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Did You Know That Many Animals Have a Third Eyelid?

If you’ve ever observed your pets during their sleepy moments or while they’re enjoying affection, you might have seen a fleshy membrane in the corner of their eye. But what exactly is its purpose, and why don’t humans have one?

The third eyelid sweeps horizontally across the eye, unlike the vertical movement of the upper and lower lids. It’s a specialized fold of the conjunctiva, the moist membrane covering the other lids, and the sclera (the white of the eye). This structure is not exclusive to mammals; birds, reptiles, amphibians, and fish also possess third eyelids.

The structure and function of the third eyelid vary across species. In many animals, it contains a cartilage skeleton for Support and glands that secrete tears. These variations help animals adapt to diverse environments, from sea and air to treetop habitats. Studies have examined third eyelids in various species, including hedgehogs, kangaroos, and brown bears, to understand their roles better.

Research shows that the third eyelid functions similarly to the upper and lower lids, protecting the eye and removing debris. It also spreads tears across the eye’s surface, keeping it moist and preventing ulcers. This function is particularly crucial for brachycephalic (flat-faced) dogs, like pugs and King Charles spaniels, whose protruding eyes are less protected. Both domestic and wild animals, including canine, feline, and equine species, require eye protection from foreign bodies. Wild animals need this protection even more as they navigate through grasslands or contend with bites and scratches from prey or rivals.

For desert animals like camels, the third eyelid is vital in preventing eye damage from sand and dirt. Their partially transparent third eyelid allows them to retain some vision during sandstorms. Similarly, aardvarks use their third eyelids to protect their eyes while rooting around for insects. In aquatic environments, the third eyelid offers protection and aids underwater vision. Manatees, for instance, have translucent membranes that help them see underwater. Larger sharks, like blue sharks, protect their eyes with their third eyelids when hunting and feeding.

Birds of prey, such as falcons, use their third eyelids during rapid flight to shield their eyes from fast air currents. This protective reflex is also observed in owls and other avian species, helping them avoid damage from sharp-beaked offspring. Woodpeckers, whose skulls undergo intense vibration when drilling into tree trunks, benefit from their third eyelids, which protect their eyes from sawdust and act as a seatbelt against forceful head banging. In polar regions, where UV light reflects off the snow, animals like polar bears might have third eyelids that absorb UV light, protecting them from snow blindness. Although not conclusively proven, their clear third eyelids assist them in their marine hunting.

Humans and most primates have evolved beyond the need for a fully functional third eyelid. Instead, we have the plica semilunaris, a small crescent-moon fold at the corner of our eyes, which helps drain tears into the nasal cavity, explaining why crying leads to a runny nose.