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What is a Nictitating Membrane?

S. Mithra
S. Mithra

Many animals have a third eyelid called a nictitating membrane. This clear eyelid can be drawn across the eyeball for protection from debris, prey, or the dryness of air, similarly to regular eyelids. Reptiles, birds, sharks, and some mammals evolved this extra layer of protection to keep their eyes moist and clean while maintaining visibility.

The nictitating membrane is part of the conjunctiva, a mucus membrane. Usually, it resides in the corner of the eye when not in use. In humans, this membrane is permanently folded into that corner; it's the visible pink nub. Most animals can control the haw, another name for the eyelid, by drawing it diagonally or horizontally across the cornea. When animals evolved to live out of the ocean, they had to develop a way to keep their eyeballs moist in dry air and free of particles in dusty wind. Predators, especially, cannot risk blinking their eyes repetitively, lest they miss a scurrying mouse or hopping frog. The nictitating membrane allows them full vision as well as the benefits of blinking.

Cats have a nictitating membrane, but it is usually hidden.
Cats have a nictitating membrane, but it is usually hidden.

The owl, like other birds, closes its haw when flying at high speeds because the additional moisture improves their vision. Certain kinds of sharks called carchariniformes, also use nictitating membranes to keep from being stabbed by thrashing prey. Amphibious creatures such as alligators, crocodiles, and other reptiles use their third eyelid while hunting, or while not underwater.

A crocodile, which has a nictitating membrane.
A crocodile, which has a nictitating membrane.

Harbor seals benefit from their nictitating membrane because they live in and out of the water. Aardvarks close their nictitating membrane when they are eating termites to keep from getting bitten. For the polar bear, the membrane actually filters ultraviolet light and reduces snow blindness. The nictitating membrane of other mammals, like cats, dogs, and horses, only protrudes if they are ill. A visible haw could be a symptom of dehydration, tetanus, low body weight, or an abscess near the eye.

Frequently Asked Questions

What is a nictitating membrane and which animals have it?

Polar bears have a nictitating membrane that helps filter ultraviolet light.
Polar bears have a nictitating membrane that helps filter ultraviolet light.

A nictitating membrane is a translucent or transparent third eyelid present in some animals, including birds, reptiles, fish, and mammals. It provides protection and moisture to the eye while maintaining visibility. For instance, birds of prey use it to shield their eyes during flight, and aquatic mammals like seals utilize it underwater.

How does the nictitating membrane function differently from regular eyelids?

Animals that live both in and out of the water benefit from their nictitating membranes.
Animals that live both in and out of the water benefit from their nictitating membranes.

Unlike regular eyelids, the nictitating membrane moves horizontally across the eye and can be drawn across the cornea independently of the vertical movement of the primary eyelids. It cleans and lubricates the eye without blocking vision, allowing animals to see while protecting their eyes from debris and harsh environmental conditions.

Can humans be said to have a nictitating membrane?

The additional moisture provided by the third eyelid helps owls when flying at high speeds and spotting prey.
The additional moisture provided by the third eyelid helps owls when flying at high speeds and spotting prey.

Humans do not have a functional nictitating membrane. However, the plica semilunaris, a small fold of tissue in the inner corner of the human eye, is considered a vestigial remnant of a nictitating membrane, indicating a common evolutionary ancestry with animals that possess a fully functional third eyelid.

What is the evolutionary advantage of having a nictitating membrane?

Certain species of sharks have nictating membranes to protect their eyes from thrashing prey.
Certain species of sharks have nictating membranes to protect their eyes from thrashing prey.

The nictitating membrane offers several evolutionary advantages. It protects the eye from potential damage due to debris, wind, or vegetation, and in aquatic environments, it helps in hunting by allowing predators to see clearly underwater. Additionally, it maintains moisture on the eye's surface, which is crucial for animals in arid habitats.

Is the nictitating membrane visible to the naked eye when observing animals?

Yes, the nictitating membrane is often visible to the naked eye, especially when it sweeps across the eye. In some species, it's more transparent and harder to detect, while in others, like birds and reptiles, it can be more apparent. Observers might notice it when an animal blinks or is in a state of alertness.

Does the nictitating membrane have any sensory capabilities?

The nictitating membrane itself is not typically involved in sensory perception, as its primary functions are protection and lubrication. However, it is sensitive to touch and can reflexively activate in response to stimuli, such as objects approaching the eye, to provide rapid protective action while maintaining the animal's ability to see.

Discussion Comments

calabama71

@christym: Just a little FYI, since we are talking about camels, they can also shut their nostrils. This keeps sand from blowing into their noses. When the sand stops blowing, the camel just opens the nostrils back up. They are pretty amazing creatures.

cmsmith10

@christym: Your son was correct! Camels are equipped with 3 eyelids. They have the upper and lower eyelids just like we do and then they have a third eyelid that moves from side to side. It could be compared to a windshield wiper. Since the camel has a lot of sand to deal with, the third eyelid helps to keep the sand out and protect the camels’ eye.

The two “normal” eyelids are very long and keep a lot of sand out of the camel’s eyes. The third eyelid is actually transparent which allows the camel to see even when the eyelid is closed.

christym

Is it true that a camel has a third eyelid? My son came home from school one day talking about it. I had never heard that.

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    • Cats have a nictitating membrane, but it is usually hidden.
      Cats have a nictitating membrane, but it is usually hidden.
    • A crocodile, which has a nictitating membrane.
      A crocodile, which has a nictitating membrane.
    • Polar bears have a nictitating membrane that helps filter ultraviolet light.
      By: JackF
      Polar bears have a nictitating membrane that helps filter ultraviolet light.
    • Animals that live both in and out of the water benefit from their nictitating membranes.
      By: Erni
      Animals that live both in and out of the water benefit from their nictitating membranes.
    • The additional moisture provided by the third eyelid helps owls when flying at high speeds and spotting prey.
      By: carmelo milluzzo
      The additional moisture provided by the third eyelid helps owls when flying at high speeds and spotting prey.
    • Certain species of sharks have nictating membranes to protect their eyes from thrashing prey.
      By: Christopher Bartlett
      Certain species of sharks have nictating membranes to protect their eyes from thrashing prey.