Science Question from a Toddler: All Eyes on the Turtle

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I like this turtle because he looks just a little deranged. Image by Flickr user audreyjm529, via CC.

The daughter of reader Amie Miller wants to know, "Do turtles have eyelashes?"

Short answer: No.

Eyelashes are, in general, kind of a mammal thing--what with being made out of hair and all. Besides their usefulness in pseudo-comic flirting and as a habitat for freakishly awesome mites (as well as their importance as an advertising revenue stream for Hulu), eyelashes also work as a trap, catching bits of dust and other scrapey-ouchy particles before they can reach our eyeballs.

But as we behold the motes eyelashes protect us from, we have to consider the beams that surely must be getting into the eyes of creatures unfortunate enough to be lash-less.* Poor, little turtle.

Or not. Non-mammals have their own way around this problem. They protect their eyeballs with a nictitating membrane--basically a third eyelid that slides in horizontally from the side. Besides having a great name, the nictitating membrane also has some pretty cool features eyelashes can't claim. For instance, nictitating membranes are translucent. Turtles can close their third eyelid completely, but still see. It's a skill that's particularly useful when you spend a lot of time swimming around with your head mostly below the waterline--kind of like having built-in goggles. Amphibians and reptiles have nictitating membranes. So do birds, who use them to protect and moisten the eye during flight much the same way that turtles use them in the water.

But mammals aren't wholly without nictitating membranes. In fact, you have the vestigial remnants of one. Called the plica semilunaris, it's that little lump on the inside of your eye, next to your nose. In animals that still use them, nictitating membranes are associated with glands that secrete eye-moistening goo. So it's no coincidence that the "sleepies" you pick out of your eye in the morning come from the same area as your plica semilunaris.

For humans, nictitating membrane may be a thing of the past, but other mammals still get some use out of it. Harbor seals, which spend plenty of time underwater, have functioning nictitating membranes. So do camels, who use the third eyelid for added protection during desert sand storms. Aardvarks, awesomely, actually use the thing to keep termites from biting their eyes while they (the aardvarks) are trying to eat them (the termites).

Cats and dogs have nictitating membranes that are somewhere in-between. Their membranes still exist--and still work--but our pets can't control them the way turtles and other creatures can. Instead, nictitating membranes only show up when the cat or dog is sick, or otherwise messed up in the head. Case in point, I last saw my cat, Red's, nictitating membranes on a drive from Alabama to Minnesota, for which the vet had prescribed a slightly higher-than-necessary dose of kitty Valium.

*Please see the Book of Matthew, chapter 7, verse 3. It's an awkward joke, yes. But give me a break, here. I've got four years of Baptist high school-worth of Bible verses memorized and I'm not just gonna let that go to waste.