Why do the eyes of some animals glow in the dark

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[edit] Introductory Note

Human eyes do reflect light, as all flash photographers know ... they just don't do it very well. Our eyes look red in some photographs. But it's not a reflector. It's the camera flash bouncing off the red blood vessels and red tissue in the back of our eye. The distinctive "eyeshine" given off by wolves and raccoons and crocodiles, among many other species, comes from the "tapetum lucidum", a mirror-like layer of cells in or behind the retina. This structure is found mostly in nocturnal animals, for whom it serves as a kind of light amplifier. The retina captures some of the light that enters the eye, but some passes through. The tapetum lucidum bounces it back at the retina, giving the animal a second chance to "see" it. It's this ricocheting light that gives off the vaguely eerie glow we've come to associate with scary tales ... all those sinister pairs of eyes staring out from gloomy thickets.

[edit] Reason

The eyes of many animals, but not humans, contain a reflective layer called the "tapetum lucidum". That helps the eye gather more light, which improves night vision. You just see what's reflected back at you; the rest of the light is absorbed by the creature's eye.But it's not a precision mirror, and it tends to give you less accurate vision during the day. Humans go in for precision (which is why we can read) at the expense of hunting or seeing predators at night.The tapetum lucidum is reflective, but it's not like a mirror. It's more like some bird feathers and butterfly wings, where it uses ridges or dots of clear material to achieve a colored effect. The distance between the ridges interacts with the frequency of light to give you different colors. The colors you're not seeing are the ones that the animal are what the animal actually sees.

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[edit] More

• The eye in all animals except pigs and man have a reflective layer along the back of the retina called the tapetum. This layer serves to reflect light to allow maximal usage of the available light. In some animals the tapetum is colored in others it is not.

• The reddish color that you see in human eyes is the reflection of the blood vessels in the back of the eye because the tapetum is NOT colored. This is the same in the "red eye" of the Yorkie.

• You can watch Yorkies, clipped down or in full coat, outside in the sunshine ... and you can easily separate them into two groups - those with a "blue cast" to the coat, and those with a "gold cast" to the coat.

• Those with gold or bronze cast to the coat have eyes that reflect a flash bulb or flashlight with the "red eye" or "shiny" red reflection to the pupil no pigment.

• What is going on is that those with the gold/bronze cast has LESS eumelanin (black) and more phaomelanin (yellow). Typically phaomelanin (yellow) does NOT get put in the tapetum. So NO tapetal pigment results in red color from the blood vessels. Whereas the blue cast dogs have more eumelanin resulting in a pigmented tapetum and the reflection that you see is bluish or green. It's very similar in chocolate and black Labs-- chocolate labs tend to have pale gold or red tapetums while the black labs have brilliant green and blue tapetums.

[edit] References

1. Glowing eye

2. luminous eye in night

3. Eye Shine

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