Do Iguanas Have a Third Eye? The Parietal Eye Explained

What is the parietal eye?

The parietal eye, also called the pineal eye or third eye, is a light-sensing organ connected to the brain. In the green iguana it lies on the midline of the head, just behind the level of the main eyes. From the outside it looks like a slightly larger, smoother, pale gray or whitish scale.

Beneath that scale is a tiny structure with a simple lens and a basic retina, a layer of light-sensitive cells. A small nerve links it to the brain. It sits in a small hole in the skull roof called the parietal foramen, named after the parietal bones that surround it.

The parietal eye was first described in lizards in 1872 by the German zoologist Franz Leydig. Later researchers noticed that its sensory cells resemble the cone cells of a normal eye, and the nickname “third eye” stuck.

What can an iguana’s third eye see?

The parietal eye cannot see shapes, colors or detail the way the two main eyes do. It works more like a light meter. It reacts to how bright it is and to sudden changes in brightness, such as a shadow sweeping across the iguana.

That makes it useful for spotting danger from above. Hawks and other birds of prey are major predators of iguanas, and a shadow passing over the parietal eye can warn a basking iguana to flee before the predator is close. Animal Diversity Web notes that the visual role of the green iguana’s parietal eye is mostly limited to detecting predatory shadows overhead.

Studies of lizard parietal eyes have found that the photoreceptor cells respond in opposite ways to blue and green light. Because the balance of blue and green light in the sky changes from dawn to dusk, this may help a lizard judge the time of day.

What does the parietal eye do for an iguana?

The parietal eye is linked to the pineal complex, the part of the brain that produces the hormone melatonin and helps control daily and seasonal rhythms. Through this link, the third eye is thought to help iguanas:

• Keep a daily rhythm, knowing when to wake, bask and rest.
• Track the seasons by sensing day length, which influences the timing of breeding and the development of the reproductive organs.
• Manage basking, since iguanas rely on sunlight to warm their bodies and absorb ultraviolet light.
• Detect predators from above through changes in light and shadow.

Experiments on several lizard species in the 20th century found that lizards whose parietal eye was covered changed how long they stayed out in the light. In some lizards the parietal eye also seems to help with orientation using the sun or the pattern of polarized light in the sky.

Do other animals have a third eye?

The parietal eye is an ancient feature. Fossils of many early vertebrates, including early fish and the first four-legged animals, show a hole in the skull roof that probably held a working third eye.

The tuatara of New Zealand is the famous example. Its parietal eye has a lens, a retina and a nerve, and in hatchlings it is visible as a translucent spot on top of the head. Within a few months it becomes covered by scales and pigmented skin, but it still responds to light. Tuataras are not lizards but the last survivors of a separate reptile order; see Is a tuatara a lizard?.

Among lizards, the parietal eye varies. It is generally well developed in sun-loving, daytime species like iguanas and is reduced or missing in many nocturnal lizards, such as most geckos.

Do humans have a third eye?

No working one. In mammals, including people, the part of the brain that forms the parietal eye in reptiles became the pineal gland, a small pine-cone-shaped gland deep inside the brain. It still helps regulate daily rhythms by producing melatonin, mostly at night. But it cannot sense light itself; it gets that information indirectly from the main eyes.

The ancestors of mammals had a parietal opening in the skull, which closed during the Triassic period, more than 200 million years ago. Birds also lack a parietal eye, although in some birds the pineal gland itself is sensitive to light passing through the skull.

Myths about the third eye

The phrase “third eye” sounds mysterious and is linked to many spiritual traditions, but in iguanas it is simple biology. The parietal eye is not hidden, magical or able to see in the dark. It is a small, primitive light sensor that has stayed useful for reptiles that depend on sunlight.