Horses endocrine system

Horses endocrine system

(From Ultimate Horse Care, published by Ringpress Books, 1999; ISBN: 1860541860)

Over many millions of years, horses evolved a physique and patterns of behaviour that were necessary for their survival in the wild. Like all animals, they have to respond appropriately to events occurring in the world around them. They need to be able to communicate socially with other members of their own species. With domestication, horses also had to adapt to the conditions and demands imposed on them by humans.

All the responses and activities which make up a horse’s behaviour are directed by the nervous system. Sensory systems continuously feed information from the external environment (sight, sound, smell, taste, touch) and from within the body (position, temperature, pain) to the central nervous system, where they are filtered and processed in various ways.

As a result of this processing, messages are sent to the skeletal muscles. Nearly all behavioural activities are carried out by the contractions of skeletal muscle. Some responses, termed ‘reflex’, are automatic, immediate and fixed. Other ‘voluntary’ responses may involve a decision to act based on a number of factors such as motivation and previous experience.

As well as its outwardly-directed activities, the nervous system has the task of co-ordinating most of the body’s internal functions. For example, it controls digestion, respiration, the contractions of the heart and of smooth muscles in blood vessels, and the release, by the endocrine system, of hormones which regulate metabolism and growth. Hormones themselves can influence behaviour by their effects on specific parts of the brain. For example, a stallion’s aggressiveness and libido is affected by the male sex hormone, testosterone.

The basic units of the nervous system are nerve cells, or neurones.

An animal is born with its full complement of neurones (tens of billions!) and produces no more during its life. While their size and shape vary with location in the nervous system, most neurones contain the same parts: a cell body; a number of highly-branched outgrowths of the cell body called dendrites; a single, long nerve fibre (or axon) extending from the cell body; and branches at the end of the nerve fibre (axon terminals).

Wherever their location, the neurones also operate in the same way. In response to signals received at the cell body and dendrites, they generate electrical signals which travel rapidly to the ends of the cell. This process can be likened to what happens when a match is applied to one end of a line of gunpowder – once lit, the flame moves down the line to the other end. Depending on the type of neurone, the speed at which this signal can travel can range from about 1mph to over 250mph.

Neurones communicate with each other, and with other cells of the body (such as muscle or secretory cells). The junction between two neurones is called a synapse. Rather than there being a direct electrical connection between neurones, a chemical called a neurotransmitter is released into the gap between the cells. Depending on the neurotransmitter released, this can either tend to provoke the second cell to fire off its own electrical signal (excitatory) or put a damper on its activity and make it less likely to fire off (inhibitory).

A nerve cell in the brain may have contacts with many other nerve cells – some have more than 100,000 contacts. Whether a particular cell fires or not depends on the combined activity of the cells which contact it at any moment in time. Furthermore, the efficiency of a synapse may improve the more it is used, and new branches and synapses may be formed over a period of time. This provides a possible cellular basis for storing memories.

In addition to nerve cells, the nervous system also contains glial cells. Some of these serve to increase the speed at which signals can travel along axons. Others help sustain the nerve cells, providing nutrients, mopping up unwanted chemicals and stimulating the growth of axons and dendrites. The may even play a role in processing information.

The structure of the nervous system

The nervous system can be divided into two main parts: central and peripheral. The central nervous system (CNS) lies within a series of protective bones: the brain inside the skull, and the spinal cord in the vertebrae that make up the spine. The peripheral system comprises the nerves.

The brain is a very complex organ with many parts. A horse’s brain is similar in shape and function to that of other mammals. Specific groups of nerve cells, or ‘centres’, in different parts of the brain, are specialised to perform different tasks.

The brain’s shape conforms approximately to the cranial cavity of the skull. Between the delicate nervous tissue and the protective bones are three membranes, the meninges. The space between the two inner membranes is filled with clear, colourless, cerebrospinal fluid which provides additional mechanical protection and a stable chemical environment for the brain.

An adult horse’s brain weighs 400-700g, accounting for 0.1 per cent of body weight. This compares with 2 per cent for humans. This percentage difference has been used simplistically to compare intelligence amongst different animal species. As each animal has its own kind of intelligence, such comparisons are ultimately rather meaningless. IQ depends on how it is defined and tested. It is more useful to recognise that horses are good at being horses, and to study their particular strengths. Like the brains of all mammals, the horse’s brain is divided anatomically into three sections: the hind-brain, the mid-brain and the fore-brain.

The hind-brain consists of the brainstem (literally the ‘stalk’ of the brain) through which pass all the nerve fibres that relay signals from the spinal cord. The medulla and pons of the brainstem contain neuronal ‘centres’ (groups of nerve cells) that control, automatically and without volition, breathing rhythm, coughing, blood pressure and heart rate. Other centres control posture and muscle tone and others promote sleep and wakefulness. The cerebellum, a globular structure at the back of the brain, is important for co-ordinating and learning movements and for controlling posture and balance.

Tucked underneath, the mid-brain is a short portion containing centres for visual and auditory reflexes and for voluntary movement. The mid-brain leads directly to the diencephalon (part of the fore brain) consisting of various structures including the pituitary and pineal glands, hypothalamus and thalamus. Another fore-brain area, the limbic system, is involved with olfactory (smell) sensations, emotions, learning, endocrine functions, and (along with the hypothalamus) the expression of sexual behaviour, fear and rage. The remaining fore-brain structure is the cerebrum, which makes up the bulk of the brain. Its two separate left and right cerebral hemispheres are connected by bundles of nerve fibres, the largest being the corpus callosum.

The cerebral cortex is the rumpled grey outer layer of the brain. It is relatively thin but accounts for almost half of the volume of the cerebrum because it is so highly folded. Ascending pathways transmit information from touch-sensitive receptors in the skin, through the brainstem and thalamus, finally projecting a map of the whole body on to a discrete (somatosensory) part of the cortex.

Neurones in another area, the motor cortex, contribute directly to movements involving the use of individual muscle groups. In these maps, the size of the projection is exaggerated according to how many nerves supply each part of the body. As the horse chiefly uses its lips and muzzle for tactile exploration, these areas are extensively represented in the brain. There are similar projections for the visual and auditory senses. Smells are not seen because the visual areas of the brain do not receive impulses from olfactory receptors! The cerebral cortex is also considered to be the part of the brain responsible for an animal’s awareness of objects and situations, and for mental activities such as problem-solving.

While it is convenient to subdivide the brain into parts on the basis of anatomy or function, all these parts are interlinked and influence each other in ways which we do not yet understand.

The spinal cord, which is the thickness of a thumb, runs down the length of the vertebral canal, on average reaching a couple of metres from head to tail. Inside the cord is a butterfly-shaped core of nerve cells (grey matter) surrounded by the tracts of the nerve fibres (white matter) which carry impulses up and down the cord.

All the ascending and descending nerve tracts that link the trunk and limbs with the brain pass through the spinal cord in the white matter. Nerve bundles emerge from the spinal cord in pairs through holes between adjoining vertebrae: 7 cervical (neck), 18 thoracic (chest), 5 or 6 lumbar (loins), 5 fused sacral (croup) and 5 out of 20 or so caudal (tail) vertebrae.

The grey matter contains the large nerve cells, the motor neurones, that innervate the skeletal muscles. In part, they respond to impulses in pathways descending from the brain, but they are also involved in reflex loops within the spinal cord.

Skeletal muscles are often called ‘voluntary’ muscles, because their actions are under conscious control. However, not all skeletal muscle activity is voluntary. Many reflex responses occur which involve specific sensory stimuli leading automatically to a fixed set of muscle contractions.

Many reflexes have a protective function. For example, an object moving close to the eye or a tap on the bone just below the eye will cause the horse to blink (the eyelid or palpebral reflex). Blinking also occurs when the surface of the eve starts to dry out or become irritated. Foreign matter in the eye also leads to the production of tears (lachrymal reflex). Bright light directed into one eye causes the pupil to narrow (pupillary light reflex), thereby preventing the light-sensitive cells in the eye from being overwhelmed. One reflex horses possess that we lack is the skin twitch (panniculus) reflex which is particularly evident when a horse is being troubled by biting flies. Reflex muscle action also is important in the control of posture and muscle tone, providing a background level of muscle activity appropriate for effective voluntary actions. Vets use the absence of standard reflexes to diagnose neurological problems, or during surgery, to determine the depth of anaesthesia.

At the cellular level, reflexes involve a sensory detector and afferent neurones which are linked, within the central nervous system, to an efferent motor neurone. This pathway is known as the reflex arc. In a few reflexes there is a direct connection via a single synapse in the spinal cord between the afferent and efferent neurone. An example is the ‘knee-jerk’ reflex, which involves a sudden twitch of a limb muscle caused by tapping the tendon.

Most reflexes, however, are ‘polysynaptic’ as they involve one or more intermediate neurones. An unpleasant stimulus, such as a pin prick, applied to a limb will cause it to be promptly withdrawn. As the flexor muscle is made to contract, signals to the antagonistic extensor muscle, which acts in the opposite direction, are inhibited allowing it to relax. This stops the two muscles from working against each other. But if the horse simply pulled one leg up, it might lose balance and fall over. So the muscles in the other limbs are commanded to re-distribute the weight of the body to maintain balance.

Co-ordination of movement in this way occurs more or less automatically. Usually, higher brain centres, and not just the spinal cord, are involved in these more complex reflexes. Although the initial response to the pin prick is instantaneous and involuntary, the message is likely to filter up to a level where the horse becomes aware of a painful or unpleasant sensation. This may then provoke a voluntary reaction, such as running away or kicking out.

The peripheral nervous system consists of nerve bundles extending out from the CNS to the body and limbs. Peripheral nerves may be classified by function into afferent nerves (bringing signals into the CNS) and efferent nerves (carrying signals out). The efferent nerves can be sub-divided into somatic and autonomic nerves.

Somatic nerves have cell bodies in the brainstem or spinal cord and run their axons directly to muscle cells. These are often called ‘motor’ neurones because their activity leads to contraction of the innervated muscle. Autonomic nerves make connections, via synapses in cell clusters called ganglia, to smooth muscle in blood vessels and to glands throughout the body.

A hormone is a chemical messenger secreted into the circulation by an endocrine gland. It is carried by the blood stream to target cells which contain receptors for the particular message.

When hormone molecules bind to receptors, a response is initiated. For example, when blood pressure falls, perhaps as a result of haemorrhage and reduction in blood volume, the pituitary gland secretes a hormone called ADH (anti-diuretic hormone) which acts on the kidneys, its target organs. This causes them to reabsorb more water and produce a concentrated urine, thus limiting further fluid loss.

The endocrine and nervous systems are similar in that both involve the transmission of a message that is triggered by a stimulus and produces a response. But because nerve impulses can travel much faster than blood-borne substances, nervous system responses are more rapid.

On the other hand, hormonal responses are often long-lasting because it takes time (anything between minutes and days) for hormones to be broken down or excreted. In contrast, nervous responses are short-lived: for example, the twitch of a muscle is over in a fraction of a second.

One other difference between the two systems is that nerve impulses are transmitted to specific destinations, whereas hormones circulate throughout the body and may affect several target organs in different locations.

This table illustrates the great diversity of endocrine glands and hormones. Some glands secrete hormones in response to a simple change in the level of a substance (e.g. calcium) in the surrounding fluid. Others respond to nervous system signals. For example, when a foal suckles its dam, receptors in the teat are activated and impulses conveyed to the pituitary gland by way of the spinal cord. This results in the prompt release of oxytocin into the blood. When this reaches the mammary gland, 20 seconds or so later, it diffuses out of the capillaries and causes contraction of the cells surrounding the alveolar ducts, leading to milk ejection or ‘let-down’. Finally, a number of endocrine organs respond to hormones from other glands.

Organs may be primarily endocrine in nature: the pituitary, the pineal, and the thyroid, parathyroid and adrenal glands. Other organs combine endocrine with other important related functions: the pancreas, testes, ovaries and placenta. A third group comprises organs with quite different primary function, hut which are also capable of producing hormones: the kidneys, liver, thymus, heart and gastro-intestinal tract.

The pituitary gland is sometimes called the ‘master gland’ because many of its hormones function chiefly to regulate the activity of other endocrine glands. It is an appendage of the brain, suspended below the hypothalamus by a thin stalk containing nerve fibres and small blood vessels.

The close connection between the endocrine and nervous systems is illustrated by the combined activities of the sympathetic nervous system and adrenal glands. In stressful situations, when an animal is forced to challenge an attacker or run from it (the ‘fight or flight’ response), there is widespread stimulation of the sympathetic system of the body. This results in a rapid pulse and rise in blood pressure. The output of the heart is increased and the flow of blood directed primarily to the skeletal muscles, at the expense of that to the skin and abdominal organs.

The middle part of the adrenal glands, the adrenal medulla, secretes the hormone adrenaline, which is chemically very similar to the substance noradrenaline produced at the endings of sympathetic nerves. Adrenaline evokes the same responses as impulses in the sympathetic nerves. It also produces a marked increase in metabolic rate.

Thus the combined effect of the endocrine and nervous systems is to prepare the body for emergency.

Horses perceive the world around them using the same senses that we do. Broadly speaking, a horse’s senses work in the same ways as ours, but differ in their capabilities and in how they are used. We will never know what it is really like to see or hear like a horse. Still, some appreciation of horses’ unique perceptivity is crucial to understanding why they behave in the way they do.

The central nervous system is supplied with input from an extensive array of sensory receptor cells.

Some are scattered throughout the body, in every muscle, tendon, ligament and joint capsule. These pick up changes in mechanical strain, and keep the brain updated from moment to moment about the relative positions of the limbs, neck and head. The skin contains a great variety of receptors too, which make possible a range of cutaneous sensations.

All these receptors are modified nerve endings, whose shape and type of encapsulation determines the kind of stimulus to which they respond. They occur singly or in small groups. In contrast, large numbers of receptors are gathered together within the eyes, ears and nose, highly-specialised sense organs which enable features of the outside world to be discerned in considerable detail.

The ancestors of modern horses avoided being eaten by fleeing first and evaluating the situation later. Vision is the primary danger detector for a horse, as well as influencing almost every other aspect of his behaviour. The size of the eyes (the largest of any land mammal), the area of the cerebrum devoted to processing visual information, and the fact that a third of all sensory input to the brain comes from the eyes, indicate the relative importance of sight.

Eyes work by focusing rays of light from external objects to form an upside-down image on a layer at the back of the eye called the retina. There photoreceptors (light-sensitive cells) transform the image into a pattern of nerve impulses which are conveyed, via the optic nerve, to the brain.

Two types of photoreceptors are present in both horse and human retina: rods and cones. Rod cells are more sensitive to light of low intensity, but provide no information about its wavelength (colour). Thus they are more suited to night vision. Cone cells need more light to be activated, so come into play more during daylight hours. They also respond differently to different colours, depending on the photopigment present in the cell. Humans have three cone types (red, green and blue); stimulation of these by varying amounts allows us to discriminate many subtly different shades.

The extent to which horses are able to distinguish colours is still the subject of inquiry. What is clear, however, is that horses have at least some colour vision, contrary to the old belief. Recent research indicates that they can discriminate between grey and four individual colours (red, yellow, green and blue), though some horses seem to have a more difficult time telling yellow or green apart from grey. This suggests that horses have only two cone types, in common with some other mammals including dogs, pigs and squirrels.

In the human retina, there is a central spot where photoreceptors and connecting neurones are packed together more tightly. Images focused at this point are seen with the greatest detail and visual acuity is at its highest. Because we cannot see equally well in all parts of the visual field, we use eye movements to shift attention from one feature to another. The equine retina also has a high-acuity spot, called the area centralis. When horses look face-on at an object, this is where the image falls. But there is also a second area of high-density photoreceptors and acute perception, which extends in a horizontal band across the equine retina. The location of this so-called ‘visual streak’ allows the horse to scan the horizon while grazing. Most of the time, the eyeball is reflexively rotated by two (of seven) attached muscles to keep the eye aligned with the horizon. Although horses have an extended field of gaze, they are not quite as able to make out fine detail as a person with average eyesight. However, they are extremely sensitive to variation in all parts of the visual field. This accounts for their tendency to become alarmed at the slightest unfamiliar movement and their ability to notice subtle body-language cues.

Equine eyes are set to the side of the head, and forward. The field of vision is obstructed only by the body and the shape of the head. Apart from a narrow blind area behind, in an arc of 10 degrees or so, a horse can see almost all of the 360 degrees around him in the horizontal plane. In the vertical plane, the retinal field of view is almost 180 degrees.

While grazing, a horse has only to turn his head slightly to check for signs of danger behind him. Although horses have laterally-placed eyes, there is a region in front of 60-70 degrees (depending on the breed) in which the two eyes can focus on objects – roughly half that of man and typical predators.

An advantage of this binocular mode of vision is that the brain can combine two slightly different images to judge the distance of an object, a process called stereopsis. In the horse, the binocular area is limited by the nose, which obscures anything directly underneath and up to four foot in front of it. Therefore, a horse must be able to turn or lower his head in order to see where he is treading and to navigate obstacles safely. It is worth bearing this in mind whilst riding and particularly when jumping. Your horse needs a degree of freedom to move his head to obtain a good view of the jump in the approach phase and also to see where his front feet will land.

For an object to appear sharp and not blurred, its image must be focused precisely on to the retina. The ability of the eye to focus on near and far objects is called accommodation. Much of the necessary bending of light rays is done by the curved surface of the transparent cornea. Changes in focus are brought about by the contraction of ciliary muscles which pull on the elastic lens located towards the front of the eye. When these muscles are relaxed and the lens is more spherical, the eye focuses on distant objects – thus horses are naturally ‘long-sighted’. However, horses are subject to the same kinds of visual defect and deterioration with age as we are. It has even been suggested that domestication and in-breeding of the horse has introduced a degree of myopia, or near-sightedness.

According to the ‘ramp retina’ theory, it was assumed that the equine lens is inflexible and that a horse must raise and lower its head to bring objects at different distances into focus on upper and lower parts of the retina. This has now been disproved by optical measurements. Any head movements probably have more to do with bringing things out of blind spots and into binocular view. Nevertheless, it is likely that horses are unable to bring about changes in focus as rapidly as we can, so sudden moves nearby may be all the more startling.

Horses are vigilant for virtually 24 hours a day, so good vision is required both in daylight and in darkness. The iris, a flat ring of tissue containing smooth muscles between the cornea and lens, limits the amount of light entering the eye through the pupil. Under reflex control, the pupil closes to a narrow horizontal slit in bright light, matching the orientation of the visual streak. Extra shading is provided by irregular lumps, the corpora nigra, on the top edge of the pupil.

In dim conditions, the pupil enlarges and becomes more circular, admitting the maximum amount of light. Horses’ eyes contain an additional shiny layer behind the retina called the tapetum lucidum, a feature shared by nocturnal animals. This is responsible for ‘eyeshine’, when the eyes are illuminated at night. Light that has passed through the retina without being absorbed, and hence not registered, is reflected by the tapetum back through the photoreceptor layer, giving it a second chance to be detected. As a result of these adaptations, horses have much better night vision than humans and are quite able to find their way around in the dark.

The eyeball itself is quite tough, given shape and stiffness by the outer, fibrous sclera. However, the eye has delicate parts, injury to which is always a serious matter as it could spell disaster for a wild horse.

The eye has a blood supply to sustain its tissues. A dense network of blood vessels forms the uvea, comprising choroid, ciliary body and the iris. Several structures protect the eye. The eyelid, with its lining of conjunctiva, is perhaps the most obvious. The orbit has a strong ridge of bone, and fat in the cavity behind the eyeball acts as a cushion. Lachrymal glands produce fluid secretion which passes through ducts to the surface of the eye, where it prevents abrasion and damage to the cornea. Tears also contain lysosyme which has an anti-bacterial effect.

Finally, horses are born with protective reflex responses (see above), although it may take several days for a neonate foal to acquire a ‘menace response’ to sudden movement.

Sound carries messages in a different way to light – as vibrations in the air. Sound can go around corners and carry over large distances. Horses have a well-developed sense of hearing, using it both to detect predators and in communicating with other members of the herd. Horses also readily learn to respond to vocal cues from their human handlers and riders.

The ear converts sound waves in the environment into action potentials in the auditory nerves by finely-tuned structures located inside the temporal bone of the skull.

The part of the ear we can see is the external ear, or pinna, which collects sound and funnels it down on to the eardrum. Within the middle ear, sound is transmitted through a series of three tiny bones (ossicles) – the hammer (malleus), anvil (incus) and stirrup (stapes) – to the oval window where it sets up vibrations in the cochlea.

The cochlea consists of a spiral canal, set in bone and filled with a fluid called endolymph. Running up the spiral is row of exquisitely sensitive ‘hair cells’ which synapse with the neurones of the cochlear nerve. High-pitched sounds are picked up towards the near end of the row; low-pitched sounds towards the far end. Very loud sounds initiate the tympanic reflex: the contraction of two small muscles in the middle ear which dampen sound transmission and protect the fragile cochlear mechanism.

In another part of the fluid-filled bony labyrinth of the inner ear are three semi-circular canals and a pair of sacs, the utricle and saccule. These serve the second major sensory function of the ear, which is to provide the brain with information about head movements and position, which it then uses to maintain balance.

Each ear can be swivelled independently through 180 degrees, or laid back, shutting it off. Such mobility is achieved with 16 auricular muscles attached to the base of the pinna. Humans have only three such muscles, all of which are vestigial.

Easily visible at the top of the head, a horse’s ears are used to signal emotional state and intent. Laid-back ears may indicate aggression, or may simply be protection against a loud noise.

In response to directional sounds, a horse flicks an ear towards the source, or, if the sound is coming from the front, pricks both ears forward. This so-called ‘Pryer reflex’ improves the acuity of hearing and gives us an indication of what the horse is listening to. It also provides a way to test a horse’s hearing abilities. In this way we find that their sense of hearing is somewhat better than ours. Horses can hear high-pitched tones with frequencies of 25kHz or above – almost an octave higher than we can. However, the frequencies at which sensitivity is best are much lower in horses and humans: around 2kHz and 4kHz respectively In this middle range, horses have been shown to be capable of discriminating very small differences in tone and loudness. Yet, like us, their hearing does decline with age.

Some horses have been reported to become agitated before earthquakes. They may indeed be responding to low-frequency sound waves, but through feel rather than hearing.

Despite their evident sensitivity, horses do not react to all sounds in their environment. Clearly, a considerable amount of filtering takes place in the brain to ensure that, in general, only relevant sounds are acted upon.

What makes a sound interesting to a horse? That will depend on factors such as novelty, biological relevance and motivation. Instinctively, horses attend to vocalisations of other horses. But they may also learn to distinguish sounds which are not part of their natural repertoire. For example, horses kept next to a main road have been known to whinny in anticipation of feeding on hearing the approach of their owner’s car, while totally ignoring all the other passing cars. Anything, such as wind, which hampers detection and pinpointing of sounds, can make a horse nervous and ‘spooky’.

As a horse approaches a strange object to investigate, not only are his eyes and cars directed towards it, but he also extends his head and neck to sniff with open nostrils. The equine skull evolved to be its present shape due to the need for a large set of grinding molar teeth. Having an elongated nose, however, also means there is a large surface area within the nasal cavity for detecting odours.

Smell is used to locate water, select food, and avoid predators. It is involved in aspects of social behaviour too: mares and foals recognise each other partly by smell, horses exchange breath on meeting, stallions assess the sexual status of mares and leave aromatic messages in their dung piles.

Air containing volatile substances enters the nasal passages when the horse inspires. Closer to the nostrils, the surface tissue (epithelial mucosa) contains few sensory cells and is more concerned with cleaning, warming and humidifying the air before it comes into contact with the olfactory areas. Further up, the air flows through tightly-rolled ‘turbinate’ bones which are overlaid with yellow-brown mucosal tissue. This is a mixture of chemo- receptor cells and supporting cells which secrete a layer of mucus, covering the epithelium.

Projecting from the peripheral end of each receptor cell are microscopic filaments which further increase the area of surface membrane available for reception of chemical stimuli. The axon of the receptor neurone join others to form the olfactory nerves, which pass through tiny holes in the ethmoid bone, converging finally on the olfactory bulbs of the brain.

In man, the olfactory epithelium is only 3-4 square cm in area; in a horse, it may be a hundred times or more larger than this. Imagine, then, the richness and importance to horses of their sense of smell!

Opening into the floor of the nasal cavity is a pair of accessory ducts that are also involved in scent sampling in horses and other domestic animals, though not in humans – the vomeronasal organ (also called the Organ of Jacobson). This screens the air for pheromones, chemical signals linked to specific behavioural reactions.

A stallion will often curl his upper lip in the ‘Flehmen’ response after sniffing the urine of a mare. This is thought to facilitate the transfer of pheromones, dissolved in nasal secretion, into the vomeronasal organ. Pungent or unusual smells may also provoke Flehmen, not only in stallions.

Taste, also referred to as the gustatory sense, plays a significant role in survival by allowing horses to avoid ingesting unpalatable and potentially harmful food (or tainted water). Horses have to be particularly cautious about what they eat because, unlike many other animals, they are unable to vomit. How something tastes also depends on its smell, so the two senses are closely related.

In the epithelium of the mouth, chemoreceptors and supporting cells are grouped together in onion-shaped taste buds, about 0.2mm across. These occur on the soft palate and epiglottis as well as in papillae on the tongue. To stimulate the receptor cells, substances dissolved in oral fluids must reach the tiny pore of the taste bud, through which hair-like microvilli project. There they interact with the cell membrane, where they generate action potentials in sensory neurones. Relatively slow-conducting axons from the taste buds are routed via cranial nerves to unite in the medulla of the brain.

In man there are four basic taste ‘modalities’: sweet, sour (or acid), bitter and salty. It would be wrong to assume that the same tastes are experienced by animals: what is sweet to us may be perceived entirely differently by a horse. One can, however, find out whether horses can tell the difference between, for example, pure water and water with something added to it. Thus we know that horses can detect the above modalities, though there are variations between individuals.

It is not surprising that salt is tasted, as a deficit of sodium in the body stimulates an appetite for salt. In one study with foals, sugar solutions were preferred to tap water, supporting the notion that horses have a natural ‘sweet tooth’. One notable difference between horses and humans is that horses seem to tolerate things which would taste extremely bitter to us.

The skin, which will be covered in detail in the next chapter, covers the body, protecting it against injury, invasion by micro-organisms and dehydration. It participates in the control of body temperature through the cooling effect of sweating. The skin has a sensory function too: the immediate environment is felt through this medium, with sensations of touch, pressure, vibration, cold, heat and pain.

Some parts of the body surface are more sensitive than others. This is partly due to differences in thickness of coat and skin. The latter ranges from 1-6mm and is thickest where the mane grows from the neck crest and on the upper surface of the tail. Skin thickness also varies with age, sex and breed and from one individual to another. Thoroughbreds tend to have a rather thin and delicate skin, whereas draught horses have a thicker, coarser skin.

In addition, the number of sensory receptors varies from hundreds to thousands per square inch of skin on different parts of the body. Many of these receptors are simple nerve endings, while some, looking like tiny bulbs or discs, are specifically sensitive to either light or sustained touch. Tactile sensitivity is particularly great around the lips, nose and eyes, due to a higher concentration of receptors and the presence of long, stiff hairs whose follicles are surrounded by nerve endings. Whiskers are important to horses because they indicate when the nose is close to an object. They may also be used while feeding to judge textures. Shaving a horse’s whiskers off just for cosmetic purposes should therefore be discouraged.

Touching plays a vital role in communication between horses, particularly between mare and foal, and in courtship. Mutual grooming helps to cement friendships within the herd. We also rely on the horse’s keen tactile sense in riding, through the use of legs, seat, hands and whip. A responsive horse remains sensitive to subtle signals. On the other hand, repeated and indiscriminate use of harsh aids is likely to result in a ‘hard’ mouth and/or ‘dead’ sides, as the sense of touch becomes desensitised.

Horses prefer to be stroked rather than patted. Gentle stroking of certain areas of the body, such as the withers, may be used both to calm and to reward. Touching is the basis of massage therapies such as Tellington Touch and Shiatsu.

In conclusion, all the hormonal, nervous and sensory adaptations discussed in this chapter are genetically programmed and, to some extent, fixed. While a horse may learn new behaviours and skills, training can never completely overcome his innate instincts and drives. Knowledge of these systems and how they affect behaviour help us work with, rather than against, the horse.

Throughout its life, a horse’s innate abilities and instinctive responses are shaped, refined and extended by learning. Experiences perceived, using the senses described, lead to the formation of memories and lasting changes in behaviour. Sometimes, the fact that a horse has learned something is not immediately apparent, although the information may be used later (so-called ‘latent learning’).

Horses naturally learn best those things which are biologically relevant to them: what is dangerous and what is not, the smell and texture of good food, where to find water and shelter, who’s who in the herd, sureness of foot, how to mate, or fight, or scratch those itchy spots. However, we also expect them to learn specific athletic skills, ‘good manners’, and how to live within the unnatural constraints we impose upon them.

Horses learn in various ways. One simple type of learning is ‘habituation’, in which the response to a particular stimulus becomes less and eventually ceases with repetition. Thus a horse becomes quickly desensitised to frequent sights and sounds which are initially alarming but turn out to be harmless. The horse may react again to the same stimulus after a long time without exposure (‘spontaneous recovery’), but typically re-habituates more rapidly. Habituation may be used to get a horse accustomed to unfamiliar objects or procedures (e.g. clippers).

In ‘flooding’, a horse is confined or restrained so that it cannot escape while being exposed repeatedly to the stimulus, until the horse ignores the stimulus and becomes calm again. A gentler alternative is called ‘progressive de-sensitisation’. Here exposure to the stimulus is carefully controlled so that the horse never becomes fearful enough to precipitate the flight reaction. This approach takes longer, but is less likely to have secondary effects on the horse’s attitude to people.

Other kinds of learning involve associating between two events. In ‘classical conditioning’, a horse learns that a signal or cue, initially of no significance, is followed by an event or stimulus which is significant (and which produces a response). For example, in the wild, the appearance of a predator may be preceded by the alarm call of a bird. By learning this natural signal, a horse’s ability to survive may be increased. In the domestic setting, a horse may similarly learn to associate the sound of buckets with feeding.

In ‘operant conditioning’, also known as trial and error learning, the performance of a behaviour is changed by the consequences of that behaviour, which may be pleasant or unpleasant. When a newborn foal discovers where its mother’s teats are located, it is immediately rewarded with its first drink. The foal’s tendency to head for a dark under-surface may be instinctive, but the most efficient ways to obtain milk are learned through trial and error. In this example, suckling behaviour is learned through ‘positive reinforcement’: something pleasant occurs after the initial action of sucking which makes the action more likely to occur again on future occasions. The ‘something’ here is the ingestion of milk, which satisfies a physiological need.

‘Negative reinforcement’ also increases the likelihood of an action. However, in this case the action is performed in order to escape or avoid an unpleasant or aversive stimulus. Thus a horse learns to yield to pressure applied through a halter. By moving his head in the direction of pull, he is rewarded by an instant release of pressure and regaining of comfort. With good timing, lighter and lighter contact can be learned.

In contrast, punishment is an aversive stimulus given after an action with the intention of decreasing its likelihood. It may succeed in stopping an unwanted behaviour, but in general it is an antiquated approach not well-suited to horses. It may make a fearful horse more afraid or an aggressive horse more aggressive. These emotional states are not conducive to learning.

Alternatively, a horse might habituate to repeated, ineffectual aversive stimuli. Some ‘punishments’, such as shouting at a horse that kicks its stable door, may actually reward the behaviour because the horse has succeeded in getting your attention. In such cases it is better to ignore the offending behaviour completely until it extinguishes itself.

In training which relies on associative learning, timing is crucial. The reward or release should be delivered as soon after the desired action as possible. If it is delayed more than a second or two, it may be useless, or worse – you may be rewarding the wrong thing. For this reason, verbal praise, stroking or some other signal (such as a click) may be used as ‘secondary reinforcers’ which can be given in a more precisely timed way They may bridge the delay between the action and the primary reward (usually a treat), or may substitute for food if perceived as pleasant. Rewarding every time (‘continuous reinforcement’) is useful in the early stages of training. However, if the rewards stop so does the trained behaviour, a process called ‘extinction’. By rewarding only once every few successful attempts (‘intermittent reinforcement’) extinction is reduced and the lesson retained for longer.

Teaching a horse a complex skill, such as jumping, is made easier by ‘shaping’. This involves reinforcing successive approximations, a step at a time, towards the final goal. Sequences of actions can be taught by ‘chaining’ simpler actions together.

Horses may learn through imitation, though convincing scientific evidence for this is lacking. However, some learning, such as the location of water, may be facilitated by following and watching other horses. It is no longer thought that vices such as cribbing and weaving are acquired through imitation.

Studies of learning in horses have revealed aspects of memory, as well as sensory and other cognitive abilities. For example, one horse learned to obtain a food reward by picking the correct choice in each of 20 pairs of patterns. At the end of the training period, the horse’s performance with four of the pairs was perfect, and even the ‘hardest’ pair was discriminated correctly 73 per cent of the time. A year later, the horse showed hardly any memory loss. Not all horses do equally well at such tests, though a poorer performance might indicate an individual was simply less motivated, rather than a lack of innate ability. Despite all the research done to date, we still have much to learn about the horse’s mind.