Yes, horses do blink, but the subtle nature of the action often makes it difficult for human observers to notice. Blinking in mammals serves the fundamental purpose of protecting the eye and maintaining the health of the corneal surface. The equine blinking mechanism is subtly different from humans, incorporating an extra structure that provides a unique layer of rapid protection.
The Physical Action of Equine Blinking
The visible part of the equine blink involves the upper and lower eyelids, which are thin folds of skin controlled by muscles like the orbicularis oculi. Unlike the human blink, which is often a rapid, complete closure, a horse’s blink is frequently incomplete, involving mainly the downward motion of the upper eyelid.
When a complete blink does occur, the lower eyelid moves upward to meet the upper lid, covering the entire corneal surface. The entire duration of a complete blink averages less than half a second. Eyelid closure is notably swift, taking about 0.162 seconds, which is roughly twice as fast as the time required for the eyelid to open again.
The primary role of these outer eyelids is to distribute the tear film, a mixture of water, oil, and mucus, across the cornea to keep it moist and clear of minor debris. The tear film also helps to slow the evaporation of moisture from the eye.
The Essential Function of the Third Eyelid
Beyond the familiar upper and lower lids, the horse possesses a specialized structure known as the nictitating membrane, or third eyelid. It is located in the inner corner of the eye near the nose, called the medial canthus.
The third eyelid is a pinkish, translucent flap of tissue containing a T-shaped piece of cartilage for structure. It is normally tucked away and barely visible, but it can rapidly sweep horizontally across the eyeball when triggered.
This action is typically a passive reflex, occurring when the eyeball retracts slightly into its socket, which pushes the membrane forward. This rapid sweep acts as a specialized windshield wiper, clearing larger foreign objects or debris from the corneal surface. A gland located at the base of the third eyelid also contributes significantly to the production of the watery component of the tear film.
Blinking Frequency and Behavioral Context
A horse’s blink rate is closely tied to its physiological state. When a horse is calm and relaxed, the average full-blink rate typically falls in the range of eight to nine times per minute. This rate is noticeably lower than that of humans, reflecting their status as a prey animal that needs to maintain constant vigilance.
The blink frequency can change dramatically when the horse is under environmental or emotional pressure. When horses are exposed to mildly stressful situations, such as being separated from their herd or having their feed withheld, their full-blink rate often decreases significantly, sometimes dropping as low as five full blinks per minute.
This reduction is often accompanied by an increase in small, rapid muscle movements around the eye, known as eyelid twitches. Conversely, the blink rate may also slow when a horse is sedated or intensely focused on a specific object in its environment.