudden gross pressure changes, such as a hard poke. When a spur touches a horse’s flank, it is Pacini corpuscles that first respond. When the spur is removed from the flank, it is Pacini corpuscles that first respond. Continuous pressure does not get these corpuscles to fire, but they do respond in a graded way to pressure: the greater the pressure, the stronger the signal they send.
Ruffini’s (Ruffii or Bulbous) corpuscles (Ruffini cylinder). An onion-shaped structure enclosed in an envelope of connective tissue deep within the skin and in joints. Respond slowly and continuously for the duration of stimulation. Responsible for kinesthetic control and movement. Respond to stretch and torque of skin and tendons. In humans, they are at their highest density around the fingernails, and may help detect whether an object we are holding is slipping from our grasp. Horses find these useful in grasping grass.
Nociceptors. [nōsēˈseptər/] (nocireceptors) Pain receptors. Located throughout the body, particularly in skin, eyes, and mouth, as well as in muscles, joints, and the digestive tract. Respond to potentially damaging stimuli by sending signals to spinal cord and brain. Serve as an alarm system. When fired, the organism reflexively withdraws the affected part, experiences pain, and has various autonomic responses. Some nociceptors are specialized to detect mechanical sources of pain, others detect chemical sources. Some don’t fire unless the pain is severe, and will be accompanied by injury. Found in leeches, sea slugs, fruit flies, fish, birds, mammals and likely any animal capable of movement.
Sensitivity of the horse’s skin
A fly lands on the back of a horse. The fly weighs 0.0004 ounces, yet your horse knows all about it. His skin twitches reflexively, the cutaneous trunci muscle reflex. This muscle sheaths his back and flanks, and seems nicely designed for flipping flies back into the air.
If a horse can detect a fly, just how sensitive is it to touch? In an unpublished study cited in Leblanc (2013), Saslow writes “We were surprised to find that horse sensitivity on the parts of the body that would be in contact with the rider’s legs is greater than what has been found for the adult human calf or even the more sensitive human fingertip. Horses can react to pressures that are too light for the human to feel.5” If you touch your horse so lightly with your calf that you can’t feel it at all, he is likely to feel it. That should be the kind of touch you should strive for, not kick-kick-kick-kick.
Distribution of skin receptors
Skin receptors are not distributed evenly across our skin. Areas such as hands, lips and tongue contain the highest concentrations. If we were to redistribute them evenly across the skin, we would need a different look:
A sensory Homunculus, demonstrating how large the hands, lips, and tongue are compared to the arms and legs.6 A horse would have an enormous face compared to the rest of its body. Its neck would also be fantastically large.
The horse has the greatest tactile sensitivity around the lips, nostrils and eyes. Vibrissae (his whiskers) are each rooted in many nerve endings, enabling him to see what is in his blind spot through touch, and to determine his exact distance from it. They protect his eyes and muzzle area.
In rat pups, whiskers are required for, or contribute to: object localization, orienting of the snout, detection of movement, texture discrimination, shape discrimination, exploration, thigmotaxis, locomotion, maintenance of equilibrium, maze learning, swimming, locating food pellets, locating food animals, and fighting, as well as nipple attachment and huddling.7 Horses that have them trimmed are more likely to suffer eye, ear, and facial injuries or lacerations, due to the lack of pre-warning they would receive when their head was too near an object.8
Horses surely use these whiskers when eating, and there are reports that they also use them to test electric fences before actually touching them.9 Trimming vibrissae is a very, very bad idea for it leaves them with much more difficulty when they eat or investigate anything, and leaves their eyes and muzzle at risk. Only 1% of horse owners trim in Germany10 (and it is illegal there). Follow their example.
In the Horse’s Mouth
A horse’s mouth and tongue is lined with papillae which help him determine what he’s got in his mouth. Four types are found: filiform, fungiform, foliate, and circumvallate papillae. Foliate and circumvallate papillae are covered with taste buds that can identify the five tastes: sweet, sour, bitter, salty, and umami.
Bits can wreck havoc with the mouth and tongue. Use of a bit can result in depapillation of the tongue, where the lingual papillae are lost, leaving a smooth, red, sore area on the top of the tongue. Mechanical irritation can also cause the foliate papillae to appear swollen. With papillae damaged or missing the sense of taste disappears, the ability to wisely choose what to eat diminishes, and appetite may be reduced.
Grooming — rhythmic touch — is an important source of pleasure and relaxation in a horse. One study reports that grooming around the withers caused the heart rate to drop.11 Many studies that measure social bonding in horses tally the number of mutual grooming episodes.12 Grooming should be a foundation for your bonding with your horse, and used often as positive reinforcement.
When grooming your horse, choose a tool from your tack box, and let your horse see and smell it, so he knows what is coming.
Benefits of Touch
Touch in any form may have therapeutic benefits. One review13 provides this summary of what touch can do in humans:
- accelerate healing,
- reduce pain,
- alleviate physical symptoms,
- reduce stress and anxiety,
- facilitate the expression of stored emotions,
- facilitate the shift from negative to positive emotions,
- induce relaxation,
- induce a sense of peace and well-being
1 Image source: McDevitt, Sarah. https://www.flickr.com/photos/smcdevitt/3722775902
2 Saslow, C.A. “Understanding the perceptual world of horses”. Applied Animal Behaviour Science 78 (2002) 209-224.
3 Montgomery, Sy. The Soul of an Octopus: A Surprising Exploration Into the Wonder of Consciousness. Simon and Schuster, 2015.
4 “Befriending an Octopus”. PRI’s Environmental News Magazine. http://www.loe.org/shows/segments.html?programID=11-P13-00046&segmentID=6
5 quoted in Leblanc, Michel-Antoine. The Mind of the Horse. Harvard University Press, 2013. P. 376
6 Image source: Price-James, Sharon WIkipedia. https://en.wikipedia.org/wiki/Cortical_homunculus#/media/File:Side-black.gif
7 “Whiskers”. Wikipedia. https://en.wikipedia.org/wiki/Whiskers See also Grant, Robyn; Mitchinson, Ben; Prescott, Tony (2011). “Vibrissal behaviour and function”. Scholarpedia. 6 (10): 6642. doi:10.4249/scholarpedia.6642. Retrieved October 29, 2011.
8 Fraser, Lauren. “Your horse – Shave and a haircut …should you?” May 8, 2012 https://iaabc.org/horse/your-horse-shave-and-a-haircut
9 McGreevy, Paul. Equine behavior: a Guide for Veterinarians and equine Scientists. Elsevier Health Sciences, 2012.
10 Emerson, L., K. Griffin, and A. Stevenson. “Practice and attitudes regarding trimming of equine vibrissae (sensory whiskers) in the UK and Germany.” Journal of Veterinary Behavior: Clinical Applications and Research 15 (2016): 92.
11 Feh, C., de Mazieres, J. 1993. Grooming at a preferred site reduces heart rate in horses. Animal Behavior, 46, 1191-1194.
12 Crowell-Davis, S.L., Houpt, K.A., Carini, C.M. 1987. Mutual grooming and the nearest-neighbor relationships among foals of Equus caballus. Applied Animal Behavioral Science 15, 112-123.; Feh, C., 1992. Alliances and reproductive success in Carmargue stallions. Animal Behavior 57, 705-713.; Keiper, R.R. 1988. Social interaction of the przewalski horse (Equus przewalski Poliakov, 1881) herd at the Munich zoo. Applied Animal Behavioral Science 21, 89-97; Moehlman, P.D. 1998. Behavioral patterns and communication in feral asses (Equus africanus). Applied Animal Behavior 60 125-169.
13 Hurwitz WL. Energy medicine. In: Micozzi, Marc S. Fundamentals of complementary and alternative medicine. Elsevier Health Sciences, 2014. Page 238-256