Some animals have an extra eyelid called a nictitating membrane. This eyelid can be found in all kinds of creatures, including mammals, fish, and reptiles. While not animals use these eyelids in the same way, the nictitating membrane usually serves a very important purpose.
What kinds of animals have multiple eyelids, and how do these eyelids work? How is a nictitating membrane different from a normal eyelid? Read on to find out more about animals with nictitating membranes and the role that these extra eyelids play in the animal kingdom.
14 Animals With Multiple Eyelids
All dogs have an extra eyelid that helps to protect the eye and spread tear film. While you won’t be able to see this eyelid most of the time, some dogs develop a condition called cherry eye that causes the third eyelid to protrude. If a dog’s third eyelid is damaged, it’s possible to replace the eyelid through surgery.
Not only do cats have a third eyelid, but that eyelid has its own lacrimal gland. This gland is responsible for producing as many as 50% of a cat’s tears. Although a cat’s third eyelid usually isn’t visible, you may be able to see the edge of the eyelid when your cat is very relaxed.
An owl’s eyes are held in place by its skull, which means it can’t move its eyes up and down, side to side, or do anything but stare straight ahead. While owls may not be able to move their eyes, they can keep them safe thanks to their third eyelid. Owls can even see through their eyelid, allowing them to protect their eyes and look for prey at the same time!
There are more than 5,000 known species of frogs, and every one of those species has three eyelids! A frog’s third eyelid is semi-transparent and can completely cover the eye. Frogs use this extra eyelid to cover and protect their eyes while they’re underwater. Since the lid is transparent, they can detect predators and search for food even with their eyes closed.
5. Hammerhead Sharks
Not all shark species have three eyelids, but hammerhead sharks do. The hammerhead shark has widely set eyes that give them a complete 360 degree view of the world around them. Since hammerhead sharks rely on their vision, they use their third eyelid to protect their delicate eyes while they attack prey.
Camels live in deserts, where it’s easy for sand to get trapped in eyes. Thankfully, since camels have a third eyelid, it’s easy for them to keep dust and sand from getting into their eyes. A camel’s third eyelid can also function like a contact lens, allowing a camel to see more clearly.
Beavers have a translucent third eyelid that allows them to retain some vision even when their eyes are close. A beaver will usually only use its third eyelid when it dives beneath the water. In addition to closing their eyes while they’re underwater, a beaver is able to close its ears and nose, allowing it to swim comfortably.
8. Bald Eagles
A bald eagle’s bottom eyelid is larger than its top eyelid, which means that an eagle shuts its eyes upwards! Eagles also have a third eyelid that helps to keep its eye moist and protect it from hazards. Since this eyelid is translucent, eagles can still see some things around them when this eyelid is closed.
9. Polar Bears
Polar bears live in arctic habitats that are covered with ice and snow. Even though these environments are chilly, that snow can reflect UV rays, which puts polar bears at increased risk for UV exposure. Thankfully, polar bears have a third eyelid that they can use like a pair of built-in sunglasses!
Most species of lizard live close to the ground, which makes it easy for debris to get into their eyes. Lizards are able to use their third eyelid to protect their eyes and to clear away any dust, dirt, or mud that’s gotten into their eye. In addition to having a third eyelid, there are some types of lizards, like iguanas, that have a third eye!
Seals are a marine mammal that’s actually able to sleep underwater! When seals sleep, they place their entire bodies under the water, but keep their snouts above the water’s surface so that they can breathe. Thanks to their third eyelids, their eyes have plenty of protection whether they’re awake or asleep.
Even though kangaroos come in many sizes, every species of kangaroo has a third eyelid. Most species of kangaroos live in hot and dry climates, which means it’s important for these marsupials to keep their eyes moist. Since kangaroos have an extra eyelid, they can easily produce tears and keep their eyes safe.
Hawks have a third eyelid that blinks horizontally rather than vertically. This eyelid is very thin, which means that hawks can still see through the eyelid while it’s closed. Hawks are fierce predators with sharp claws, and they use their third eyelid to keep their eyes safe while they swoop in to attack.
14. Alligators and Crocodiles
Like other reptiles, crocodiles and alligators have multiple sets of eyelids that protect their eyes. Alligators and crocodiles have 2 eyelids for each eye and when they submerge an additional membrane covers their eyes for added protection, making 3 eyelids for each eye.