In contrast to snakes, which largely experience the world through smell and taste, lizards are more visually oriented creatures. Lizards rely on vision for survival, and their eyes are well developed. A review of the evidence shows that most lizards can see color better than humans can; some will use color to communicate and make decisions, and some can even see colors in very dim light.
Lizards have typical vertebrate eyes -- the pupil allows light to pass through a lens, which focuses the light on the back of the retina, where the light stimulates photoreceptive cells. The photoreceptive cells change the light impulse into an electrical impulse that is transmitted to, and interpreted by, the brain. Vertebrates have two types of photoreceptive cells: rods, which detect light when levels are low; and cones to detect color. The cones contain pigments that filter the light hitting it, allowing animals to see a variety of colors. In some lizard species, some of their cones are calibrated for seeing ultraviolet light.
Nocturnal animals often see colors poorly, but have good vision in low light. These animals often have eyes containing many rods and relatively few cones. Geckos are different, however, and see in dim light by using three sets of cones. A 2004 study by Lina S.V. Roth and Almut Kelber of the University of Lund, Sweden, demonstrated that helmeted geckos (Tarentola chazaliae) were able to discern grey and blue shades in light comparable to dim moonlight.
Many lizards have a third eye, located on the top of their heads. Termed the parietal eye, this eye is very simple, and its primary function is to determine light levels. Lizards are thought to use data from this parietal eye to influence basking behavior. Interestingly, a 2006 study by King-Wai Yau of John Hopkins Medicine, published in the journal “Science,” demonstrated that the parietal eye of some lizards is even able to see two different colors: green and blue. It's possible that distinguishing between these colors allows the lizards to determine the time of day as the colors of daylight shift with the sun.
Chameleons and anoles both exhibit remarkable adaptations that enable color-based communication. By adopting any of a species-specific subset of color patterns, chameleons can communicate aggression, dominance, submission, receptivity and non-receptivity to other members of their species. Anoles identify conspecifics by a combination of head bobs and dewlap extensions. These dewlaps are often brightly colored and patterned, and anoles use them as flags to advertise their identity to nearby lizards.
Besides species identification, anoles can use color to make decisions. A study seeking to understand decision making ability in anoles was conducted by Manuel Leal and Brian Powell, of Duke University, and published in an issue of “Journal of the Royal Society: Biology Letters.” Leal and his colleagues presented anoles with two differently colored discs; under one of the discs was a meal worm -- a treat for an anole. In most cases, the anoles learned to check the appropriately colored disc for the tasty morsel first.
- Broadinstitute.org: Creature Feature: Green Anole Lizard
- Melissa Kaplan's Herp Care Collection: Reptile Vision
- Hopkinsmedicine.org: Lizard "Third Eye" Sheds Light on Evolution of Color Vision
- Lizards: Eric R. Pianka, Laurie J. Vitt
- Nbcnews.com: Don't Underestimate the Brainpower of a Lizard
- Broadinstitute.org: First Lizard Genome Sequenced
- Proceedings of the Royal Society: Biological Sciences: Nocturnal Color Vision in Geckos
- Newscientist.com: Chameleons' Colorful Flashes are Social Signals
- Jupiterimages/Photos.com/Getty Images