The Fascinating World of the Third Eyelid

The third eyelid, also known as the nictitating membrane, is a transparent eyelid found in many animals. Its purpose is simple yet remarkable: it protects and lubricates the eyes while allowing vision to remain unobstructed. This extra eyelid keeps the eyes moist in the face of wind, dust, or debris—without the need to blink. It also helps animals clear their eyes of dirt, see underwater, and sharpen their vision in challenging environments.

While fully developed third eyelids are common among fish, amphibians, reptiles, birds, and some mammals, they are rare in primates. Humans, for instance, have lost this feature through evolution.

Here’s a look at some of the most incredible animals that rely on their third eyelid.

Pangolins

Despite being colorblind, pangolins possess a fully functional third eyelid. Their small eyes contain rod-dense retinas, which limits their color perception. Pangolins feed on ants and termites, often using their claws to open insect nests. Their third eyelid protects their eyes from bites while allowing them to see clearly as they feast—a perfect example of evolution in action.

Bald Eagles

Bald eagles have sharp vision and a nictitating membrane that keeps their eyes moist and free from dust as they soar through the sky. This third eyelid sweeps horizontally across the eye every three to four seconds. Interestingly, the membrane also activates when there’s a risk of eye injury—like when catching prey, flying through dense brush, or feeding chicks—ensuring that bald eagles never compromise their sight.

Beavers

For beavers, the third eyelid is like built-in goggles. When diving underwater, the membrane closes to protect their eyes while remaining transparent enough to see. This adaptation allows beavers to swim, hunt, and navigate underwater with ease and precision.

Camels

Camels are desert specialists, and their third eyelid is crucial for keeping sand and dust out of their eyes during sandstorms. Unlike human eyelids, a camel’s third eyelid moves side to side rather than up and down. It’s also thin enough to allow vision even when fully closed, letting camels traverse harsh deserts without losing sight of their path.

Cats and Dogs

Many pet owners may have noticed a small, semi-transparent film in their cat or dog’s eyes—this is their third eyelid. While usually hidden, it can sometimes appear due to a condition called “cherry eye,” where the membrane bulges and becomes red. This condition is treatable, but it’s a reminder of how essential the third eyelid is for keeping pets’ eyes healthy.

Crocodiles

Crocodiles rely on their third eyelid for eye protection underwater. The membrane is mostly transparent, except for the front edge, which is pigmented and reinforced with cartilage. It both lubricates and shields the eye while diving, sometimes even adjusting the way light refracts to improve underwater vision.

Lemurs

Among primates, lemurs are a rare exception with a functional third eyelid. Their large eyes require extra protection, and the membrane helps keep the cornea moist and clean, allowing these creatures to see clearly while foraging or moving through dense foliage.

Polar Bears

For polar bears, the third eyelid guards against snow, wind, and saltwater irritation. It also improves underwater vision, enabling these arctic predators to hunt efficiently even in extreme cold conditions.

Salamanders

For salamanders and other ground-dwelling amphibians, the third eyelid protects the eyes from debris and serves as underwater goggles. By raising the membrane before diving, they can safely explore aquatic environments without losing visual clarity.

Sea Lions and Seals

Marine mammals like sea lions and seals have third eyelids that sweep across the eyes to remove sand and debris while improving vision underwater. These membranes also enhance night vision and shield the eyes from harsh sunlight when on land.

Sharks

Some sharks, including tiger and hammerhead sharks, have a special third eyelid that protects their eyes while hunting. Species without it can simply roll their eyes back into their heads for protection, but those with a nictitating membrane benefit from a fully functional shield against damage.

Woodpeckers

Woodpeckers face a unique challenge: drilling into trees at high speeds without harming their eyes. Their third eyelid acts as a protective barrier, moving across the eye to clear debris and prevent injury from flying wood chips.

The third eyelid is a remarkable evolutionary adaptation that demonstrates nature’s ingenuity. From desert wanderers to underwater hunters, these membranes allow animals to see, survive, and thrive in environments that would otherwise threaten their vision.