There are a lot of ways that reptiles are similar to other animals, but perhaps even more ways that they are different. Most people know that reptiles are cold-blooded critters that are covered in scales, but anything beyond that can get pretty mysterious. So of course, there are plenty of questions about the anatomy of these strange pets.
As a reptile owner, one of the most common questions I get is, “Do reptiles have eyelids?”
Many reptiles have eyelids, but some reptiles lack them. Snakes and many lizards lack eyelids entirely and instead have a clear eye covering known as a brille. Other lizards, crocodilians, and turtles have a third eyelid known as a nictitating membrane in addition to a normal pair of eyelids.
In this article, we’ll go over basic reptile eye anatomy, which critters have eyelids, and some unique reptile eye adaptations.
Reptile Eye Anatomy
As we’ll cover in depth below, reptiles have a lot of variety in terms of their eye structure. That said, they have a lot more in common than they do different. Like humans and most other vertebrates, reptiles have a type of eye structure known as camera eyes. This type of eye is very complex, but can be boiled down to several major features.
Light enters the eye at the cornea, which is the transparent, refractive surface that covers the eye. Then, it travels to the iris, which is the colored portion of the eye that has an opening that can constrict or expand to allow specific amounts of light to enter. This opening– the pupil– leads into the lens, which bends and stretches to focus light onto the retina, containing rods and cones to detect light and color.
These structures are as fragile as they are complicated and require constant moisture and protection to function. This is where eyelids come in, or in the case of several reptiles we’ll discuss below, several unusual structures.
Do Snakes Have Eyelids?
Rather than eyelids, snakes have a fused, transparent structure covering their eyes known as an eyecap or brille. This specialized ocular scale evolved during the time of the dinosaurs due to the fusion of the upper and lower eyelid. Brilles are unmovable and serve as excellent protection from dust and dirt for these low-moving animals.
Brilles are unable to be seen the majority of the time, but when a snake is about to molt, their brilles and other scales will begin to detach. This gives their eyes a blueish, cloudy haze, known by many snake keepers as “being in the blue.” When in the blue, snakes have a difficult time seeing and are particularly vulnerable.
Many snakes become anxious, refuse to eat and may even be defensive if you try to handle them. Luckily, once the snake is ready to shed in two to four days, the brille molts off along with the rest of their skin. If a snake does not properly shed its brille, a condition referred to as a “retained eyecap,” it can develop blindness. Luckily, upping the humidity and even a quick soak can help prevent this.
Do Lizards Have Eyelids?
Lizards are an incredibly diverse group of animals with plenty of anatomical differences from species to species. Most lizards have either brilles or a combination of normal eyelids and nictitating membranes depending on their family.
Which Lizards Lack Eyelids?
Many families of lizards lack eyelids altogether and instead have brilles like their snake brethren.
Gekkota– better known as geckos– are defined by their sticky toe pads, crepuscular tendencies, vocalizations, and, of course, lack of eyelids. Geckos are incredibly skilled and specialized hunters, with a unique pair of eyes according to their unique niche. Specifically, geckos have cone-filled eyes (as opposed to rods in humans), which may be as much as 350 times more sensitive than human eyes in low light! These curious critters are acute visual hunters that constantly keep their eyes open to detect prey, even when sleeping.
Like snakes, geckos typically have complete sheds, involving the shedding of their eyecaps, and may become more anxious when they are about to shed. Unlike snakes, meanwhile, they regularly use their tongues to lick their own eyes to clean them and keep them moist. The exception to the rule is the Eublepharidae family, commonly referred to as eyelid geckos. Members of this family, which includes the famous leopard gecko and the African fat-tailed gecko, are able to move and close their eyelids, and also have a nictitating membrane, which we’ll describe below.
There are a few other notable species outside of the gecko order that lack eyelids. Night lizards, which are close relatives of the more commonly kept skink, also have brilles covering their eyes. Despite being occasionally diurnal, these live-bearing lizards have eyes that are very similar to most geckos, with a strong ability to see and hunt in the tiny crevices and covered areas they call home.
Which Lizards Have Eyelids?
Other than the species mentioned above, most lizards have eyelids. In fact, one of the major reasons legless lizards are not considered snakes is due to their ability to blink. So what exactly are lizard eyelids like?
Most lizards have the normal pair of eyelids found in humans and similar animals, but they also have a third, bonus eyelid. This eyelid, called the nictitating membrane, is a translucent or transparent membrane that moves across the eyelid horizontally. These membranes can be opened and closed at will and serve to protect and moisten the eye.
Outside of this third eyelid, there are a few other anatomical differences between lizard eyelids and human eyelids. Unlike humans, only the lower eyelid of a lizards is moveable, and may even be transparent to allow the lizard to see with its eyes closed.
Lizard groups that have eyelids include skinks, monitors, iguanas, and bearded dragons. As we’ll discuss below, some lizards have more specialized eyelids.
Do Alligators and Crocodiles Have Eyelids?
While technically a distinct genus from lizards, crocodilians such as alligators, crocodiles, and caimans have similar eye anatomy to lizards with eyelids.
Crocodilians have normal, paired eyelids as well as an extensively used transparent, nictitating eyelid. Rather than only being used to sweep away any sort of dirt, this eyelid is used whenever the crocodilian is underwater to see clearly underwater and keep out water or debris, much like swimming goggles. As with lizards, crocodiles are able to close both lower eyelids at will, but typically cover their eyes with both lids in a complicated process that use in a long list of situations including sleep.
To blink, crocodilians draw in their nictitating eyelid for lubrication, then physically retract their eyelids into their skull. This causes a crocodilian’s eyelids to close over their sunken eyes without actually moving on their own. Below is a look at the bizarre phenomenon!
Do Turtles Have Eyelids?
Despite myths that claim they are blind or have poor visual acuity, turtles actually have excellent tetrachromatic vision and even prefer some colors over others. Much like crocodilians, aquatic turtles have translucent nictitating membranes that they heavily utilize to swim. Aquatic species are also known to demonstrate the blinking pattern of crocodilians, involving the retraction of the eyes into the skull.
That said, even terrestrial turtles and tortoises have eyelids for protection, and nictitating membranes are used to clean their eyes rather than as goggles. As with other reptiles, the lower eyelids of turtles are larger than their upper eyelids, and the only one that moves, with the exception to the rule being marine turtles. Sea turtles have unusually large upper eyelids that are able to blink, as well as nonmobile keratinized sections.
Outside of the two major eyelid types, some lizards have some pretty unique adaptations.
While they may be known for their dramatic color changes, perhaps the strangest thing about chameleons is their incredibly specialized eyes. With the ability to rotate independently, swell and shrink, and countless other adaptations, chameleons take the cake for the most complicated reptile eyes. In addition to all of these traits, chameleons have very unique eyelids.
Chameleon eyelids are actually fused to their pupils, which gives them sharp, telescopic vision that allows them to see up to a distance of one kilometer and focus four times more quickly than humans. These eyelids are actually partially imbedded within the eyes themselves and are incredibly muscled to allow such complex movements. In addition to their fused eyelid, chameleons also have a nictitating membrane which can clear their eyes by pushing any debris up and out of the eye.
The world of reptiles is full of some strange and fascinating species, but some of the strangest are members of the Dibamidae family, which are nearly impossible to classify due to their bizarre features and origins. Also known as blind skinks, these legless lizards have a pair of eyes that are covered by an entirely fused scale.
It may seem a bit strange for a lizard to lose its eyes and legs, but these enigmatic critters– which are regularly discovered to this day— are actually adapted for life underground. Dibamidae lost their limbs to more easily wriggle through dirt like a worm, and their eyes have reverted to a reduced form that lacks any internal structure By simplifying and fusing their eyelids, blink skinks are able to conserve energy and nutrients that would otherwise be used to develop and power a set of unnecessary eyes, much like cave-dwelling fish.
On the snake side of things, the not-too-sharp brahminy blind snake and other members of the blind snake infraorder have adapted eyelids to help conserve nutrients. More specifically, the brilles of Scolecophidia are opaque, resulting in lower visual abilities and much less energy spent on seeing.
Finally, worm lizards, which are a strange group of reptiles that are neither worms nor lizards, are underground dwellers defined by their wormlike appearance, shockingly powerful jaws, and rudimentary eyes.
While parietal eyes aren’t exactly a type of specialized eyelid, they’re certainly worth mentioning when discussing unusual reptile eye adaptations. Parietal eyes are simplistic third eyes found on top of the heads of many amphibians and reptiles. More specifically, these unusual eyes are considered relics of the past, and have been lost in snakes, crocodilians, and most turtles.
These tiny eyes, which lack normal eyelids and are instead covered by a thin sheet of skin, can be found in the tuatara and most lizard species. Parietal eyes are actually able to detect light with a complex mechanism that is more like that of an insect than the rods and cones of their other eyes. While the exact function of these eyes are unknown, they are theorized to be used in light-specific hormone regulation (much like humans need darkness to produce melatonin) and have been proven to aid in finding light.
Reptiles are a diverse and fascinating group of creatures, and there’s a lot of anatomical differences between species. Crocodilians, turtles, and most lizards have eyelids, as well as a third eyelid known as a nictitating membrane.
Snakes and geckos, meanwhile, lack “normal” eyelids and instead have brilles. There are some specialized reptile eyes, from complex chameleon spectacles to the vestigial eyes of blind snakes and skinks.
Of course, there’s always more to learn about these amazing animals, especially when it comes to reptile anatomy. Luckily, there’s plenty to read about snakes, lizards, and more on this site or at your local library!