ANIMAL EYE
Animals that see well in the dark usually have big eyes with large pupils. The bigger the eyes, the more light they can collect. ... Their retinas have many more photoreceptors called cones and rods that can convert light waves into information for its brain.
Most animals active at night have a cell layer inside the back of their eyes called the tapetal reflective layer. This layer reflects light back onto the retina so light hits it twice at night. It has many colors in it, sometimes green, yellow, or red.
Amazing Facts
* A worm has no eyes at all.
* An owl can see a moving mouse more than 150 feet away.
* Guinea pigs are born with their eyes open!
* Scorpions can have as many as 12 eyes, but the box jellyfish has 24!
* Camels have three eyelids! This is to protect their eyes from sand blowing in the desert.
* Most hamsters only blink one eye at a time.
Owls are the only bird which can see the color blue.
* Goats have rectangular pupils to give them a wide field of vision.
* A scallop has around 100 eyes around the edge of its shell to detect predators.
* Snakes have two sets of eyes – one set used to see, and the other to detect heat and movement. They also don’t have eyelids, just a thin membrane covering the eye.
* The four-eyed fish can see both above and below water at the same time.
* Owls cannot move their eyeballs – which has led to the distinctive way they turn their heads almost all the way around.
* A dragonfly has 30,000 lenses in its eyes, assisting them with motion detection and making them very difficult for predators to kill.
* Dolphins sleep with one eye open.
* The largest eye on the planet belongs to the Colossal Squid, and measures around 27cm across.
* Geckos can see colors around 350 times better than a human, even in dim lighting.
* The eyes of a chameleon are independent from each other, allowing it to look in two different directions at once.
* A camel’s eyelashes can measure up to 10cm long, to protect its eyelashes from blowing sand and debris in the desert.
* An ostrich’s eye is bigger than its brain.
* Dogs can’t distinguish between red and green.
* Polar bears have a third eyelid that helps filter UV light.
* Human eyes are not the most highly evolved. The mantis shrimp has four times as many color receptors as the human eye and some can see ultraviolet light.
* Pigeons can see millions of different hues, and have better color vision than most animals on earth.
* Cat’s eyes have almost 285 degrees of sight in three dimensions – ideal peripheral vision for hunting.
* Although color blind, cuttlefish can perceive light polarization, which enhances their perception of contrast.
* A moth’s eyes are covered with a water-repellant, anti-reflective coating.
* An ant only has two eyes, but each eye contains lots of smaller eyes, giving it a “compound eye.”
* Eagles have 1 million light-sensitive cells per square millimeter of the retina – humans only have 200,000.
* A honeybee’s eye is made of thousands of small lenses. A drone may have up to 8,600 and the queen be can have 3,000-4,000 lenses.
* The night vision of tigers is 6 times better than humans.
* Eyes on horses and zebras point sideways, giving them tremendous peripheral vision, to the point of almost being able to see behind them, but it also means they have a blind spot right in front of their noses.
A classification of anatomical variants of tapeta lucida defines four types:
1. Retinal tapetum
seen in teleosts, crocodiles, marsupials and fruit bats. The tapetum lucidum is within the retinal pigment epithelium; in the other three types the tapetum is within the choroid behind the retina.
2. Choroidal guanine tapetum
seen in elasmobranchii (skates, rays, and sharks) and chimaeras. The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of guanine.
3. Choroidal tapetum cellulosum
seen in carnivores, rodents and cetacea. The tapetum consists of layers of cells containing organized, highly refractive crystals. These crystals are diverse in shape and makeup.
4. Choroidal tapetum fibrosum
seen in cows, sheep, goats and horses. The tapetum is an array of extracellular fibers.
The functional differences between these four different types of tapeta lucida are not known.
This classification does not include tapeta lucida in birds. Kiwis, stone-curlews, the boat-billed heron, the flightless kakapo and many nightjars, owls, and other night birds such as the swallow-tailed gull also possess a tapetum lucidum. This classification also does not include the extraordinary focusing mirror in the eye of the brownsnout spookfish.
Like humans, some animals lack a tapetum lucidum and they usually are diurnal. These include most primates, squirrels, some birds, red kangaroo, and pig. Among primates only the strepsirrhines, with the exception of several diurnal Eulemur species, have a tapetum lucidum.
When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head, such that in all cases the tapetum lucidum enhances night vision in the center of the animal's field of view.
Apart from its eyeshine, the tapetum lucidum itself has a color. It is often described as iridescent. In tigers it is greenish. In ruminants it may be golden green with a blue periphery, or whitish or pale blue with a lavender periphery. In dogs it may be whitish with a blue periphery.
Functions
The tapetum lucidum functions as a retroreflector which reflects light directly back along the light path. This serves to match the original and reflected light, thus maintaining the sharpness and contrast of the image on the retina. The tapetum lucidum reflects with constructive interference, thus increasing the quantity of light passing through the retina. In the cat, the tapetum lucidum increases the sensitivity of vision by 44%, allowing the cat to see light that is imperceptible to human eyes.
It has been speculated that some flashlight fish may use eyeshine both to detect and to communicate with other flashlight fish.
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