Science Question from a Toddler: All Eyes on the Turtle

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.

Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.

Maggie goes places and talks to people. Find out where she'll be speaking next.

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pinehead

It isn’t my place to answer anyone else’s questions, but it just looks like fun. Please pardon my impropriety.

@stylinghead:
Depends on how hard you push.

@Anon, about the trees:
Trees are made of elements found in the earth, so they cannot increase the earth’s mass by reproducing. Nothing can.
- Snig
  
  But they do harvest photons from space during photosynthesis, doesn’t this make them heavier?
  - jackalopemonger
    
    Photons are massless.
Caroline

Yeah, my cats show theirs when they’re half-asleep. It seems like the membrane closes as the cat closes its eyes, but a little bit faster, so it’s possible for the membrane to be closed but the eyelids not-quite closed.

They can’t close the membrane all by itself, with eyelids fully open, though.
bklynchris

Pinehead your answer to anon fascinated me for a moment. But if what you are saying is true than I guess you don’t believe in global warming either. All matter in exothermic reactions is subject to some loss. EXCEPT you are including our atmosphere as well. I want to believe you, but I just don’t think I can.
jackie31337

I have a question I asked my mother’s pregnant belly as a very young toddler: “Hello baby, is it dark in there?” Just what does a baby in the womb see?
- Maggie Koerth-Baker
  
  Oooh, Jackie, that’s a fun one. I’ll put that on my query list.
  - Cnoocy
    
    There’s actually some neat research on the pre-natal visual capacity of animals with transparent egg-sacs. For example, scientists have trained cuttlefish to prefer shrimp as prey by showing shrimp to them as embryos.
Anonymous

Okay, wait, dog nictitating membranes only show up when the dog is sick or messed up? I think my dog needs a doctor or a shrink, then, because he is always flashing his. Often when he lies down to sleep, he closes his membranes but leaves his eyelids open, treating everyone to his creepy eyeless psycho face. When he does this, it looks like his eyes have been gouged out because his eyes actually retract a little into his head, perhaps because of the pressure of the membranes, which look like raw pink flesh. As he is my first dog, I assumed this was just a thing dogs do. Am I wrong?

Maybe “sleeping” counts as “messed in the head” for the purposes of this discussion….
fartle

what about ostrich
- alowishus
  
  Ostrich? They’re delicious.
Anonymous

how do you call a baby turtle
Matt Deckard

Based on experience from our two dogs, their nictitating membranes can also show up while they are in REM sleep, particularly when they are having dreams and their legs, paws, mouths and such start to twitch. I’ve often been fascinated by watching their eyes while this is happening because their outer eyelids will often be partially open, and the inner eyelids will be highly visible. It’s freaky.
harry forearms

Why doesn’t the wind blow the clouds apart?
stylinghead

here’s one a three year old and i had a smart back and forth with: when you fart do you get lighter or heavier? we never achieved consensus.
Anonymous

Does the earth get heavier if you plant a tree? As in, if you plant a forest full of trees/one tree, would that affect the earth’s weight?
SkullHyphy

Aside from the yellow markings on the head, this looks like my western three-toed box tortoise, which has orange and black skin.
ROSSINDETROIT

That looks like my Eastern Box Turtle named Porky. And yeah, they’re deranged.
roger_the_shrubber

Since the third eyelid is so useful, why do turtles (and all those other creatures) have the other two?
Snig

All turtles need Latisse to be truly beautiful. If you’re a turtle suffering from the heartbreak of hypotrichosis, ask your doctor if Latisse is right for you.
Jesse M.

“Eyelashes are, in general, kind of a mammal thing–what with being made out of hair and all.”

A number of types of birds have eyelashes…this Cornell ornithology page says:

“Several bird species, such as ostriches, hornbills, rheas, cuckoos, and some owls in the genus Bubo, which includes the Great Horned Owl, are known to have eyelashes. These eyelashes actually consist of bristles resembling mammalian eyelashes, and possibly serve to protect the eye against dust and other debris. Bristles are simplified feathers that consist only of a stiff, tapered rachis with a few basal barbs. The feathers have both sensory and protective functions.”

(doing some googling, some other birds not mentioned above which seem to have lashes include eagles, cockatiels, and secretary birds)
Anonymous

It would be cool if we could genetically (or otherwise) engineer our own nictitating membrane to be functional again. And change it to be semi-transparent, polarized, and with a mirrored finish. It would be like built-in sunglasses!
Anonymous

You can also see the nictitating membrane sliding across cats’ eyes when they’re sleepy or have just woken up. I never knew what that was.
andreinla

Would a toddler really understand the long answer? No.