A Close Look at Lizard Eyes and How They Work

Lizard eyes are a favorite pattern for contact lenses because they have such an alien appearance. The wrinkled and colorful iris, the slitted pupils, the glassy stare…Reptile eyes are fascinating to look at. But are they so different from our own eyes?

Lizard Eyes: Form and Function

A lizard eye is similar to our eye in some ways and wildly different in others. Lizards are very well adapted for living in both bright and low light environments. Their eyes have a pupil, iris, cornea, as does ours.

Lizards also have a structure called the sclerotic ring. It is a ring of bone around the eye that supports and protects it. Most vertebrates have a sclerotic ring; only mammals and crocodilians lack one.

How Do Lizards See?

Lizard vision is in full color but its also a little different from ours. They are tetrachromat animals, meaning they have four different kinds of cones in their retinas. Humans and other primates are trichromats; we have short, middle, and long wave cones covering the visible light spectrum. 

But we don’t have the ability to see into the ultraviolet spectrum, like lizards do. Tetrachromat animals also have a greater ability to distinguish between shades of colors that would appear identical to us. 

The visual experience of lizards is a richer one than ours, with millions more shades of colors visible to them. Not to mention ultraviolet colors that we can’t see at all.

We shouldn’t be too disappointed, though. Most mammals have worse color vision than we do. They are almost all dichromats. The reason for this is not fully understood, but one hypothesis is that mammals evolved as nocturnal creatures. 

The nocturnal bottleneck theory explains several of the features of our eyes, including dichromatic and trichromatic vision. Light sensitivity was valued over acuity, placing little value on color differentiation. 

Why primates have trichromatic vision may be because our ancestors needed to determine how ripe fruit was for maximum calorie value. As well as determining which leaves are the youngest and the most nutritious.. But the research is ongoing in this field also.

Third Eyes of Reptiles

Lizards also have a third eye called the parietal eye. It does “see” but its function is to measure the length and brightness of each day and night cycle. The parietal eye has a direct link to the pineal gland. Which in turn regulates circadian rhythms and hormone production in animals.

Other animals with a parietal eye include the tuatara, sharks, some bony fish, salamanders, and frogs. Warm-blooded animals like birds and mammals don’t have one. Suggesting that thermoregulation is another  important function of the parietal eye.

The parietal eye can also see sky polarization. Helping lizards navigate using polarized light. Taken from article summary: “Lizards with black-painted parietal eyes were completely disoriented. Thus, the present data show for the first time that the parietal eye plays a central role in mediating the functioning of a putative sky polarization compass of lizards.”

Snakes don’t have a parietal eye but several do have a different kind of third eye. They have heat-sensitive pits that see thermal radiation. Pythons, boas, and pit vipers all have pit organs located at the front of the head. 

All of these snakes prey on mammals and birds, which are warm-blooded creatures. Heat vision gives them a major advantage when hunting in dark forests or at night.

Horned Lizard Eyes

A few lizards even use their eyes for protection. North American horned lizards (Phrynosoma sp.) have one of the most unusual defenses in the animal kingdom. 

Horned lizards are already covered in sharp spikes, making them an unappetizing meal. But food is scarce in the desert; coyotes and other predators will take a chance on these slow lizards.

Horned lizards have another trick up their sleeves when pressed: they shoot blood from their eyes. The blood seems to cause predators distress. Researchers suspect the lizards may have a compound concentrated inside it that tastes foul. 

Horned lizards eat ants and it’s possible they store formic acid or another defensive compound from their meals. Releasing it later as a projectile weapon.

Are Lizards Nocturnal?

Some lizards are nocturnal but the majority are diurnal animals. Since their metabolism is regulated by the sun’s heat, it makes sense for them to be the most active during the day. 

Geckos and skinks are some of the only nocturnal lizards and they have several adaptations that day dwelling lizards don’t. Their metabolisms are measurably higher than diurnal lizards, creating body heat to counteract the chill of the night.

Skinks and geckos also have excellent night vision. And that’s even without a tapetum lucidum. The reflective layer that mammals use to direct incoming light back for a second pass through the retina. 

The “eye shine” of nocturnal mammal eyes gives them a sinister look if a bright light source is reflected. But a nocturnal lizard eye has no shine at all.

Lizards with Big Eyes

There are several lizards with big eyes. Geckos are the first kinds of lizards that spring to mind. These nocturnal lizards have immense eyes with vertical pupils like a cat’s. We see vertical pupils often in nocturnal animals because vertical pupils open wider than circular ones

Gecko lizard eyes are also unique in having a hard membrane covering them. Geckos don’t have eyelids like other lizards do. Instead of blinking, a gecko will lick their eye membrane using their long tongues.

Can lizards see in the dark? Well, geckos are one group able to. Their color vision is 350 times more sensitive than ours in the dark. Their eyes are also multifocal, allowing them to focus on multiple distant objects at once.

One family of geckos do have a true eyelid: the Eublepharidae. These ground dwelling lizards include several great pet reptiles. The most popular Eublepharid lizards are the leopard gecko (Eublepharis macularius) and African fat-tailed gecko (Hemitheconyx caudicinctus). Both of these lizards blink like humans.

Lizard Eyes Different Directions

Chameleons are the lizards with the world’s best eyesight. Their eyes have several unique features worth exploring. 

Chameleons developed their amazing eyes in response to the demands of living in the trees. Arboreal life takes place in 360 degrees, where both prey and predators may come from any direction.

Their eyes protrude out from the head, giving them a complete view around their body. Chameleons can also move their eyes independently of one another. And are able to process the visual information from both at the same time.

Each eye is large but only the pupils are exposed. The rest of the eye is covered by a scaly protective eyelid fused to each eyeball.

Which Animal has the Best Vision?

Chameleons and geckos would be good additions to lists of animals with good vision. But the animal with the best vision isn’t a lizard. It’s not even a vertebrate: the mantis shrimp is the king of animal eyesight.

Mantis shrimp eyes are so complex that it’s hard to imagine what their perception of the world is like. For starters, they don’t have three, or even four types of color-detecting cones. 

They have twelve to sixteen types of cones, expanding their color vision into the near-infrared (right below our threshold for red) and into the deep ultraviolet range.

Mantis shrimp also see polarized light; light that vibrates along a single fixed plane. Some animals can see polarized light but humans can’t. Several species even see circular polarized light, which opens up many advantages. 

The main one is that mantis shrimp can communicate to each other using invisible signals. Even animals that see polarized light only detect linear polarization.

As wild as mantis vision is, however, lizard vision is more like our own. With deeper color gradation and an extra ultraviolet vision added, of course.

Conclusion

Lizard eyes are a marvel of evolution. Day dwelling lizards like bearded dragons have better color vision than we humans do. But some lizards, like chameleons and geckos, have visual experiences that we can’t even imagine.

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