Many animals have the ability to close their eyes and still see their surroundings to some degree. This allows them to protect their eyes while maintaining awareness of predators or prey. The extent to which different animals can see with closed eyes depends on the biology and evolution of their visual systems.
Sees With Closed Eyes
Here are some animals that can see even with closed eyes:
- Frogs – Frogs have a nictitating membrane that protects their eyes while allowing light to pass through. They can see, though with poorer image quality, when the membrane is closed.
- Reptiles – Many reptiles including crocodiles have a brille, a transparent scale that covers their eyes. It protects their eyes while allowing vision.
- Birds – Birds can see with nictitating membranes closed over their eyes. Seabirds that plunge into water use this ability.
- Cats – Cats have a third eyelid called a nictitating membrane. It sweeps horizontally across the eye to keep it moist and protected.
- Horses – Horses have a nictitating membrane that can cover the eye but still allow dulled vision.
- Sharks – A nictitating membrane protects sharks’ eyes while swimming but still enables them to see.
- Mammals – Many mammals including dogs, polar bears, and seals have a nictitating membrane that can cover the eye while allowing vision.
These animals all have a translucent third eyelid called a nictitating membrane that can cover the eye while still allowing some sight. It protects the eye from damage while maintaining visual awareness.
Better Vision When Open
Though these animals can see with closed eyes, their vision is certainly better when their eyes are fully open and unobstructed. The nictitating membrane or brille reduces light intake and blocks some visual detail and clarity. So while closed eye vision provides advantages like eye protection and moisture, fully open eyes give the sharpest, clearest sight.
Here are some of the ways closed eye vision is more limited:
- Reduced field of view – The nictitating membrane obstruction blocks some peripheral vision.
- Impaired ability to judge distance – With only dulled vision, depth perception is weakened.
- Loss of visual acuity – Fine details appear blurred when seen through the membrane.
- Lower light levels – The translucent membrane reduces light entering the eye.
- Less color sensitivity – Colors are perceived as paler and less vivid.
So while animals with nictitating membranes or brilles can see with closed eyes, their vision is muted and incomplete compared to eyes wide open. Full, unobstructed vision provides the highest visual acuity and light sensitivity.
Evolutionary Advantages
The ability to see with closed eyes likely evolved to provide protection while maintaining some awareness of surroundings. This conferred survival advantages:
- Avoids eye injuries – The membrane shields the vulnerable eye surface.
- Keeps eyes clean and moist – Blinking spreads tears and the membrane wipes debris away.
- Camouflage for hunting – A closed membrane hides eye position from prey.
- Defense against predators – Eyes can be concealed while retaining some vision.
For frogs, reptiles, birds, and underwater animals, see-through eyelids prevent irritation from water, debris, or dry air. In mammals, nictitating membranes shield eyes during running, fighting, or grazing when eyes are especially prone to being poked or scratched. Though closed eye vision is impaired, the protective benefits outweighed the visual tradeoff in these species.
Human Vision Comparison
Humans lack nictitating membranes, so when our eyes are closed we experience no vision apart from light penetrating the eyelids. Some key differences between human vision and closed-eye animal vision include:
| Human Vision with Eyes Closed | Animal Vision with Nictitating Membrane Closed |
|---|---|
| No image perception, only darkness | Can see blurry images through membrane |
| No ability to judge distance or depth | Some impaired depth and distance perception |
| Total loss of visual detail | Some blurry detail visible |
| No peripheral vision | Partial peripheral vision |
| Fully blocks light | Dims but does not fully block light |
Humans sacrificed the advantages of see-through eyelids. But our large, forward-facing eyes provide excellent binocular vision, color perception, and visual acuity when fully open. Our eyelids can blink frequently to keep eyes clean and moist. We relied more on our sophisticated visual cortex to process complex sight rather than partially protective membranes.
Unique Specializations
Beyond basic nictitating membranes, some animals have specialized eyelid adaptations:
- Reindeers have semi-transparent eyelids to see in blizzard conditions.
- Woodcocks have eyes set far back in their head with eyelids that close horizontally to conceal their eyes when their beak is poked in mud.
- Emperor penguins have a membrane for filtering ultraviolet sunlight in snowy environments.
- Hippos secrete a reddish oily secretion thought to block out water and improve underwater vision when their eyes are closed.
These species evolved modified membranes or secretions tailored to their niche environments. This demonstrates the importance of retaining some sight with eye covering membranes across animal taxa.
Conclusion
Many animals ranging from frogs to cats to seals have translucent third eyelids that allow a degree of vision while the eye remains protected and moist. Compared to fully open eyes, closed eye vision has reduced acuity, field of view, light sensitivity, and color perception. But even blurred vision gave evolutionary advantages like avoiding eye injuries and maintaining visual awareness. Humans forfeited these benefits for wider-ranging binocular vision made possible by unobstructed eyes. Unique nictitating membrane adaptations in animals like reindeer and penguins underscore the value of retaining dim closed-eye sight over total blindness when eyes need protection.