nic·ti·tate

animal, vo•cab•u•lar•y

(v.) to wink

Whether you’re a bird or a human, a nap during the day is pure luxury. It only can be accomplished when there is nothing urgent to be done, when there is time to relax and shut out the world. For me, though, the best part of a nap is when I wake just enough to recognize that I am no longer sleeping and slowly become aware, but not yet a part, of the world around me. In this regard, I envy birds and their third eyelids.

The remnant of our human nictitating membrane (called the plica semilunaris, membrana nictitans, or palpebra tertia) is the little pink triangle at the inner corner of each eye. For most of us, it serves only to occasionally irritate us through allergies or infection. On birds, however, it serves as both a window or a curtain, depending on the situation.

Ibis nictitating membrane covering half of eye

Whether diving for a meal, feeding chicks, hammering into a tree, or enduring bad weather, a bird can voluntarily draw the membrane to cover the delicate and moist eyeball from damaging dust or dryness.

Ibis nictitating membrane fully covering eye

In the wild, a peaceful moment is only the illusion of safety.

Giving in fully to sleep is risky. By drawing the third eyelid closed, a bird retains the ability to sense movements. They might lose the fine details of vision, much like how we can no longer see the leaves on the tree behind blinds in our window. But in both the cases of the blinds and membranes, we can still see the shifting shadows caused by the branches moving with the wind… or the silhouette of another animal lurking nearby.

Different species of a fish, reptiles, birds and mammals have varying degrees of thickness and opacity of their nictitating membranes, but humans and other primates have retained little to no functionality of this organ.

Evolution often seems to follow the minimalist mantra of “use it or lose it.”

We primates don’t tend to hunt with our faces (like a bird). Or forage for food in tall grasses (like cats). Or eat underwater (like a fish). Losing the functional third eyelid through mutation didn’t hamper our ability to survive as a species… but we’ve definitely had to come up with solutions to deal with its loss. Sunglasses and eye drops serve many of the same functions, but without the built-in convenience.

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