A third eyelid isn’t as uncommon as it sounds. Dozens of animals have a nictitating membrane, commonly referred to as a third eyelid. Camels are one of the largest mammals that boast third eyelids; the desert darling’s eyes need them to ward off the Sahara’s swirling sandstorms. Animals with a third eyelid use the thin, protective membrane to blink away debris, sharpen vision and enable themselves to see clearly underwater.
Some of our most popular pets sport third eyelids, though you may not have noticed. Dogs and cats generally hide their third eyelids. If you happen to see an opaque film covering your buddy’s eye, that’s his nictitating membrane. Some dogs and cats let their third eyelids down, so to speak, when they’re totally relaxed or falling asleep; however, they generally retract them when alerted or awoken. If you notice your dog or cat’s third eyelid for a prolonged period, this may indicate a medical problem. Cherry eye is one of the most common problems associated with the third eyelid. This ailment presents itself as a reddened bulge in the inside corner of your pet’s eye. Consult your veterinarian if your pet exhibits any sign of third eyelid protrusion.
Many birds, such as owls and bald eagles, possess some of the most complete and simple third eyelids in existence; avian third eyelids cover almost the entire eyeball and do not obstruct vision. As birds glide through the air at high speeds, they sweep their semi-transparent nictitating membranes across their eyes so quickly that their vision is never impaired. Clear vision is extremely important for birds during hunting—they need to spot prey miles away. It’s also necessary to help them avoid obstacles as they fly at high speeds.
Reptiles and Amphibians
Frogs, lizards and snakes all use their third eyelids to protect their eyes from dust, mud and injury. Because they spend their lives so close to the ground, their eyes have developed nictitating membranes to clear away any debris that may cause irritation. Amphibians, such as frogs and salamanders, use their third eyelids as goggles when underwater; they draw them up to protect their eyes from the H2O and dive in. Their third eyelids not only protect their eyes but also allow them to see as clearly underwater as they do on land.
Sharks always seem to have their eyes wide open under water; how do they do that? Though sharks possess both an upper and lower eyelid, they never meet to form a protective optical barrier. Great whites roll their eyes back for protection but some sharks, like tigers and hammerheads, have third eyelids. These sharks employ their nictitating membranes to protect the delicate eye area when attacking prey.
- Sharkiologist: Shark Senses
- ReefQuest Centre for Shark Research: Biology of Sharks and Rays
- WebMD: The Nictitating Membrane (Third Eyelid) in Dogs
- WebMD: Third Eyelid Problems in Cats
- National Center for Biotechnology Information: A Comparative Study of the Nictitating Membrane of Birds and Mammals [PDF]
- Clark College: Frog Dissection [PDF]
- camel image by Andrey Ivanov from Fotolia.com