Why do animals’ eyes shine in dark?

First time I noticed this when I went to this village and I saw a hyena. It looked fierce and it frightened me. It was one of those topics which have always made me curious. I’ll tell you bluntly that this is Tapetum lucidum which causes such phenomenon. It is present in animal’s eyes just behind the retina.

eyeball-eyeshine.png

Another thing, if you’re thinking that we have tapetum lucidum in our eyes too, you’re wrong. We also get red eyeshine when we take a flash picture but that is for an entirely different reason. We don’t have this tapetum lucidum layer in our retinas. If you shine a flashlight in a person’s eyes at night, you don’t see any sort of reflection. The flash on a camera is bright enough, however, to cause a reflection off of the retina — what you see is the red colour from the blood vessels nourishing the eye.

What is tapetum lucidum and why is it needed?

Tapetum lucidum a layer of tissue present just behind the retina of an animal’s eyes. When light strikes the tapetum lucidum, it reflects back visible light through retina thus increasing the light for photoreceptors. We see this effect on the pupil of animal’s eye. Animals that display the brightest eyeshine have more rods (light receptors) and fewer cones (color receptors) in their retinas than animals with no eyeshine. It contributes in excellent eye sight but it makes them colour blind. The tapetum lucidum cells are leucophores which mean that they are retroreflector of the transparent sphere type.

Kinds of tapetum lucidum

  • Retinal Tapetum – This is present within the retina and it is found in reptiles like crocodile.
  • Choroidal guanine tapetum – It is present within the choroid and it is a collection of cells containing stacks of flat hexagonal crystals of guanine. It is found in animals like shark.
  • Choroidal tapetum cellulosum – It is also present within the choroid and it is a collection of cells containing organized, highly refractive crystals. It is found in carnivorous animals.
  • Choroidal tapetum fibrosum – Also present within the choroid and it is an arrangement of extracellular fibers. It is found in animals like cows, sheep, etc.

Exception

Humans have visible eyeshine when camera flash light strikes. There two possible ways of eyeshine. First, the red eye effect (which I explained above) and second, leukocoria. The red eye comes from the sudden flash in low light conditions.

eyeshine

The leukocoria will be also visible in same conditions although it is a medical sign for number of conditions.

After doing this research, my fear of animals disappeared. I used to think this was some kind of supernatural element. Although I have no fear now, this image is still frightening.

Refrences

  1. Tapetum lucidum – Wikipedia. Retrieved from:
    http://en.wikipedia.org/wiki/Tapetum_lucidum
  2. Image Sources
    http://bigfootevidence.blogspot.com/
    http://www.nerdnirvana.org/
  3. Slider image via Bayou Lady. Retrieved from:
    http://msbayoulady.blogspot.co.uk/2012/11/the-glowing-eyes.html
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