How horses sense the world

Senses shaped by being prey

Horses are prey animals. Their senses evolved to give them the best chance of noticing a predator early and escaping. When something unexpected happens, a horse's instinct is to run first and check later. Understanding what a horse can and can't see, hear and feel explains a lot of its behaviour, and helps you handle it safely.

Sight

Almost all the way round

The Royal (Dick) School of Veterinary Studies describes horses as having virtually 360-degree vision, mainly so they can keep watch for predators approaching from any direction while they graze. That all-round view comes from having large eyes on each side of the head.

There is a price. Horses have blind spots directly in front of their face and directly behind them. This is why the British Horse Society teaches riders to approach a horse from the side, talking gently as they go, and not to walk directly in front of or behind it. A horse may not have seen you there.

Colour, light and dark

Horses don't see colour in the same way as people. They also find it difficult to adjust between light and dark, so a horse may become nervous going into a dark stable.

Inside the eye

The horse's eye has the same main parts as ours, plus one extra:

• Eyelids sweep the whole surface of the eye, clearing dust and spreading a thin film of tears that keeps the clear front of the eye (the cornea) moist and healthy.
• A third eyelid also sweeps across the cornea when the outer lids close. You can just see it as a pink, fleshy structure in the inner corner of the eye.
• The cornea has no direct blood supply, which keeps it transparent, but it has many sensory nerves. That makes it less able to fight infection and slower to heal than other parts of the body.
• The iris, usually brown, opens and closes to control how much light gets in. Along its upper edge is a granular structure called the corpora nigra, which is thought to filter the light further; some horses have a smaller one on the lower edge too.
• The lens focuses light onto the retina, whose millions of light receptors send images to the brain through the optic nerve.

The eye's prominent position makes it vulnerable to knocks and scratches. Even minor damage can quickly worsen and threaten the horse's sight in that eye, within hours or days through infection and inflammation, or over months or years through scarring.

Hearing

Horses hear higher-pitched sounds than people do: over 33 kHz, compared with under 20 kHz for humans, although they may not hear the very lowest sounds that we can. The British Horse Society notes that a horse's ears pick up sounds at a lower volume and from further away than ours do, and that, as in people, some horses hear much better than others. Hearing can decline with age or some health conditions.

This matters for handling and riding. A horse may hear a sound nobody near it has noticed, freeze to listen, then run from it. It may stop and turn back to check the source, or spin round to work out where the threat is. Horses can get used to noise, but noise they aren't used to is likely to distress them. And when a horse spooks, it may be reacting to something a person hasn't heard or seen at all, perhaps behind it.

Touch: whiskers and sensory hairs

Horses have coarse sensory hairs, or whiskers, mainly around the muzzle and eyes, and hairs in the inner ear. Whisker follicles are deeper and larger than other hair follicles, with their own blood and nerve supply, so whiskers are sensitive to the lightest touch, even air movement.

• Around the muzzle, whiskers help a horse feel for food, investigate unfamiliar food and detect small objects it shouldn't eat. Their length helps the horse judge a safe distance from objects in the blind spots in front of its face and under its nose. Often whiskers are its only source of information about those areas. Foals are believed to use them from birth to find their mother's teat.
• Around the eyes, whiskers on the eyelids trigger an automatic blink when something such as a fly touches them, which protects the eye.
• In the ears, hairs inside the ear give feedback and protect delicate structures, and the fluffier outer hair shields the inner ear from insects, sun and foreign objects.

The upper lip is also strong, mobile and sensitive, and moves forage towards the front teeth.

Whiskers were long trimmed off horses for showing. The British Horse Society advises that they are an important sensory organ and shouldn't be removed for cosmetic reasons. It notes that whisker removal has been banned in Germany and Belgium for many years and that many UK governing bodies have now banned it too. If you trim the outer ear hair, hold the ear gently closed so you don't take off too much.

For more on feeding and the mouth, see The digestive system.