Why is it that snakes do not have eyelids?

Is it due to that fact that they are not as evolved as we humans and other organisms that have eye lids, or is there any other significance behind it if any?

  • 4
    $\begingroup$ "the fact that they are not as evolved as we humans" - there is no such fact. Evolution is not a scale going from "less" to "more". All organisms are equally evolved. So, it can't be due to that "fact" by definition. $\endgroup$
    – rumtscho
    Oct 30 '15 at 17:58
  • $\begingroup$ @Aneesh For more discussion on the concept of "more/less evolved", have a look at the post Are we “more evolved” than present-day bacteria? $\endgroup$
    – Remi.b
    Mar 31 '18 at 5:47

Snakes have no eyelids. Each eye is covered with a single, transparent scale. These eye scales protect the eyes from mechanical damage, and prevent the eyes from drying out, just as an eye lid would do. In effect, snakes developed another solution to protect their eyes.

As a result, snakes cannot blink and they sleep with their eyes open.

A snake’s eye scales are part of its skin. This means that when a snake sheds its skin, it must also shed its eye scales. When it is time for shedding, the eye scale turns opaque, reducing vision (Fig. 1).

opaque snake eye
Fig. 1. Opaque snake eye before skin shedding. Figure source: Snakes are long blogspot

Source: Museum of Victoria

  • $\begingroup$ Good answer +1. It would be great (although that would probably take some work) to have some mapping of this trait on a phylogenetics tree (or just some information about their snakes and mammals sister species). $\endgroup$
    – Remi.b
    Mar 31 '18 at 5:49
  • $\begingroup$ @Remi.b - thanks heaps. But the phylogenetics I leave up to you my friend. I'm in sensory systems you see :) $\endgroup$
    – AliceD
    Mar 31 '18 at 9:47

I don't know WHY snakes have no eyelids, but the lack of eyelids doesn't mean they aren't as highly evolved as animals that do have eyelids. Rather, snakes simply followed their own evolutionary path.

Whether or not snakes have eyelids may be a matter of perspective. According to this article, snakes' eyelids have evolved together into a single transparent scale called a brille.

I'll venture a couple guess to explain WHY this trait evolved. 1) Eyelids may have posed a special problem when molting. 2) Since snakes are extremely long and narrow, they're well adapted to crawling through tight spaces, into holes, etc. Eyelids could be a nuisance in such situations; imagine them getting clogged with dirt, for example.

Small, burrowing mammals typically have very tiny eyes; I believe some are blind, or nearly so. But what about their eyelids?

I haven't researched it in depth, but a cursory search revealed this clue: "Grant's golden moles have no pinnae and have poorly developed eyes, with fused eyelids covered with skin."

Sounds eerily similar to snakes!

  • $\begingroup$ except there are not , those moles are rendered blind by their condition, They don't have eyelids becasue they can't open their eyes in the first place. Alternatives to eyelids that don't render the eye non-functional exist, snakes have one of them. $\endgroup$
    – John
    Mar 31 '18 at 3:10

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