There is nothing quite as disorientating as finding yourself plunged into the inky depths of pitch blackness. Humans are heavily reliant on sight, the loss of which causes significant psychological stress and disability. Yet, most of us happily leave our horses to roam their paddocks during the darkest night hours or switch off the stable lights, leaving the barn cloaked in impenetrable darkness. Are horses disabled in the dark, or can they see well enough to avoid crashing into obstacles?
Horses can see in the dark, up to a brightness setting of 23.77 mag/arcsec2; horses are capable of navigating uneven terrain and obstacles in class 1 dark skies. A 20:1 ratio of rod to cone photoreceptors, widely dilating pupils, and tapetum lucidum give the horse superior night vision.
Unless you have worked with a night blind horse, most people don’t give a second thought to how horses navigate their environment during the night hours. Do horses shuffle along, feeling their way around obstacles, or are they capable of seeing objects in the dark?
Can Horses See In The Dark?
Prey animals have three defense mechanisms that they can use to avoid predators:
Unlike the elephant or rhino, which will charge predators, or a tree frog that blends into its surroundings, horses have evolved to be the masters of flight. A horse’s first instinct is to run far away as fast as possible when faced with danger!
However, flight only works if horses can spot the predator before the would-be hunter is within pouncing range; it’s too late to escape a hungry predator if the horse only gallops off after it’s been bitten!
Any land-based animal that uses flight to avoid dangerous situations can see in the dark; efficient night vision is imperative for their continued survival. If horses could not see in the dark, the species would have gone extinct a long time ago, and there would be no horses today.
Is There Scientific Proof That Horses Can See In The Dark?
Common sense dictates that for horses to survive, they must be able to see in the dark. However, knowing that horses can see in the dark does not indicate how much light a horse needs to see; Afterall darkness exists on a scale from pure light to a total absence of light.
A 2009 study run by Hanggi and Ingersoll investigates how well horses can navigate an obstacle course in different light conditions (i.e., brightness).
A basic obstacle course was set up, and horses were tested in different brightness settings (i.e., 10.37 to 24.12 magnitudes per square arc second 2). Brightness and darkness are measured on a scale ranging from 0 to 100, with 0 being the brightest and 100 being the darkest.
All horses included in the study were able to efficiently navigate the obstacle course up to 23.5 magnitudes per square arc second 2 (mag/arcsec2) brightness; a few exceptional horses managed to successfully work their way around the obstacles with a brightness setting of 23.77 mag/arcsec2.
Unless you are a scientist, these results are difficult to interpret. Most people have never heard of the units “magnitudes per square arc second 2,” and thus, the results may seem irrelevant.
To put these results into a context that is easy to understand, it is essential to know the Bortle system of dark sky classification. Astrophysicists have developed a 9-point scale to rate the relative darkness of the night skies.
Class 1 night skies are rated as the darkest of dark skies; a class 1 sky has a brightness measure of 21.99 to 22.0 mag/arcsec2. Most horses have functional night vision up to 23.5 mag/arcsec2, i.e., even in the blackest parts of the night, horses can still see enough to navigate their surroundings.
Eye Structures Necessary For Equine Night Vision
The evolutionary pressures placed on animals have led to complex but exquisite adaptations and biological systems; one of the most advanced systems in any higher-order animal is the visual system.
To understand how horses see in the dark, it is essential to know what anatomical structures are involved and how they work together to create functional sight.
Due to the complexity of the visual system, only the structures relating to basic light interpretation are included here; all structures relating to movement or protection of the eye have been excluded from the following table.
|Cornea||Transparent dome forming the anterior (i.e., front) wall of the eyeball||Allows light rays to enter the anterior chamber of the eye and focus it onto the retina.|
|Iris||The colored portion of the eye (usually dark brown in horses) surrounding the pupil||Controls the amount of light entering the light-sensitive part of the eye by adjusting the pupil size|
|Aqueous humor||Watery fluid located between the cornea and lens in the anterior chamber||Refraction of light|
|Pupil||A horizontal oval hole in the iris center||Allows light to enter into the posterior (vitreous) chamber|
|Ciliary body||Attached to the lens and walls of the eyeball||Controls the thickness of the lens and thus light refraction|
|Lens||A disc-like structure, positioned behind the iris and pupil||Focuses light onto the retina|
|Vitreous humor||Gel-like fluid located behind the lens and in front of the retina in the posterior chamber||Refraction of light and holds the retina in position.|
|Retina||Located on the back interior wall of the eyeball||Contains light-sensitive structures|
|Cone Photoreceptors||Retina||Responsible for color vision|
|Rod Photoreceptors||Retina||Responsible for grayscale vision in low light|
|Choroid||Located behind the retina||Protects the retina and contains the tapetum lucidum|
|Tapetum Lucidum||Choroid||Reflection of light onto the photoreceptors|
|Optic Nerve||Retina and posterior wall of the eyeball in the eye’s natural blind spot||Conducts the electrical impulses from the photoreceptors to the brain|
If you’re struggling to create a mental picture of the anatomy of a horse’s eye, you can click here for a detailed diagram.
How Do Horses See In The Dark?
As light enters the eye, the light rays are bent (i.e., refracted) by the cornea, aqueous humor, lens, and vitreous humor. The amount of refraction is varied according to the lens thickness, which is controlled by ciliary muscle contraction.
Once the light has passed through these structures, it is focused onto the photoreceptor dense region (visual streak) within the retina. During bright light conditions, the cones are most active; however, the rods are most active during low light conditions.
The rod and cone photoreceptors convert light rays into chemical energy, which is then transformed into electrical energy. The optic nerve conducts the subsequent neural impulses and synaptic processes (i.e., electrical and chemical messages) to the brain, where the message is interpreted.
Only once the brain has decoded the electrical and chemical signals generated by the photoreceptors and nerves is the horse able to make sense of the visual information.
To effectively see in the dark, the eye needs to achieve the minimum threshold for photoreceptor reactivity; this is achieved in two ways:
- Increasing the amount of light hitting the retina and photoreceptors
- During dark adaptation, the sensitivity of rods is increased, lowering the threshold for stimulation
How Do Horses Adjust Between Light And Dark Conditions?
Vision is divided into three phases depending on which photoreceptor cells are active:
- Photopic: sharp color vision in bright light is due to the stimulation of cone photoreceptors without rod activation
- Mesopic: color vision in dim light due to duel stimulation of cone and rod photoreceptors
- Scotopic: greyscale vision in low light conditions is due to the stimulation of rod photoreceptors without cone activation
As a horse moves from light to dark areas, its eyes gradually adjust; the dilator muscles within the iris contract causing the pupil to dilate and expand, allowing increased light into the eye.
In bright conditions, scattered light rays that do not land on the retina remain undetected and do not stimulate the active photoreceptors. In low light conditions, the “unused” scattered light rays are reflected by the tapetum lucidum back onto the retina.
The tapetum lucidum is critical for efficiently utilizing the small amount of light available in dark (i.e., low light) conditions.
Lastly, the chemical sensitivity of rod photoreceptors is gradually increased during prolonged exposure to dark conditions. It takes horses approximately 20 minutes to switch from mesopic vision to fully functional scotopic vision during dark adaption.
Rod photoreceptors are 10 000 times more sensitive in scotopic vision than during mesopic or photopic vision.
How Does A Horse’s Night Vision Differ From Humans?
Every strength is countered by a weakness, which is no more evident than when comparing human and equine vision.
The human visual system emphasizes color differentiation, acuity (i.e., sharpness of the image), and accommodation. However, humans have a limited field of vision and poor night vision compared to horses.
The widely dilated pupils, tapetum lucidum, and rod sensitivity in horses have ensured that they can function in low light conditions that leave humans sightless.
Not only do horses see better in the dark, but they also adapt to low light conditions more quickly than humans. Dark adaptation in horses takes approximately 20 minutes, whereas, in humans, it can take as long as 30 minutes.
What Is Night Blindness In Horses?
In nature, it’s survival of the fittest; this ensures that no unhealthy genes or harmful characteristics are passed onto future generations. Humans are more merciful and short-sighted than nature.
Men and women frequently intervene to save animals that would otherwise have died and thus allow these genetically inferior animals to enter the breeding pool and pass on their harmful genes.
Congenital Stationary Night Blindness (CSNB) is one such condition. Affected horses are functionally blind when exposed to dark conditions.
Phototransduction In Healthy Horses
In low light conditions, horses need to be able to convert light energy into a neural impulse via the phototransduction pathway (i.e., rod photoreceptors to bipolar cells to ganglions)
When light hits the rod photoreceptors in healthy horses, voltage-gated sodium-calcium cation channels are closed, causing the rod cell to become hyperpolarized. As the rod photoreceptors become hyperpolarized, the amount of glutamate released by the photosensitive cell decreases.
The absence of glutamate causes OFF bipolar cells to become hyperpolarized as calcium-sodium channels are closed. ON bipolar cells become depolarized as calcium-sodium channels are opened.
These bipolar cells communicate with ganglion cells at synaptic junctions. The bipolar cells’ chemical signal will be converted into an action potential at the ganglion membrane.
Once the chemical message is converted into an action potential (i.e., electrical impulse), the neural impulse is conducted to the brain via the optic nerve. The firing rate within the optic nerves determines how the brain interprets visual information.
Phototransduction In Night Blind Horses
In CSNB, the altered flow of cations (i.e., calcium and sodium ions) negatively affects phototransduction between rod photoreceptors and ON-bipolar cells.
The poor communication between rod photoreceptors and ON-bipolar cells renders the horse functionally blind during dawn, dusk, and the intervening night hours. Affected horses often struggle to navigate low light situations, including walking into a dark stable, horsebox, or heavily shaded area.
What Horse Breeds Are Prone To Night Blindness?
CSNB in spotted horse breeds is caused by the recessive gene, transient receptor potential cation channel subfamily M member 1 (TRPM1) gene. Affected spotted horse breeds include:
- Spotted Miniature horses
In Tennessee Walking Horses, CSNB is caused by the metabotropic glutamate receptor 6 (GRM6) gene.
The gene causing Congenital Stationary Night Blindness in Paso Fino horses and Thoroughbreds has not yet been identified.
Is Night Blindness In Horses Curable?
CSNB in horses is not curable; however, it is a relatively simple condition to manage. When working around CSNB affected horses, it is essential to remain conscious of the fact that these horses cannot function in low light conditions.
As mentioned previously, horses are prey animals, and as such, loss of sight and mobility are the two conditions most likely to trigger uncontrolled panic in a horse.
From dawn to dusk, your CSNB horse is rendered blind and will be vulnerable to injury and debilitating levels of stress. A CSNB horse can be effectively managed in low light conditions by ensuring that artificial light is always available.
Some tips for managing affected horses include:
- Stable horses during the night and always switch the barn lights on when the horse is in the stable.
- If your horse lives outdoors, it can be managed by laying a wide gravel track around the perimeter of the paddock. Based on the feel and sound gravel makes when the horse walks over it, the horse can avoid crashing into the paddock fences.
- Don’t ride affected horses during dark hours unless you are riding or working your horse in a well-lit indoor arena.
- When transporting your horse, make sure the horsebox or truck is well padded and lit. Affected horses should be fitted with protective boots and a poll protector to prevent injury if they bump themselves.
A healthy horses’ night vision is significantly better than humans due to an increased number of rod photoreceptors, the reflective tapetum lucidum, and a widely dilating pupil.
A small percentage of horses are affected by Congenital Stational Night Blindness, which leaves them sightless during low-light conditions. During daylight, these horses have normal vision and can function as well as non-affected horses.