Blinking is an essential process for maintaining eye health across the animal kingdom. While humans blink vertically with upper and lower eyelids, many animals use a different strategy. Some species blink horizontally, sweeping a specialized membrane across the eye. This adaptation provides advantages for eye protection and function in various environments.
The Nictitating Membrane: An Anatomical Marvel
The structure enabling this horizontal blink is the nictitating membrane, often called a “third eyelid.” This transparent or translucent layer is located in the inner corner of an animal’s eye. Unlike conventional eyelids, it moves across the eyeball from the side, sweeping horizontally or diagonally. This specialized eyelid is composed of a thin layer of tissue. Its transparency allows animals to maintain some vision even while the membrane is extended.
Who Blinks Sideways? A Survey of Animal Kingdom
The nictitating membrane is widely distributed across various animal groups, including fish, amphibians, reptiles, birds, and some mammals. Among birds, nearly all species possess a functional nictitating membrane, including birds of prey, woodpeckers, and various waterfowl such as loons and ducks. Reptiles commonly exhibit this feature, with examples found in crocodiles, lizards, and most snakes, though some snakes have a fixed transparent scale over their eyes instead. Most amphibians, such as frogs and toads, also use a nictitating membrane.
Many fish species, including sharks, also possess a nictitating membrane. Among mammals, it is less common but present in several species. Notable examples include polar bears, seals, camels, beavers, and aardvarks. While domestic cats also have a nictitating membrane, it is usually only visible when they are sleepy or unwell. Humans and many other mammals retain only a small, vestigial remnant of this membrane in the corner of the eye.
The Adaptive Purpose of Sideways Blinking
The nictitating membrane provides significant adaptive advantages for eye protection and maintenance. It acts as a shield against debris, dust, and sand, beneficial for animals in harsh environments like the desert-dwelling camel. This third eyelid also plays a crucial role in lubricating the eye by spreading tears across the cornea, much like a natural windshield wiper.
For aquatic animals, the membrane allows for clearer vision underwater, as seen in beavers and manatees. Birds of prey, such as peregrine falcons, use their nictitating membranes to prevent their eyes from drying out during high-speed dives. Woodpeckers tighten this membrane before impacting trees, protecting their eyes from impact and wood chips. In polar bears, it helps prevent snow blindness by reducing glare. Sharks use their nictitating membrane to protect their eyes when striking prey, and birds of prey also use it to shield their eyes from struggling prey.