Science & Technology

Ever Seen a Three-Eyed Lizard? They're Everywhere

If you ask us, the sudden appearance of a three-eyed lizard on a hiking path or anywhere else is cause to drop everything and immediately inform officials of some sort of radioactivity-based mutation. But put that phone down. Most lizards have a third eye located right smack at the top of their heads. And in a way, so do humans.

Eye Eye Eye, Captain

If you happen to have the opportunity to get up close and personal with a living lizard's head, then we encourage you to do so. If you draw an imaginary line connecting one of its eye to the other, you'll probably notice that as the line crosses the lizard's forehead, it intersects a tiny indentation right in the middle. Just don't look too hard at it — it's rude to stare somebody in the eye. The parietal eye is very common among lizards, and it can also be found in other reptiles, including sea turtles and tuataras. You'll also find it in many fish and amphibians. But what is it for, and why don't humans have one?

The parietal eye isn't quite the same as the animals' main eyes, since it can't focus or form images. But it does have a rudimentary retina and lens, along with a cluster of light-sensitive cells, that make it perfect for sensing minute changes in light and dark. That comes in handy in a lot of ways, some of them quite unexpected. First, the obvious benefits: because of the parietal eye, lizards are able to sense if there's something high above them (though they can't tell if that something is a hungry hawk or a harmless goldfinch). Also, the eye gives the lizards a pretty solid sense of the sun's progress across the sky, which is important if you rely on the sun to keep your body warm.

One study from the University of Ferrara in Italy actually suggested that the parietal eyes play another, less intuitive role for the lizards: navigation. For the study, the researchers trained lizards to swim from the center of a water maze to reach a platform near the edge. Even though the platform was hidden beneath the water, the lizards could use their sense of direction to find it every time. Next, they kept one group of lizards in a room where the light dimmed and brightened in sync with the sun, while another group lived in a place where the light would flare up and down at an unpredictable rate. When they were placed in the same water maze, the ones who had been accustomed to normal light cycles were able to navigate to the platform no problem. But the ones who were bewildered about what time it actually was had a much harder go of it. That suggested to the researchers that the parietal eye syncs up with the lizard's internal clock so that the little guys can tell what direction they're going via the location of the sun.

Prehistoric Parietals

The fact that there are so many parietal eyes in so many different families of vertebrates suggests that the organ must be pretty old. Think of it this way: if a bunch of people in your family have red hair, they probably all inherited it from somebody further back on your family tree. The same is true of parietal eyes. In fact, if you go back far enough on the human family tree, you'll find parietal eyes as well — you just have to go back far enough that we aren't mammals anymore. Some researchers think that the parietal eye is intimately connected to cold-bloodedness, and that mammals lost the need to keep such close watch on the movement of the sun when we developed the ability to keep ourselves warm. Interesting, we still have the remains of the parietal eye locked inside our brains. It's become the pineal gland, which helps us regulate our sleep cycles. That means it's still intimately connected with how and when we decide to rest to conserve energy.

Finally, we just couldn't in good conscience end this article without mentioning the most recent strange discovery in unexpected eyes. In 2018, German researchers going through some old fossils discovered the skull of a prehistoric monitor lizard that was originally uncovered in 1871. Saniwa ensidens roamed the Earth 49 million years ago, and back then you had to think bigger than three eyes to make an impression. That's why Saniwa had four. Although the scientists still aren't sure why, they believe that this prehistoric giant may have had an even better navigation system than its eventual descendents.

2-Minute Neuroscience: Pineal Gland

Written by Reuben Westmaas May 17, 2018

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