By Kelly Davis (Scripps College) and Alex Mauro (Claremont McKenna College) [edited by Lars Schmitz, as part of BIOL 167 "Sensory Evolution", an upper division class at the Claremont Colleges]
When you were young, did your mother ever tell you that she has eyes in the back of her head? You probably thought that there is no such thing as eyes in the back of your head. Well, it’s time to reevaluate that perception because there are reptiles that can prove you wrong. Indeed, many species of lizards and the tuatara have just that—an eye on the top of their heads known as a parietal eye (Figure 1). However, this eye can’t “see” in the same way that we generally think an eye does. In fact, its function remains enigmatic, although researchers have been trying to uncover the true purpose of this photosensory organ for many years. Theories on the subject abound, but no conclusive evidence exists to confirm any of them. In 2010, Antonieta Labra and colleagues set out to further investigate these ideas and figure out the exact function of this eye. The group conducted various comparative studies that investigated the possible functions of the organ in relation to climate, thermoregulation, and thermophysiology in an attempt to conclusively determine the function of this mysterious sensory organ.
As of yet, little is known about the parietal eye regarding its function and evolutionary history, but there is a general consensus regarding its basic physiology. The parietal eye is known to be part of a larger sensory system, which also includes the pineal complex. This organ’s job is to help regulate circadian rhythms and seasonal cycles. The pineal complex has been retained in all vertebrates except crocodilians and a few mammals whereas the parietal eye has been lost in almost all clades except the lizards and tuataras. Notably, the parietal eye has been lost in snakes, which evolved from a common ancestor to lizards. So why do lizards still have the parietal eye? Why do they still have a sensory organ that is no longer necessary for other vertebrates, or even other reptiles?
As aforementioned, there is a long history of research on the parietal eye before Labra and colleagues started their investigation. For example, Foa and colleagues suggested that the parietal eye can function as a “time-compensated sun compass,” meaning it can be used to orient the animal based on the intensity of the sun’s rays. This was consistent with prior evidence that suggested the parietal eye was sensitive to polarized light. Foa and colleagues tested their hypothesis with a truly fascinating study in which they trained iguanas to swim through a maze to hidden targets at different times of the day. There were no forms of visual cues other than the sun, so the lizards were forced to use the sun as a guide. Furthermore, they conducted the trials at different times of the day to demonstrate that the lizards could account for different levels of sunlight (time-compensated). Although the lizards were successful in finding the targets, Foa et al. concluded that more evidence was needed to support this hypothesis.
Tosini and Menaker provided another hypothesis regarding the parietal eye function and carried out a study in which they surgically removed the parietal eye from iguanas in order to test its thermoregulatory capabilities. They uncovered some useful data, namely that thermoregulation was only slightly affected by the removal of the parietal eye. However, they also found that when they removed the entire pineal complex the results were a lot more dramatic: the iguanas could no longer thermoregulate properly at all. Thus, they left the door open for further investigation of the thermoregulation hypothesis.
Labra and colleagues’ 2010 study probed further into these and other hypotheses of thermoregulation and orientation by sampling the parietal eye sizes of individuals from the genus Liolaemus (Figure 2) and comparing these values to aspects of the lizards’ habitats. Liolaemus was chosen because it is a genus that is found in many different climates and thus could provide a lot of data on many different habitats while still maintaining a high degree of relatedness between individuals (it’s difficult to compare very different species with accuracy). Additionally, the phylogeny for this genus is fairly well established, which was constructed using Bayesian methods. Labra et. al.’s study was different than previous ones because it took a comparative approach in examining the function of the parietal eye. Instead of directly testing its function (e.g. a maze or by vivisection), parietal eye size was measured with the idea that larger eye size would indicate increased function/importance. This eye size was then compared to the individual’s altitude, environmental temperature, and territory size. It was hypothesized if the parietal eye had thermoregulatory capabilities, it would be used more in populations that lived at higher altitudes where light levels varied more in order for the lizards to properly thermoregulate. Hence, individuals at higher altitude should have bigger parietal eyes. It was also hypothesized that larger territory size would require more orientation abilities, meaning bigger eyes should be found in individual with larger territories. Lastly, the temperature of the lizard’s habitat was also examined to see if thermophysiological correlations could be found.
With these previous studies and hypotheses in mind, Labra and her colleagues took on this ambitious quest. After sampling thirty species of Liolaemus, they found several correlations, but still no conclusive answers. Parietal eye size was not significantly correlated with altitude, but there was a correlation between eye size and the minimum temperature of a population’s environment. Before moving on, it is important to emphasize the correlations that were attempting to be made. Altitude was attempted to be representative of a greater range of temperatures in the lizard’s environment whereas temperature examined the maximum, minimum, and average temperatures of the lizard’s environment. Lower minimum temperatures occur at higher altitudes so their approach appears to be slightly convoluted and more direct data is needed to make significant correlations. More data was also needed to make conclusions about parietal eye size and territory size (which meant the orientation hypothesis could not be supported). Ultimately, the most concrete evidence gathered was not between parietal eye size and environmental factors at all, but simply between eye size and species: The parietal eye size variance between species was significantly lower than would be predicted if parietal eye mutation was random. Labra’s data also suggested that there was an absence of phylogenic affects. Combined, these facts indicate that the parietal eye is constrained evolutionary because of function or because of many evolutionary changes in a limited sampling range. In a nutshell, we still do not know the exact function of the parietal eye, and there is very limited data which indicates it has a function at all. The thermoregulation hypothesis seems to be the most likely, but it still needs further investigation. However, Labra and her colleagues finished the study by suggesting that the data they did obtain on orientation was promising enough to deserve more investigation and that this seems like the most likely function of the parietal eye as of now.
Foa, A., Basaglia, F., Beltrami, G., Carnacina, M., Moretto, E., & Bertolucci, C. 2009. Orientation of lizards in a morris water-maze: Roles of the sun compass and the parietal eye. Journal of Experimental Biology, 212(18): 2918-2924.
Labra, A., Voje, K. L., Seligmann, H., & Hansen, T. F. 2010. Evolution of the third eye: A phylogenetic comparative study of parietal-eye size as an ecophysiological adaptation in liolaemus lizards. Biological Journal of the Linnean Society, 101(4), 870-883.
Tosini, G., & Menaker, M. 1998. Multioscillatory circadian organization in a vertebrate, iguana iguana. Journal of Neuroscience, 18(3), 1105-1114.