HORSES bear sensitive whiskers on both their muzzle and around their eyes. It’s not uncommon for owners to trim these off for cosmetic reasons, especially in show animals. However, there’s evidence to suggest that these specialized hairs contribute to a horse’s sense of touch and spatial awareness.

Germany, Switzerland and most recently France have banned whisker trimming in horses, based on concerns that their removal is potentially detrimental to the horse.

Facial vibrissae – the technical term for whiskers – are specialised sense organs which function as short-distance vision replacement in many species of mammals, and are used for tactile perception.

These whiskers are much larger than other hairs and have a prominent bump or follicle at the base, which is rich in nerve endings and blood vessels. The nerve endings are mostly pressure-sensitive mechanoreceptors, making whiskers extremely sensitive to touch.

As the whiskers brush along a surface the animal is informed about the distance, direction and texture of the object. Because of their location; protruding from the skin surface for several centimetres, they extend an animal’s sense of touch.

Most whiskers have a circular cross section with tapered tips (apart from seals and walruses, who have oval whiskers). The narrow tip appears to help the animal to gather more detailed information about the surface texture of objects it comes in contact with. Whiskers are prone to wear and tear and are replaced continuously but never all together, even when an animal sheds a heavy winter coat or moults.

Immobile whiskers

Horses (and cattle) have immobile whiskers while other species (cats, seals) have whiskers on their muzzle that they can move due to the action of muscles attached to the base. Animals may also have immobile whiskers elsewhere on their head, such as around the eyes in the case of horses.

Rodents can typically ‘whisk’ the whiskers on their upper lip rapidly back and forth, and in circles. Rat whiskers are the most closely studied and we know that rats can distinguish tiny openings and changes in surface textures and angles very rapidly and with a high degree of accuracy.

This allows them to easily navigate their environment, even at night. Likewise, Etruscan shrews are a nocturnal species that eat mostly locusts (small, fast and agile insects).

The shrews use their whiskers to find and capture their prey in the dark. However, these rodents need multiple intact whiskers to achieve this – their accuracy is dramatically reduced if some or all of their whiskers are trimmed off.

Seals can use their whiskers to locate the currents in water left by fish, helping them to hunt.

A European research group discovered that a trained Harbour seal called Henry could detect such ‘fish trails’ in the water for up to 35 seconds after a fish had swum past. He could also distinguish the size and speed of objects in the water, allowing him to decide if a potential meal was likely to be worth pursuing and eating.

Immobile whiskers are used to help animals locate items near the head/body and orientate in response. This serves to protect the face/head by informing the animal of nearby objects, especially in the animal’s ‘blind spots’ around the head, or a dark environment.

We don’t have much specific information about immobile whiskers, especially in horses, but we do know that they are used to inform the animal about items close to its head, especially in the dark. This helps the horse to avoid banging into things and reduces the risk of head injuries.

Given the prominent location of equine eyes on the side of the head, and a horse’s reliance on their wide field of vision to help them avoid predators, the presence of whiskers around the eyes appears to reduce their risk of ocular trauma. Horses can readily navigate woodland or scrub in low light conditions and their whiskers appear to help them achieve this without injury.

Whisker trimming in horses is usually done for cosmetic reasons. However, given the importance of whiskers in informing the animal about its environment, and the evidence we have from studies in other species that whisker trimming or removal is detrimental to this function, perhaps we should be leaving them alone.

Domestic horses in dark stables, trailers or pens are potentially at a greater risk of head or eye injury, especially if there are fittings such as hay racks, hinges or door frames at eye level.

Without their whiskers, it’s more difficult for the horse to detect and avoid such items, before they make contact with their head, so it appears to make sense to leave them well enough alone.

Other ideas

Why all the interest in whiskers? Robotic whiskers could enable machines to obtain information about an object prior to lifting or manipulating it. This would be very useful in warehouses for example, where designing robots to move or pack a wide variety of items without damaging them is a challenging design problem. Mechanical whiskers could help search-and-rescue robots to navigate safely through dark, dusty and smoke-filled spaces.

Karen Dunne, MA, CertEM (StudMed), MVB. Veterinary nursing programme director at Dundalk Institute of Technology (DkIT). Member of the education, research and industry (ERI) and equine groups of Veterinary Ireland.

Her interests include large animal nursing, equine and stud medicine and education.