Lizards are fascinating creatures that have adapted to thrive in a variety of environments. One aspect of lizard anatomy that enables their success is their specialized eyes. But do lizards have eyelids like humans? Read on to uncover the answer.
If you’re short on time, here’s a quick answer: Most lizards do have eyelids, but they are different in structure compared to human eyelids. Lizards have a transparent lens that protects their eyes and helps keep them moist.
The Main Differences Between Human and Lizard Eyelids
Lizards Have a Transparent Spectacle Instead of Eyelids
Unlike humans, most lizards lack movable upper and lower eyelids. Instead, they have a clear, fixed transparent scale known as a spectacle that protects their eyes. This spectacle is an immobile structure made of modified scales that covers the eye like a lens.
It serves a similar function to eyelids in keeping the eye moist and protecting it from debris, but does not move up and down.
Some key differences between the human eyelid and the lizard spectacle include:
- The human eyelid is a fold of skin that can open and close over the eye, while the lizard spectacle is an immobile scale structure.
- Humans blink frequently by raising and lowering their eyelids, while lizards have a permanently fixed spectacle with no blinking mechanism.
- The human eyelid contains the thin skin of the eyelid as well as the levator palpebrae superioris muscle that controls opening and closing. Lizards lack these anatomical features in their spectacle.
So while the human eyelid and lizard spectacle both serve to protect the eye, only the human eyelid can dynamically blink and move up and down over the surface of the eye.
Lizards Can Also Have a Nictitating Membrane
In addition to the spectacle scale, some lizards like geckos and iguanas have a secondary see-through eyelid called a nictitating membrane. Located inside the inner corner of the eye, this membrane sweeps horizontally across the surface of the eye to moisten and clean it.
Unlike the spectacle, the nictitating membrane can actively move, functioning like a windshield wiper to maintain eye health and remove debris.
A 2018 study published in the Journal of Anatomy found that 52% of lizards have a nictitating membrane, with the highest rates seen in geckos (96%) and varanids like monitor lizards (100%). So while most lizards lack eyelids, over half supplement their fixed spectacle with an additional transparent protective membrane that actively slides across the eye.
The Advantages of the Lizard Eyelid Structures
The Spectacle Helps Keep Eyes Clean and Moist
The spectacle is a clear scale that covers and protects the eye of lizards and snakes. It helps keep the eye clean and moist in several key ways:
- It prevents dust and debris from getting into the eye, acting like a windshield wiper to keep the surface of the eye pristine.
- The smooth, transparent surface allows the eyelids to sweep across and distribute the tear film evenly over the surface of the eye.
- It reduces evaporation of moisture from the eye’s surface, especially in dry environments like the desert.
Without the spectacle, lizards would constantly have to clean their eyes and replenish moisture. The spectacle makes this process far more efficient. Researchers have found the spectacle allows certain lizards, like the Texas Horned Lizard, to maintain vision even in extremely dry conditions [1].
The Nictitating Membrane Protects Eyes
The nictitating membrane is a thin, opaque eyelid many lizards have that can sweep across the eye. It provides protection in several ways:
- It acts like a windshield wiper, sweeping away debris and spreading moisture across the cornea.
- It protects the eye when lunging at prey, preventing scratches or poke injuries.
- Some lizards will use it to help keep sand out of their eyes during sandstorms.
Research on lizards like the Moorish gecko found the nictitating membrane allows them to maintain visual acuity even in sandy environments [2]. Without it, their vision would constantly be compromised by getting sand in their eyes when hunting.
Variations in Eyelid Structures Among Different Lizards
Geckos and Night Lizards Lack Eyelids
Geckos and night lizards such as Coleonyx variegatus astonishingly lack movable eyelids. Instead, they have a transparent scale called a spectacle that permanently covers their eyes. This clear covering protects their eyes from dust and debris.
The spectacle is kept clean through the gecko’s long tongue, which has special brush-like features enabling it to wipe away particles from the spectacle.
Having a fixed spectacle instead of movable eyelids is an adaption that likely helps geckos and night lizards be more alert at night when many predators are active. Unlike eyelids that open and close, a spectacle allows the gecko continuous visual access to watch for predators and prey.
Chameleons Have Unique Eyelids
Chameleons have independently movable eyelids, with each eye able to operate separately from the other. While humans can move both eyelids together, chameleons can move one eyelid down while keeping the other open.
This gives them a larger visual field to detect prey and predators without needing to move their eyes or head.
Additionally, chameleons have a transparent spectacle membrane like geckos do. So their eyes are protected by both the spectacle and their specialized eyelids. The upper and lower eyelids roll horizontally back into the eye socket, unlike human eyelids that close vertically.
Conclusion
In summary, most lizards do have specialized eyelid structures including a transparent spectacle and sometimes a nictitating membrane. These eyelid adaptations allow lizards to keep their eyes clean and protected in their varied habitats.
The next time you see a lizard basking on a rock or climbing a tree, take a closer look at their eyes and admire the unique anatomy that enables their success.