What is the Parietal Third Eye of an Iguana? (Iguana 3rd Eye)

Old fossil records tell us a fascinating story about how many animals used to have an extra eye on top of their heads. As time passed and animals evolved, this extra eye disappeared in mammals and birds. But interestingly, some reptiles and amphibians still have this residual ‘parietal’ eye, giving us a glimpse into the ancient past of the animal world. One such reptile that still has the third eye is the iguana.

Understanding the Anatomy of the Iguana’s Third Eye

Iguanas have something known as a parietal eye. It is commonly referred to as a “third eye,” although it actually looks more like a round gray scale. The term “parietal” is used because it is situated between the two parietal bones of the iguana’s skull. This third eye is sensitive to light cycles, and while it does contain a lens, the lens detects light and dark, not actual images, shapes, or colors.

The location of this third eye is very useful to an iguana in his native habitat. While he is basking, he can keep his “third eye” open so he can detect light changes and shadows that may be indicative of a lurking predator or danger.

The Function of the Parietal Eye in Iguanas

Scientists have performed research to discover the full purpose of the parietal eye. Some of these tests were more humane than others. One experiment involved blocking the iguana’s regular two eyes, then using a bright artificial light to create a shadow over the parietal eye. This caused the iguana to wince and shy away.

Another experiment involved the actual surgical removal of the parietal eye altogether. This process did not seem to alter the iguana’s everyday habits such as eating, moving, sleeping, etc. The iguana did not, however, seem to know when to get out of the direct sun or light.

Parietal research to date reflects that the iguana uses the parietal eye to evaluate and condition itself and its actions according to the environment. The third eye helps the animal know when to go from sun to shade or vice versa. It is also believed to affect iguana breeding behavior.