In this third installment of understanding reptile vision, we’ll be discussing the parietal eye that is present in many species of lizards, and its impact on how your lizard perceives the world. The parietal eye is often referred to as a “third eye”, and in some species of lizards you can even see the scale or spot on the top of the head where the “eye” is present. The parietal eye in lizards is tied to their pineal gland; a gland that produces, among other hormones, melatonin – the hormone that helps you sleep at night. The pineal gland is a fascinating organ, and one that we don’t know nearly as much about as we’d like to. Even in humans, the functions of the pineal gland are still somewhat of a mystery, so it’s understandable that in reptiles, we struggle to understand fully what impact it has on their day to day lives.
If you have never seen a parietal eye, or are unsure of what exactly one is if you wanted to look for it, in our pet lizards it is usually a small, circular scale in the center of the top of the head. It can be grey in color, or just a slightly different shade than the rest of the lizard. The third eye is most pronounced in the prehistoric tuatara lizard – their third eyes have similar parts as their two main eyes, including a lens similar to a cornea. The third eye is quite primitive, “ much more like the retina of an octopus rather than that of a vertebrate” (Schwab and O’Connor, 2005). This eye cannot see in quite the same way as the main eyes, instead likely only detecting shapes and shadows rather than full pictures. They are also highly sensitive to light – producing markedly different hormones based on time of day, with one study showing a system of neurons reversing their reactions based entirely on the daily photoperiod of the lizard. (Engbretson and Lent, 1976)
Older studies done on lizards to examine the purpose of their parietal eye experimented with removing the eye as well as simply covering it up. In humans, the pineal gland is what helps control our circadian rhythm, and in lizards the combination of the third eye and pineal gland serve a similar function. Experiments that removed the third eye from common North American fence lizards found that lizards missing their parietal eyes were more active for a longer period of time than their counterparts with intact eyes. On the surface, this doesn’t seem like such a bad thing – why wouldn’t a reptile want to be out and active for the most amount of time? Turns out, that’s only a good thing if you’re a mammal. We are active as much as possible to get as much food and energy as possible. For us, just sitting costs energy, while for reptiles, the less they move, the less energy they expend.
So, when a lizard is active for a longer period of time, but is not necessarily consuming more food, being that active becomes a hindrance, not a help. Parietal eyes helped limit the amount of time that fence lizards were spending out basking or moving around. In addition to the differences in activity between lizards that had parietal eyes and those that didn’t, lizards with their third eyes removed were harder to startle and scare away, with speculation in one study that “the pronounced heliothermism perhaps works antagonistically to the normal retreat reaction.” (Stebbins, pg 35)
Not only do the parietal eyes help set a lizard’s internal clock, control hormone production, and help them determine needed activity levels – they also use their parietal eyes to navigate. A study done with Italian wall lizards found that they used the sun to navigate through a “Morris water-maze” (click link for the wikipedia article), and by tricking their biological clock to be 6 hours faster or slower, the lizards were no longer able to reach their goal at the end of the maze. Painting over or removing the parietal eye entirely caused the lizards to no longer be able to navigate the maze at all. (Carnacina, 2009)
All of this just scratches the surface of what the parietal eye and, through extension, the pineal gland, are responsible for and control in a lizard’s life. This sensitivity to light is one reason for the common recommendation to provide basking lizards with bright, white lights to bask under – you are helping your lizard to keep its biological clock ticking at the right speed. Any diurnal lizard is particularly sensitive to light, and understanding how heavily they rely on external sources to help guide their lives will help you as a keeper provide them with a rich captive life. This is, of course, most relevant to true lizards, such as iguanas, skinks, lacertas, bearded dragons, and other similar reptiles. There are few, if any, lizards that are nocturnal, and in fact searching online for “nocturnal lizard species” will instead bring up the gecko family. Geckos do not appear to have the same parietal eye as diurnal lizard species, but as we saw in the last article, that does not at all mean that their sight is less attuned to light!
For your diurnal lizards at home, please be sure to provide them with a regular day/night cycle, including a nice bright, white basking light for them to heat up under. As studies have shown, diurnal lizards rely heavily on lighting as well as heat to determine activity levels as well as hormone levels. Good lighting not only encourages natural behaviors, but enables your pets to thrive all the way down to a cellular level. Lighting is vital to proper husbandry, and an integral part of any set up for diurnal lizards. Asking for help from any experienced lizard keeper in a sick, non-feeding, constantly sleeping lizard will immediately earn you questions about your lighting set up – and is it any wonder?
Augusto Foa, Francesca Basaglia, Giulia Beltrami, Margherita Carnacina, Elisa Moretto, and Cristiano Bertolucci (June, 2009) “Orientation of lizards in a Morris water-maze: roles of the sun compass and the parietal eye”, Journal of Experimental Biology, 212 Retrived from:http://jeb.biologists.org/content/212/18/2918.short
Gustav A. Engbretson, Charles M. Lent (February 1976) “Parietal eye of the lizard: Neuronal photoresponses and feedback from the pineal gland”, Proc. Nat. Acad. Sci USA Vol 73, No 2, pp 654-657 Retrieved From: http://www.pnas.org/content/73/2/654.full.pdf
I.R. Schwab and G.R.O’Connor (March 2005) “The lonely eye”, British Journal of Opthalmology, V. 89(3), 256 Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1772576/
Robert C. Stebbins and Richard M. Eakin (February 1958), “The Role of the “Third Eye” in Reptilian Behavior, American Museum Novitates, Number 1870 Retrieved fromhttp://digitallibrary.amnh.org/dspace/bitstream/handle/2246/4659//v2/dspace/ingest/pdfSource/nov/N1870.pdf?sequence=1