Dr Rebecca Hamilton-Fletcher MRCVS investigates the science behind horses’ coats
An equine coat is socially significant, with a lustrous, glossy appearance indicating the health, virility and genetic fitness necessary for herd dominance. The coat also has other physiological roles that are important for a horse’s survival and can be manipulated through adaptation and management.
Hair is classified in three types: permanent, such as the mane, tail and feathers; tactile, such as the hairs on the muzzle and inside the ears; and temporary, referring to everything else. Each hair originates from a follicle under the skin’s surface, which is supported by a sweat-producing sebaceous gland plus a vascular, sensory and muscular system.
A horse’s coat colour is determined by the amount of black, brown or yellow melanin pigment in each hair, and is genetically determined. Other hereditary characteristics include coat length and texture.
These features are unique to each individual and may also be breed-specific. The Akhal-Teke, for example, has a cream gene that produces a metallic sheen. The Bashkir Curly, as its name suggests, has an unusual curly coat and furry ear lobes.
Regardless of colour or type, the equine coat has a remarkable ability to perform an array of useful functions.
Let it grow
Hair growth is not continuous but occurs in cycles, controlled by pre-determined physiological responses to environmental factors. The term anagen describes the active growth phase. Next comes the resting stage, or telogen, when the hair is technically dead. The transition period between the two is called the catagen.
This ability of the hair to grow to different lengths, in cycles — and to stand up or lie flat — means that it is well adapted for various roles:
- Thermoregulation: hair grows in response to daylight hours, rather than ambient temperature. The diminishing length of daylight in autumn causes the equine brain to increase melatonin production which, in turn, stimulates the extra hair growth necessary for winter warmth. The level of this response is breed-specific, with native types producing a heavier coat than other warmblood types.The reverse process applies in spring. This response can be manipulated by the use of artificial lighting to induce show horses to lose their coats earlier in the season.Interestingly, studies have shown that donkeys have a significantly lower hair weight and coat length compared with horses and mules. Donkeys show relatively little seasonal change, which explains why they cope less well in wet and cold conditions than hot, dry environments.In cold weather, the arrector pili muscle is stimulated to raise the coat hairs, creating multiple, body-warmed air pockets that act as a layer of insulation. Interfering with these processes by clipping or excessive rugging can make it difficult for a horse to maintain a stable body temperature and may, instead, encourage steady shedding throughout the year.
- Protection: the coat provides a barrier for chemical, physical and microbial challenges, such as wind, sun, rain, mud, UV light and insects. In addition, the mane and tail are invaluable for whisking away troublesome flies.
- Sensory perception: the tactile hairs of the eyelids and muzzle act as pre-warning “feelers”, sending important information to dedicated areas of the brain. It has been shown that if these hairs are trimmed, the horse is more likely to suffer facial injuries.
- Camouflage: coat colour and pattern can be integral to a horse’s survival in the wild.
- Communication: hairs are responsible for producing and transporting pheromones, which are natural chemicals that can impact the behaviour of other horses towards the individual.
Top coats
Hair is composed of protein and requires energy to grow and flourish.
A balanced diet with sufficient levels of zinc, iodine, biotin and methionine is necessary for good health and a gleaming coat. Grooming will further enhance the coat by removing dirt and any shed hairs, stimulating the circulation, distributing the natural oils from the skin and “polishing” the hair shafts.
Numerous cleaning and enhancing products are available, from shampoos and conditioners to glosses, polishing sprays and oil-based highlighters. While selective use of these can create a mirror-like finish, overuse can strip away natural protective oils.
Furthermore, consider the possible welfare implications before excessive trimming of tactile hairs or the use of oil-based products that can cause skin blistering in hot weather.
Just as a good coat suggests a healthy horse, so the reverse is true — the coat can be an accurate indicator of underlying problems.
Alopecia (hair loss) may be “primary”, where the hair shaft is directly affected, or “secondary”, relating to hair loss as a consequence of another condition.
An example of primary alopecia is ringworm, where the hair shaft is weakened by a dermatophyte fungus and breaks. Alopecia areata, in which an immune mediated inflammatory reaction around the hair bulb leads to hair loss, is far rarer and most likely to be seen in quarter horses.
Secondary alopecia tends to occur with an underlying skin condition, commonly resulting in patchy hair loss. Causes include rain scald, mud fever and allergies, as well as self-trauma due to fly bites, lice or sweet itch. Other more systemic skin problems reflected in the coat include auto-immune conditions, such as pemphigus.
Abnormal hair growth or appearance may indicate an inadequate nutritional status, while any general unthriftiness or systemic illness will result in a dull, lacklustre coat. Examples include excessive parastism, hypothyroidism and PPID (known as Cushing’s disease) — where a shaggy, curly coat can result from the imbalance of hormones from the affected pars pituitary intermedia gland. This “hirsutism” is due to a persistent anagen phase, which disrupts and delays normal hair shedding.
White hairs can result from injury and scarring, or natural ageing, although abnormal fading or sudden greying of the coat (termed vitiligo) could point to something amiss. The Arabian fading syndrome is an example of a progressive skin depigmentation condition that can also lead to localised areas of hair whitening, or leukotrichia.
Some breeds, such as the Appaloosa, have inherently reduced permanent hair growth, which accounts for the shorter, thinner tail often seen.
Splitting hairs
Vital information about a horse can be gleaned from his hair.
Since it is the bulb at the base of the hair shaft that stores this data, strands must be carefully plucked rather than cut. Extracted hair can then be used for DNA testing to confirm parentage or coat colour genetics, or as an aid to selective breeding by identifying hereditary diseases, such as fragile foal syndrome, foal immunodeficiency disease and hoof wall separation disease. While blood and urine are traditionally used for dope testing, hair is more sensitive and can indicate whether drug administration has been short- or long-term.
There is little evidence of the validity of using hair for allergy testing. It can, however, provide an insight into a horse’s nutritional status, revealing trace mineral levels plus possible over-exposure to environmental toxins or heavy metals, such as cadmium, lead or selenium.
Increasingly, hair is viewed as a source of DNA for future genetic manipulation and even for the cloning of horses.
Ref Horse & Hound; 6 February 2020
