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The retina is the organ in the eye that transmits light stimuli to the brain where it becomes vision. Within the retina, there are two structures that determine colors that are seen: rods (black and white) and cones (color). Color vision helps animals with seeing minor contrasts among colors. With color vision, it is easier to pick objects and other animals out from their background. Color vision may help an animal in prey capture, to see predators quickly, and to be able to communicate more efficiently with other members of its species.
Animals with color vision include many species of fish, frogs, most reptiles and birds, some primate species, and others.
Birds have an abundance of oil droplets in cone cells.
Remember that vision is caused when the rods (black and white) and cones (color) are stimulated by light. If an animal can’t see at night, it is because not enough light enters the eye-ball and strikes the rods and/or cones in the retinal layer. So, if animals can see well in the dark, there must be special features that make them different from those that cannot.
One important feature is that the pupil must be able to get very large. In humans, the pupil expands in the dark and contracts in the light. The same is true for most animals. Some, like cats, pit vipers, and others with elliptical pupils, have large round pupils in the dark and narrow slit pupils in the light.
Since it is the stimulation of the retina that translates to vision, anything that increases that stimulation will be good. Many night-active animals have a mirror-like (composed of platelets of guanine crystals) layer, called the tapetum lucidum, located behind the retina. When light passes through the retina, it stimulates vision. It is then reflected off the tapetum lucidum and stimulates the retina again. It may well ricochet about in the eye-ball, thus stimulating the retina many more times.
It is this tapetum lucidum that one sees when lights shine on a dog, cat, raccoon, opossum, bullfrog, shark, etc. With practice, one can recognize the common local species by the color of their eye shine - alligators are red, bullfrogs are blue, etc.
Animals that have a tapetum lucidum must have a mechanism that allows them to be active in daylight. Most animals simply make the pupil very small or slit-like. Sharks have a layer of melanin (dark pigment) that can cover the tapetum lucidum and prevent it from reflecting light.
WHY DO BIRDS OF PREY TILT THEIR HEADS WHEN THEY LOOK TOWARD THE SKY?
An adaptation that gives birds of prey better distance vision is that many of the sensory cells are concentrated in the upper half of the eye. This works well while flying or perching and looking toward the ground. If they want to look toward the sky, they can see better if they tilt their heads and look at the sky “upside down.”
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