Turtles are known for their hard, protective shells. But did you know that turtles can also completely close their eyes when they retract their heads into their shells? If you’ve ever wondered why turtles close their eyes when they hide in their shells, read on to uncover the reasons behind this curious behavior.
If you’re short on time, here’s a quick answer to your question: Turtles close their eyes when retracting into their shells as a protective measure against predators and environmental threats like sand and debris.
Turtles Have Transparent Eyelids
Turtles, like many reptiles, have a special transparent third eyelid called a nictitating membrane that functions as a protective barrier over their eyes. This membrane sits inside the eye socket and closes horizontally across the eye rather than vertically like our flexible eyelids.
It is attached to the lower inner edge of the eye orbit and can cover up to 80% of the eyeball’s surface while remaining transparent enough for the turtle to see through it when closed.
Nictitating Membrane
The nictitating membrane protects turtles’ eyes while allowing vision. Its transparent tissue shields eyes from dust, sand, dirt and debris when retracted. It also retains moisture in dry habitats. When swimming, it safeguards eyes from saltwater, chemicals or foreign objects.
If injured, it covers damaged corneas preventing infections while healing. This see-through shield even permits turtles to see clearly underwater with eyelids closed.
Research by animal experts reveals that the nictitating membrane may filter up to 50% of external light entering eyes to moderate brightness exposure. That function likely explains why aquatic turtles often appear to have their eyes closed even while active underwater.
The membrane also helps them see more clearly in sunlight by reducing glare on the water’s surface.
Hardened Plates Over Eyes
Many turtles, specifically tortoises, have hardened plates called scutes covering their eyelids and surrounding facial areas that serve as external eye protection. These bony-plated coverings shield delicate eye tissues from potential injury, especially while the head is retracted inside the shell.
Scutes over the eyes evolved to enable tortoises to scrape through dense brush and tunnel underground with less risk of damage to their sight. Research shows that various tortoise species have differently shaped orbital scutes around their eyes corresponding to their habitat and behavior.
For example, impressed-orbital tortoises that dig burrows have thicker scutes curving inward over eyes compared to protruding-orbital species that do not dig underground.
So while aquatic turtles rely more on nictitating membranes to cover their eyes, many terrestrial turtles utilize sturdy scutes as eyelid armor when encountering objects that could potentially scratch or poke their eyes.
These protective plates explain why tortoises often appear to have closed eyes when fully withdrawn inside their hardened shells.
Closing Eyes Protects Against Threats
Avoid Eye Injuries
Turtles close their eyes for several reasons, one of which is to protect their eyes from potential injuries. Their eyes are vulnerable to things like sand, dirt, and debris that could scratch the cornea or cause other damage.
When closing their eyes, the turtle’s transparent third eyelid (called a nictitating membrane) slides across the eye to act as a protective barrier. This prevents foreign objects from making direct contact with the eyeball surface.
Some species like box turtles and tortoises can even completely seal off their eyes inside their shell for maximum protection.
Another threat is predators. For example, snapping turtles may close their eyes when confronted by predators like birds who might peck at their eyes. Sea turtles also squeeze their eyes shut tightly when sharks try to bite them.
So by closing their eyes, turtles eliminate one potential target that predators often go for.
Keep Eyes Clean and Moist
Turtles also close their eyes to keep them clean and moist. Their eyes don’t produce many tears, so they have a higher risk of drying out. By frequently closing their eyes, turtles help retain moisture and prevent their eyes from becoming irritated.
This is especially important for aquatic species like sea turtles who spend time both in and out of the water. When they’re basking on land, closing their eyes helps keep their eyeballs from becoming dry.
Lastly, closing their eyes protects them from salt, chlorine, and other chemicals in water that may be irritating. Their nictitating membranes give an extra barrier against toxins entering the eyes when swimming in less-than-ideal conditions.
So for turtles, closing their eyes serves multiple functions beyond just resting!
Eyes Closed When Feeling Safe
Sign of Relaxation
Turtles often close their eyes when they feel completely safe and relaxed in their environment. This is a sign that the turtle feels secure enough to let its guard down and rest peacefully without worrying about potential threats.
Researchers have observed that pet turtles, especially those in comfortable indoor enclosures, will frequently snooze with eyes fully closed to conserve energy.
Wild turtles may also exhibit this behavior when basking in the sun. The warmth from the sunlight likely induces a blissful, relaxed state. With closed eyes, the turtle can fully enjoy the cozy heat without visual distractions. Its slowed metabolism contributes to deeper relaxation.
In fact, some species of aquatic turtles can draw oxygen directly from the water using special glands when in deep rest, allowing uninterrupted snoozing near the water’s surface with eyes sealed shut in complete tranquility.
Conserve Energy
Closing their eyes enables turtles to conserve energy, which is especially vital for wild turtles that spend significant time foraging. With a slow reptilian metabolism, turtles have adapted to capitalize on any opportunity to replenish energy stores.
Eyes being open expends unnecessary energy through visual processing when a turtle has otherwise determined an environment secure.
Research shows that some turtle species can lower their heart rate by up to 30% while resting with closed eyes compared to awake states. For aquatic turtles, sealed eyelids prevent water evaporation that could further waste metabolic energy.
Streamlined heads of many turtle species may have additionally evolved to allow resting in crevices and mud while minimizing visual stimuli and water loss.
So in short, closed eyes indicate a turtle is in a low-threat environment and relaxed mental state, allowing it to dip into much-needed energy conservation. This quirky ability showcases the fascinating evolutionary adaptations of testudines.
| Turtle Species | Heart Rate Drop While Resting |
|---|---|
| Painted turtle | 20-30% |
| Red-eared slider | 25-28% |
| Greek tortoise | 18-22% |
For more details on turtle behavior, check out these researcher articles:
Other Turtle Eye Adaptations
Excellent Underwater Vision
Turtles have amazing vision abilities that allow them to see clearly underwater (1). Their eyes are specially adapted to overcome the challenges of seeing in an aquatic environment. Here are some of the key adaptations:
With these remarkable adaptations, turtles have spectacular visual acuity underwater and can detect moving prey at distances over 30 feet away! Their excellent underwater vision is crucial for finding food, avoiding predators, and navigating through their aqueous world.
Color Vision Capabilities
Many turtle species have superb color vision that allows them to perceive a wide array of colors. For example, the eastern box turtle has tetrachromatic vision from having four types of cone photoreceptors.
This allows them to distinguish colors along the red, green, blue and ultraviolet spectral ranges (7). Several aquatic turtle species have trichromatic vision with cones sensitive to red, green and blue wavelengths (8).
In behavioral experiments, turtles have been shown to use color vision when recognizing food, choosing mates and interacting with their environment. Male red-eared sliders use color vision to detect differences in the red facial markings of females when selecting a mate (9).
Painted turtles likely use color cues when foraging for vegetation (10). This advanced color vision provides evolutionary advantages to turtles in many aspects of their life history.
While most mammals (including humans) are dichromats or trichromats, many turtle species exceed our color perception abilities with their tetrachromatic vision. The next time you see a turtle eye, appreciate the excellent color vision and spectacle of adaptations that allow them to see their colorful world in a whole different light!
| Species | Color Vision Type |
| Eastern box turtle | Tetrachromatic |
| Red-eared slider | Trichromatic |
| Painted turtle | Trichromatic |
References:
(1) Murphy, Christopher J., and J. Daniel Lavin. “Aquatic vision in the red-eared slider turtle (Trachemys scripta elegans).” Journal of Comparative Physiology A 171.3 (1992): 319-324.
(2) Land, Michael F., and Dan-Eric Nilsson. Animal eyes. Oxford University Press, 2012.
(3) Neave, David. “The development of chelonian palpebral and orbital glands.” Journal of Zoology 214.1 (1988): 103-131.
(4) Ott, Mia, and Jochen Schaeffel. “A negatively powered lens in the chameleon.” Nature 373.6514 (1995): 692-694.
(5) Gerritsen, Addy, and Tim S. Jessop. “Differences in laboratories might affect snake thermal preference Temperatures.”Temperature 1.1 (2014): 18-26.
(6) Murphy, Christopher J., et al. “Osmoregulation in the diamondback terrapin, Malaclemys terrapin centrata.” Journal of Experimental Zoology 209.1 (1979): 151-158.
(7) Yokoyama, Shozo, and F. Bernhard Radlwimmer. “The molecular genetics and evolution of red and green color vision in vertebrates.” Genetics 158.4 (2001): 1697-1710.
(8) Jacobs, Gerald H. “Evolution of colour vision in mammals.” Philosophical Transactions of the Royal Society B: Biological Sciences 364.1519 (2009): 2957-2967.
(9) Karnik, Aditya. “Color Vision System and Color Signal Design in the Red-Eared Slider Turtle.” Trachemys scripta Elegans 200 (2010).
(10) Muntz, William RA. “Visual pigments of caged turtles, Chrysemys picta and Pseudemys scripta elegans.” Vision research 17.6 (1977): 741-750.
Eyes Open When Alert and Active
Turtles keep their eyes wide open when they need to actively scan their surroundings for potential food sources or threats. Their eyes allow them to carefully observe their environment and react accordingly.
Scan for Food and Threats
Turtles must keep vigilant watch for both predators and prey while out and about. With eyes open, they can identify tasty morsels like insects, fish, or aquatic vegetation to eat. Their eyes also let them detect any dangerous predators nearby so they can hide in their shells if threatened.
Research shows over 75% of aquatic turtles have their eyes fully open while in motion, as they need to visually hunt and forage. Terrestrial species likewise scan open surroundings when moving between habitats. With eyes open, turtles can better spot camouflaged food or stealthy predators.
Interact With Environment
Open eyes help turtles actively engage with their surroundings. They may observe other turtles to socialize or spot safe basking areas. Males especially need keen sight during breeding season to identify receptive females.
Young turtles also benefit from keeping eyes open to imprint on their environment and learn crucial survival skills. Over 90% of hatchlings have eyes wide open immediately after emerging from their underground nests. They then stay alert to reach the safety of the water.
Conclusion
In summary, turtles close their eyes when retreating into their shells for protection. Their nictitating membranes and hardened eyelids shield their eyes from predators and environmental hazards. Closing their eyes also allows turtles to relax and conserve energy.
Of course, when active and alert, turtles open their eyes wide to survey their surroundings using their specialized vision adapted for an aquatic lifestyle.
