Why Can a Diving Bird See Underwater? It Has a Third Eyelid
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Many animals have a transparent eyelid that sweeps sideways across the eye, cleaning and protecting it without blocking vision. Humans retain a shrunken remnant of it in the corner of each eye.
What it is and how it moves
The structure is a thin fold of tissue, usually translucent or transparent, that lies folded at the inner corner of the eye and sweeps horizontally across the surface rather than dropping from above. Because it is see-through, the animal keeps some vision while it is closed, which the ordinary eyelids do not allow. It is drawn across by muscles and springs back, and the movement is usually much faster than a normal blink, which is why it is easy to miss. In birds the sweep takes a fraction of a second and happens many times a minute without the outer lids moving at all.
What it is for
The uses differ between groups and explain where it is well developed:
- •Spreading tear fluid and wiping debris without losing sight
- •Shielding the eye during a dive, a strike or a struggle
- •Protecting against wind and grit at speed, in fast-flying birds
- •Acting as a lens underwater in some diving birds and seals
- •Covering the eye while feeding young, in birds of prey
- •Guarding against thrashing prey, in crocodiles and sharks
The underwater lens
The most interesting version does something beyond protection, and it solves a problem every diving animal faces. An eye adapted for air relies on the curved front surface to do most of the focusing, and that surface only works because air and tissue bend light very differently. Underwater that difference largely disappears, so the eye loses most of its focusing power and the animal is severely long-sighted. Some diving birds, including cormorants and diving ducks, have a thickened transparent region in the centre of the membrane that acts as an additional lens when it is drawn across, restoring the focus that the water removed. The animal effectively puts on a pair of goggles before it enters the water.
Which animals have one
The distribution across the animal kingdom is uneven in a way that says something about what it is for. Birds have a well developed one almost universally. Reptiles mostly have one, and it is conspicuous in crocodiles, which close it as they submerge. Amphibians have a version. Fish mostly lack one, since a permanently wet eye has nothing to wipe away, although several sharks have one that closes over the eye at the moment of a bite. Among mammals the picture is mixed, with a functional one in cats, dogs, camels, seals and polar bears, and a reduced remnant in most primates and in some rodents. Camels use theirs against blowing sand.
What humans still have
The small pink fold in the inner corner of the human eye is the remnant of this structure, reduced to a stub with no muscles and no function, and it is a standard example of a vestigial feature. Other primates have it in the same reduced form, and the reduction appears to have happened early in primate history. Why it was lost is not settled, though the leading suggestion is that primates with good binocular vision, hands to rub their eyes and no need to strike at prey or dive simply had no use for it. Domestic cats and dogs retain a working one, which is visible when a sleeping animal's eyes open partly, and a condition in which a dog's gland prolapses through it is common enough to have a familiar name.
The takeaway
A transparent fold sweeping sideways across the eye cleans and shields it while leaving vision largely intact, which suits diving, striking, fast flight and handling struggling prey. Some diving birds carry a thickened central region that acts as an extra lens underwater, restoring focus the water removes. The pink fold at the inner corner of a human eye is what remains of the same structure.