Many animal species, including our pets, possess a third eyelid, also known as the nictitating membrane, while humans do not. The reason for this difference stems from the evolutionary process and adaptation to the habitat of each species.
Our family dogs have a quite noticeable third eyelid, which is particularly evident when they doze off. This is the fleshy membrane seen at the corner of the eye, closest to the nose. It is often referred to as the nictitating membrane.
But what does this unusual structure actually do? And why don’t we have it?
Many animal species, including pets, possess a third eyelid.
Many animals, including familiar pets like dogs, cats, lizards, birds, and sharks, have a third eyelid. This structure plays an important role in protecting and maintaining the health of their eyes, including: Protecting the eyes from dirt and debris; Keeping the eyes moist; Removing dirt and mucus; Protecting the eyes from bright light.
The third eyelid typically sweeps horizontally across the eye, rather than vertically like the upper and lower eyelids. They are actually a specialized fold of the conjunctiva – a thin, moist membrane covering the sclera. They can be found in many species of mammals, birds, reptiles, amphibians, and fish, which may also possess a third eyelid.
The structure of the third eyelid in them also varies – in many species, a cartilaginous skeleton supports the third eyelid, while others contain glands that secrete tears. This variation likely helps animals adapt to different environments.
Several studies have examined the third eyelid to better understand its role in species such as hedgehogs, kangaroos, and brown bears.
Research has shown that the third eyelid functions similarly to the upper and lower eyelids. It protects the eye and sweeps away any intruding debris. It also distributes tears across the surface of the eye, keeping it moist and preventing ulcer formation. This is particularly important in brachycephalic dog breeds (flat-faced), such as pugs and Cavalier King Charles spaniels.
The third eyelid functions similarly to the upper and lower eyelids.
According to evolutionary hypotheses, the third eyelid may have gradually lost its important function for humans over evolutionary time. As humans transitioned from wild habitats to artificial environments, which are less dusty and dangerous, the third eyelid may have become less necessary.
In the Wild
Both domesticated and wild animals (including species from the dog, cat, and horse families) possess a third eyelid that helps shield their eyes and protect them from foreign objects. Wild animals may even need them more due to the harsher nature of their environments.
Preventing and removing debris from entering the eyes is crucial for animals living in deserts, such as camels, where sand and dirt can damage the eyes. Their third eyelid is transparent, allowing camels to maintain visibility during sandstorms while protecting their eyes.
In the wild, wombats also have a third eyelid, possibly to protect their eyes while foraging for insects.
Anteater.
Some scientists suggest that humans traded the third eyelid for better three-dimensional vision. The positioning of two eyes at the front of the face allows humans to better gauge depth and distance, which is especially important for activities like hunting, foraging, and navigation.
The third eyelid can protect against water, and a translucent membrane may assist aquatic animals’ vision underwater, including manatees. Larger shark species (e.g., blue sharks) often protect their eyes with a third eyelid while hunting and feeding.
For birds, fast-moving air can be equally damaging. Therefore, in predatory birds such as hawks, the eyelid is used when flying quickly during hunting. Typically, gusty winds will cause the third eyelid to blink in these birds (including owls) as a natural protective reflex.
In predatory birds like hawks, the eyelid is used when flying quickly during hunting.
Although humans do not possess a third eyelid like many other animal species, we still have effective mechanisms for protecting and cleaning our eyes, including eyelids, tears, and an immune system.
In polar regions, where the white landscape reflects sunlight, ultraviolet rays can damage the eyes. This can lead to temporary vision loss – a condition known as snow blindness. Therefore, some Arctic animals like polar bears may have third eyelids that absorb UV rays.
Evolutionary Loss
Humans and most primate species (except for lemurs and the Calabar angwantibo, which belong to the Lorisidae family) have evolved to the point where a third eyelid is no longer necessary. Human and primate eyes are less vulnerable to hunting, competition, and environmental threats. Additionally, human eyes are highly sensitive and can recognize and respond to danger by blinking faster.
However, the third eyelid has not completely disappeared. Humans retain a remnant of it known as the plica semilunaris. This crescent-shaped fold can also be seen at the corner of our eyes.
Even without a third eyelid, humans still have effective protective and cleaning mechanisms for the eyes.
The third eyelid, or nictitating membrane, is a thin structure resembling an eyelid that covers part or all of the eye in some animal species. It often appears as a translucent membrane at the inner corner of the eye. Although humans do not have a complete third eyelid, we still have a small remnant called the plica semilunaris.
Some scientists believe that the plica semilunaris may still assist in tear drainage due to its two small ducts located at the corner of our eyelids, allowing excess and old tears to drain into the nasal cavity. This explains why your nose runs when you cry.
