Lizards possess a specialized structure often referred to as a “third eye,” known scientifically as the parietal eye or pineal eye. This organ is located on the top of their head, but it does not function like their two primary eyes. It is not capable of forming images or perceiving the world visually.
What is a Parietal Eye?
The parietal eye is typically found on the top of a lizard’s head, appearing as a small, scale-less spot. It is positioned along the midline, often nestled between the two lateral eyes. This photoreceptive organ is much smaller than the primary eyes and is usually not readily visible externally, though in some species like the tuatara, it can be clearly seen.
While the parietal eye possesses a lens and retina-like structures, it lacks the complex musculature for movement and the sophisticated neural pathways needed for image formation. Unlike conventional eyes, the parietal eye detects light using different biochemical methods. It primarily senses changes in light intensity and wavelength, rather than creating a detailed visual image. Many lizard species, including iguanas and bearded dragons, have a parietal eye, while others, like snakes and crocodiles, do not.
Functions of the Third Eye
The primary function of the parietal eye is to detect changes in light intensity, including day-night cycles and UV light. This light sensitivity regulates the lizard’s internal biological clock, known as circadian rhythms. It influences sleep-wake patterns, hormone production, and various physiological processes, helping synchronize the animal with its environment.
The parietal eye is also important for thermoregulation. As ectothermic animals, lizards rely on external heat sources, and this eye helps them sense sunlight intensity, prompting them to move into or out of direct sun to maintain an optimal body temperature.
Additionally, the parietal eye may offer a limited form of predator detection. Changes in overhead light, such as a shadow cast by a bird, could signal a potential threat. This basic light detection provides a non-visual warning.
Evolutionary Journey of the Parietal Eye
The parietal eye is an ancient structure with deep evolutionary roots, found in many early vertebrates. Fossil evidence indicates that many of the oldest vertebrates, including ostracoderms and early tetrapods, possessed skull sockets suggesting the presence of functional third eyes. This structure is an outgrowth of the pineal gland, a part of the epithalamus in the brain. In humans, the pineal gland is primarily involved in melatonin production and circadian rhythms, receiving light information indirectly.
Over evolutionary time, some species retained the parietal eye due to its ecological advantages, while others lost or reduced it. For instance, the tuatara, a reptile distinct from lizards, has a particularly well-developed parietal eye. Mammals, including humans, lost this external photoreceptive function as they evolved endothermy and developed different mechanisms for light detection and internal regulation. The presence of the parietal eye in modern lizards serves as a remnant of this long evolutionary history, continuing to perform vital, non-visual functions.