Many people may have observed the glowing in the eyes of various animals and wondered what caused it. Studies have revealed that the glowing is produced by the layer of tissue known as tapetum lucidum, which is located behind the retina (Benson & Suter, 2013). The tissue’s special physical characteristics allow it to function as a reflective surface, i.e. much as a mirror. It reflects a portion of the light entering the eye after it is cast on the retina, which is the area where the image of what is before an animal is constructed to be further transferred to the brain.
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When dealing with biological phenomena, it is always appropriate to ask the question, “What is it for?” Although some phenomena observed in different animal species are purposeless (e.g. atavisms or vestiges), in most cases, particular abilities and features of organisms develop in the process of evolution to facilitate animals’ survival. Therefore, they have a purpose. Land and Nilsson (2012) suggest that the reflection of light within the eye is needed to increase the chances of seeing particular objects. The reflected light is cast back on the photoreceptors to give them a second chance to detect the light, i.e. convey the image of what is an insight to the brain.
This mechanism is especially needed if there is a high risk that the light does not hit photoreceptors. Such risk occurs when there is little light around. Therefore, the tapetum lucidum is most frequently found in animals who lead nocturnal lifestyles. Interestingly, the color of the reflective layer may differ from animal to animal, not only from species to species. Although green glowing is the most widespread, some animals’ eyes also glow in yellow or blue.
Benson, K., & Suter, R. B. (2013). Reflections on the tapetum lucidum and eyeshine in lycosoid spiders. The Journal of Arachnology, 41(1), 43-52.
Land, M. F., & Nilsson, D. E. (2012). Animal eyes. New York, NY: Oxford University Press.