Contact and how horses interpret it is crucial to training, yet surprisingly little research has been done into equine sensitivity to touch, according to scientists.
Researchers, in a recently published review, examined more than 180 studies exploring elements of equine vision, hearing, smell, taste, and touch.
Maria Vilain Rørvang, Birte Nielsen and Andrew McLean, writing in the journal Frontiers in Veterinary Science, sought to gain an overview of current knowledge of the sensory abilities of horses and their importance for equitation science.
Most vertebrates share the five senses employed by humans. With these senses, the animal receives information about its environment.
How this information is organized, interpreted, and experienced is known as perception.
“Horses and humans share the five most common sensory modalities, however, their ranges and capacities differ, so that horses are unlikely to perceive their surroundings in a similar manner to humans,” the trio said.
“Understanding equine perceptual abilities and their differences are important when horses and humans interact, as these abilities are pivotal for the response of the horse to any changes in its surroundings.”
Sensory ability, perception, and behavior are closely linked.
In summary, they reported that horses have panoramic visual abilities with acuities similar to those of red-green color-blind humans. Their hearing abilities in some respects exceed human hearing. They have a highly developed sense of smell. All of these influence how horses react in various situations, they said.
They found that research to date has focused mainly on hearing and vision. Smell and tactile sensitivity, on the other hand, have been the subject of limited study.
“Equine sensitivity to touch has been studied surprisingly sparingly, despite tactile stimulation being the major interface of horse training,” the researchers said.
Equine perception is affected by factors such as breed, individuality, age, and in some cases even coat color, emphasizing that different horses may need different types of management.
“Understanding the sensory abilities of horses is central to the emerging discipline of equitation science, which comprises the gamut of horse-human interactions,” they said.
“Therefore, sensory abilities continue to warrant scientific focus, with more research to enable us to understand different horses and their various needs.”
Their key conclusions on how horses perceive the world:
Equine sight is well adapted to provide optimal vision for predator detection and escape.
The eye of the horse is among the largest of land vertebrates. Contrary to the binocular and typically narrow vision of most predators, the evolution of the horse’s visual field has favored a more panoramic view with only limited binocular capability.
The visual input is narrow and wide, which provides a panoramic outlook with only a small blind spot at the rear.
Horses have poorer acuity than most other terrestrial mammals.
“Hence, at first glance, it seems somewhat surprising that horses are so capable in showjumping and eventing competitions, where jumping obstacles indisputably requires substantial visual abilities to gauge both distance and height of obstacles.”
However, studies of depth perception in horses reveal that horses possess true stereopsis — that is, the ability to perceive depth and three-dimensional structure obtained on the basis of visual input from both eyes. However, this can occur only within the binocular field of vision.
In terms of equitation science, the relatively limited binocular vision and visual acuity may help explain the higher frequency of faults at obstacles with successive elements in combination.
Unlike the human retina, the equine retina has no central fovea but instead has what is known as a “visual strip”.
“This gives the horse the ability to broadly and most likely equally see the entire horizon, but much less above or below.
“From an adaptive viewpoint, this horizon-focused vision has obvious benefits for an open grassland prey animal with no aerial predators and little threat from beneath.”
To bring an object into focus, the horse will usually lift, lower, or tilt its head to make use of the visual strip.
“Head and neck position are therefore important factors found to affect the visual abilities of horses,” the review team said.
In 1999, a research team questioned whether the over-arched neck of the ridden horse in dressage would hinder the horse’s ability to see what is directly in front of it.
“The trend in dressage over the last few decades has been for increasing arching of the neck, resulting in the nasal planum keeping behind the vertical line.”
Research, they said, has highlighted the visual deficits that occur when the angle of the nasal planum increases beyond the vertical line. However, another study challenged this assumption and found that 16 riding school horses were not visually impaired when ridden with a vertical nasal planum because a horse is able to rotate its eyeball, enabling a horizontal eye position and hence a horizontal field of vision.
What the authors did not investigate, however, were head/neck positions great than 90°, also called being “behind the bit”.
“More recent findings suggest that the rotation of the eyeball can compensate for some head and neck rotation, but not the most extreme hyperflexed positions.”
The review team noted that riders in showjumping and eventing typically allow their horses sufficient rein so that they have the freedom to choose their own head carriage appropriate for clearing the obstacle.
“This is particularly important just before and during the jumping effort as it enables the horse to achieve optimal athleticism and balance when negotiating an obstacle.”
From an adaptive viewpoint, the ability of horses to perceive depth and three-dimensional structure within their binocular field of vision would most likely help their sure-footedness where predators have the advantage of surprise. It enables the horse to pinpoint and evaluate the potential threat.
“This may also explain the alert behavior where the horse stands vigilant with an elevated neck, and with head and ears oriented toward the stimulus (that is, a potential threat).”
At night, horse pupils can dilate greatly to capture sparse photons at night, and the retina is generally rod dominated. There are also adaptions in what is known as the tapetum lucidum in the back of the eye, which increases the number of photons being picked up by photoreceptors.
Horses, therefore, have good vision in low light. It has also been established that horses see details better on overcast days than bright sunny days.
Horses have a higher proportion of retinal rod cells than humans, giving them superior night vision. Indeed, researchers have found that horses can solve two-dimensional discrimination tasks in nearly complete darkness.
Horses also possess good visual capacity under both natural and artificial light conditions.
Equine color vision is dichromatic — that is, color vision that is deficient in one of the three cone pigments. Their color ability resembles that of red-green color-blind humans.
“This is an important aspect to consider in eventing and showjumping when choosing the colors of obstacles, as these may not be as obvious to the horse as they are to the rider.
“Several studies have also shown that obstacle color affects how likely the horse is to detect, and hence how successful the horse jumps the obstacle.”
One study showed that it can be difficult for horses to jump obstacles that are all light or all dark, as uniform light may cause an optical illusion, which overestimates the size of the obstacle, and uniform dark may make the horse disregard the obstacle altogether.
“These authors suggested that the color of the surroundings plays a crucial role. The horses they tested seemed to detect blue-colored obstacles easier than green, which they conclude is due to the coloring of the (grassed) jumping arena.”
Other researchers tested racehorses on obstacles colored orange, fluorescent yellow, bright blue, or white, and found that the obstacle color influenced both the angle of the jump and the distances jumped.
“Altogether, these studies highlight that coloring of obstacles is of high importance, especially when evaluating the level of difficulty of the tracks.
“Although obstacles might seem easy/hard to the human eye, the coloring alone might result in an easier/harder challenge than anticipated (for the horse).
“Such considerations are important in eventing, stadium showjumping and other jumping sports, including steeplechasing and hurdling.”
Horses show visible reactions to sounds, with one or both ears typically moving toward the direction of the sound source.
The hearing ability of horses was first studied in the 1980s, and surprisingly little research has been done in the area since, the review team said.
Researchers in the 40-year-old study tested the range of frequencies horses can detect and demonstrated that while larger animals tend to be adept at hearing lower frequencies, horses are an exception.
The lowest frequency detectable by horses is 50 Hz, which is higher than the lowest human detection threshold of 20 Hz.
Conversely, equine hearing exceeds the highest frequencies that can be heard by humans (33 kHz compared to 20 kHz for humans), indicating that there will be situations where a horse can detect sounds that humans are unable to hear and vice versa.
The funnel shape of the equine ear provides an acoustic pressure gain of 10 to 20 decibels, improving the acuity of equine hearing.
“From the viewpoint of horse-human interactions, it is important to consider that the higher frequency hearing abilities of horses compared to humans may explain some of the unwelcome and otherwise inexplicable behaviors that are regarded as problem behaviors.”
The review team said high-frequency hearing is undoubtedly adaptive in horses and is likely to provide the horse with important information regarding, among other things, the stealthy advance of predators.
Horses, they said, appear to possess a cross-modal recognition of known individuals. For example, a horse is able to match a particular voice to a particular person.
“In practical terms, this means that the horse is able to recognize a familiar person based on vocal cues (e.g., voice), even when unable to see this person.”
Old age is known to affect hearing ability in many animals, including humans. In horses, only one study has investigated hearing ability in relation to age. The authors found that older horses (15–18 years old) showed fewer behavioral reactions to sounds than younger horses (aged 5–9 years).
Since then, no published studies have investigated age and hearing impairment in horses.
“It has been suggested that as deafness progresses, the horse can compensate by enhancing other senses such as vision and by learning daily routines to still behave as per usual.
“Detection of partial or complete hearing loss in horses can be difficult, but it is nevertheless important for people working with horses to be aware that hearing ability can weaken with age.
“Horses,” they said, “are commonly trained to react to voice commands from the rider/trainer and such commands will become less detectable as the horse ages.”
Likewise, horses communicate with each other through vocalization.
“Depending on the type of deafness (high or low frequency) horses may show no signs when ridden (high-frequency sounds), but still be constrained in their social communication or vice versa.”
Dealing with the impact of sound, the review team said it is well known that loud noises can cause stress responses.
“In many horse barns and riding stables, it is common for a radio or other music devices to be playing during the time when people are active.
“The effect of such music has not been widely studied in horses, and it is therefore unknown if these sounds are perceived as attractive or aversive by the horse.”
In everyday horse management situations, the effects of music have only been studied by one research team, which found that classical music reduced the intensity of stress responses of horses subjected to either short transportation or a farrier treatment.
This suggested that background music can have practical implications.
“One study investigated the potentially calming effects of music on ponies, but found no effects of either classical, jazz, country or rock music.”
Another study found that instrumental guitar music can have a positive influence on Arabian racehorses when played for five hours a day for one to three months, after which the positive effect diminished.
“The same type of music was tested in a study that showed that the positive effects of playing the music were greater when played for three hours per day than for one hour per day, confirming the positive effects of instrumental guitar music.”
Collectively, however, these studies only compared music against silence (in this case meaning no music, which is not necessarily silence), and hence the treatments were a more general “sound” versus “no sound” comparison.
The review team said music may simply mask sudden noises, such as machinery or doors slamming. “Such noises have previously been found to be stressful in other species.”
Can horses move to a musical beat?
“From an evolutionary perspective it would seem an unlikely phenomenon that would entail the recruitment of higher mental processes than those so far found to be possessed by horses,” the review team said.
One study investigated horses moving to music and recorded the footfall and the beats of the music to analyze if horses possessed the ability to synchronize their tempo to a musical beat.
“The preliminary results (based on one horse) suggest that a horse may be able to spontaneously follow a rhythm, but more studies with larger sample sizes .. are needed to refine the method and confirm the findings.”
Their highly developed nose indicates that information from odors is important to horses but, despite this, research on olfaction is relatively scarce for the species.
Only a handful of studies have examined the area, and these have focused mainly on reproduction and social recognition.
Odors are used to some extent by horses for social recognition, as they are able to distinguish between different individuals by their smell. It has also been shown that horses of both sexes can distinguish individual competitors among their group mates by the smell of their dung.
Volatile organic compound profiles from hair samples have been found to differ among horse breeds, and these odor profiles are different in groups of related compared to non-related horses.
“The odor profiles indicate a degree of kinship, suggesting that each horse has its own odor profile with a certain degree of similarity among related individuals.”
This ability to recognize other horses based on odor can be used by the horse to guide its response with other horses in the group based on previous experiences so that odor profiles become an aid in determining the potential outcome of a given interaction.
Individual olfactory recognition can be considered a beneficial evolutionary trait, which still exists in domestic horses.
“Odors from different horses should be taken into consideration during their handling, as they will leave a scent trace on the human handler.
“A person training many horses a day will end up with many different odor traces on their clothes, hands and on equipment, and these odors may affect horses handled subsequently, such as if an early-handled horse is a known aggressor.”
The review team said the nose of horses could be exploited in different situations if more knowledge about odor detection and preferences were known.
“Attractive smells could potentially draw horses to certain places/locations scented with these odors, limiting the need to manually move the horses.
“Moreover, conditioning horses to associate a certain odor with a pleasant experience could take the use of odors a step further.”
However, this had been barely been explored in livestock.
In mammals, the most well-known innate response to an odor is the avoidance of, or flight from, the smell of a predator.
Studies indicate that the ability is even preserved in species living where no predators have been present for centuries.
“Horses have also shown vigilance behavior when exposed to an unknown odor (eucalyptus oil), and to a predator odor (wolf urine).
Pairing a predator odor with a loud noise elicits significantly higher heart rates in horses than when exposed to only one of the stimuli, suggesting that the mere presence of a predator odor can increase the response to fear-eliciting situations.
“This would indicate that when in an environment where predators roam, the horse may be more reactive than usual, and detection of predator odors may be one of the reasons why horses react unpredictably or more abruptly in some situations,” the review team said.
Riding in or close to environments where the likelihood of encountering predators is higher may pose a safety risk to horse and rider, they said.
It is commonly speculated that humans, when scared or stressed, secrete odorous compounds associated with fear, which can affect the horse.
“Several studies have shown an increase in heart rate of horses when either handled or ridden by a nervous person and similar increases have been seen in horses when stroked by a negatively thinking person, which in this study were male subjects with a negative attitude toward companion animals in general.”
Contrary to these findings, and perhaps surprising to many, another study found that horses react more calmly when accompanied by a stationary nervous or physically stressed person than a calm person.
“Although these are preliminary results, the authors question the common saying that horses will be scared if the person is scared. Even when a person is stationary, subtle movements and body language of the human is likely to affect horse/human interactions, and this may have influenced the results.”
Horses express more relaxed behavior in the company of humans who express a positive attitude toward horses, another study found.
However, none of these studies considered the potential effect of human odors on horse behavior.
It is likely that humans wearing artificial odors, arising from shampoos, soaps and deodorants, will hide the natural human odors. “This could have both positive and negative consequences as it can mask potential human odors connected with fear and stress, but also limit imprinting and other familiarity benefits.”
Humans are able to associate some odors with a certain taste, and vice versa, and refer to the combined effect of smell and taste as flavor.
“Unlike humans, horses only breathe through their nostrils, and oral breathing only occurs if the horse is physically prevented from nasal breathing.”
The tasting organ of horses is linked to the olfactory epithelium, but it is not known if horses are able to associate odor and taste and form a concept of flavor, like humans.
“However, even though horses do not smell retro-nasally while masticating, odors are likely to escape the mouth, allowing the horse to smell what it eats, and this may connect taste and smell.
“Indeed, horses are capable of detecting four of the five taste components i.e., sweet, sour, salty, and bitter, whereas detection of umami (a kind of savory taste) in equines is as yet unknown.”
Like many other ruminant species, individual horses are quite variable in their responses to a particular taste.
“The greatest variation in individual taste preferences (in this case pellets) was found in purebred Arabian horses, indicating that breed differences are present.
“Generally,” they continued, “flavor affects diet acceptance and consumption time of horses, but when comparing taste, odor, and nutrient contents, the latter has been shown to be the main driver for horse diet choices.”
The skin is the largest organ in horses as well as humans, and the body surface of the horse is thus the largest of its sensory organs.
As a prey species, it is unsurprising that the horse is a touch-sensitive animal.
Such sensitivity may have served the adaptive purpose of resisting and disabling entrapment by predators. However, it also foreshadowed the horse becoming the most popular ridden animal.
“Tactile stimulation of the surface of the skin is the main interface of communication between a horse and a rider, and also between a horse and human handler.”
The sensitivity of the skin is thought to vary across the body of the horse as the distribution of sensory nerve receptors vary, with areas such as the muzzle, neck, withers, coronets, shoulders, lower flank and rear of the pastern typically being most sensitive.
In the facial area, where the epidermis is thin, tactile sensitivity is particularly high around the eyes, nostrils and mouth.
Like many mammals, horses have whiskers around the muzzle, as well as around the eyes, but only a few studies have looked into their role. However, they are considered as sense organs and removing or thinning them for aesthetic purposes has negative welfare implications.
“Another tactile concern for the area around the nose and mouth of the horse, is the use of restrictive nosebands. Recent studies have shown that nosebands in several equestrian sports are excessively tightened to the extent that natural oral behavior is inhibited, stress can be induced, and tissue damage may occur.
“Interestingly, while nosebands are believed to lead to lighter rein tension and to improve control, the modern trend in dressage, eventing and jumping of increased noseband tightness has welfare implications and warrants further investigation.”
Grooming by humans, or mutual grooming by horses, is commonly considered a positive behavior, as is massage. However, some tactile stimuli are perceived as unpleasant.
“It is thought-provoking that, as horse trainers, we expect the horse to readily habituate to the pressure of the girth, whilst at the same time remain sensitive to pressure from the rider’s legs at approximately the same location.”
The reaction of horses when trying to avoid unpleasant tactile stimulation (such as a fly landing) can include tail swishing, skin rippling, ear flicking, foot-stomping, head shaking, and biting directed at the particular spot.
“These behaviors are also typically the behaviors used as indicators of possible conflict between the rider and the horse.”
A significant issue in equitation science surrounds the use of the whip in horse racing.
Another commonly used aid, based on unpleasant tactile stimulation, is the use of electric shock in the form of non-lethal electric fences to control horses on pasture. Horses (and animals in general) quickly learn to avoid the negative experience of contact with an electric fence.
The review team said tactile stimulation should be used with caution, especially when the force applied is high, such as during twitching.
“More knowledge about the tactile sensitivity of horses both during handling and riding is needed to safeguard the welfare of horses and refine our handling techniques.
“It is likely that horses vary with regard to tactile sensitivity, with individual levels of tactile sensitivity being relatively constant.
“It is believed by many horse people that different breeds of horses differ in their sensitivity. For example, what are colloquially known as ‘cold-blooded’ horses are generally deemed less sensitive as they have been selectively bred to endure forces on their bodies from carrying and/or hauling heavy loads, while at the other end of the scale, Arabians and Thoroughbreds are known as ‘hot-blooded’ because of their tactile sensitivity, a catalyst for successful racing.”
They said future research should focus on mapping the tactile sensitivity of the horse’s body, and reveal how age, breed, experience and personality may influence the way in which tactile stimulation is perceived by the horse.
The authors said many of the studies they examined noted large individual variations in sensory abilities and sensitivity among horses.
This may come down to personality, which in horses has been studied only sparsely
“Identifying specific types of horses according to their specific sensory sensitivity could be a way to optimize management and training and may help to improve the welfare of individual horses.”
They said seasonal and circadian rhythms also have an influence. Future work could focus on determining peak times for training and competing horses in relation to both circadian rhythms and seasonality, to estimate the best training periods and durations throughout the year.
“It may even be possible to manipulate some aspects of seasonality and circadian rhythms, such as using blue light to stimulate estrus in anestrus mares.”
The authors suggest there may be unexploited potential in using sensory enrichment/positive sensory stimulation to improve the welfare of horses in various situations — for example, using odors, touch or sound to enrich their environment or to appease horses.
“Considering the popularity of horses in leisure, sport and other activities, there is a significant need and scope for further research into the sensory abilities of the horse,” they said.
“Knowing how horses perceive their surroundings will help improve awareness of what they find aversive or pleasant and will enable more efficient, welfare-friendly training and handling techniques as well as improve human safety.”
Rørvang is with the Swedish University of Agricultural Sciences; Nielsen is with the University of Paris-Saclay; and McLean is with Equitation Science International in Tuerong, Victoria, Australia.
Rørvang MV, Nielsen BL and McLean AN (2020) Sensory Abilities of Horses and Their Importance for Equitation Science. Front. Vet. Sci. 7:633. doi: 10.3389/fvets.2020.00633