|Our ability to see greatly influences our perception
of the world around us. Do all mammals see in the same way that we do? Is
sight as important to their way of life? All mammals depend on light to see.
Objects in the world reflect light at different levels in the form of electromagnetic
waves. Our eyes pick up the waves and, with the help of the brain, convert
them in to visual images.
How the Eye Works: The eyes of all mammals have the same basic design, so they all work the same way. At the front of the eye is the transparent cornea, which acts as a window allowing light to enter the eye. Behind the cornea is a colored area known as the iris, which can change its diameter to vary the intensity if light entering the eye. Behind the iris is the lens, which directs the light rays onto a light sensitive surface at the back of the eye, known as the retina. The lens is surrounded by muscles which expand and contract, allowing the eye to focus on an object.
The retina is made up of closely packed, light sensitive cells. They are connected to the optic nerve, which is in turn connected to the brain. These cells receive an upside down image, and they pass it on to the brain through the optic nerve. The brain interprets, or decodes, this image, turns it right side up, and transforms it into a three-dimensional picture. Thus the brain can be considered as much an organ of sight as the parts of the eye are.
Nighttime Vision: If vision depends on light reaching the retina, how can some animals see in the dark? The design of the eyes belonging to nocturnal prowlers is almost exactly the same as in other mammals. The most obvious difference, though, is that the eyes of mammals with excellent nighttime vision are larger and more bulbous, and thus more sensitive to light. Nocturnal mammals also have a reflective layer behind the retina, which bounces light back through the eye, giving the light sensitive cells –called – in the retina another chance to absorb the light waves. This effect is visible when car headlights shine on a cats eyes; they shine back. In addition, the retinas of these animals generally contain more rods, increasing their sensitivity in dim light.
Seeing in Color: How do our eyes interpret color, and do all mammals see different colors to the same extent? The electromagnetic waves sent out by objects vary in length: the longer ones are interpreted by our eyes as reds and oranges, and the shorter ones as greens and blues. The light sensitive cells in the retina are divided into two different types – rods and cones. The rods cannot distinguish colors, but they are sensitive and pick up even very low levels of light. The cones, however, interpret and relay information about the colors of the wavelengths they receive. Mammals with the best color vision have three different types of cones.
The brain transforms the messages received by the cones into the multicolored images of the objects we see in front of us. The cones receive color wavelengths only in good light conditions. This may be why it has long been thought that nocturnal animals do not see in color. However, it is now generally accepted that all mammals have some degree of color vision.
What Different Mammals See: All mammals have the same basic eye construction. But it is the position of the eyes on the head that most dramatically affects the way in which different mammals see. Successful predators, such as cats, must be able to focus their vision exactly in order to pinpoint prey. They generally have eyes that face forward. Although their peripheral vision is limited, their binocular vision (the ability to focus both eyes together on an object) and depth perception allow them to gauge the exact distance between themselves and their prey.
Animals that are preyed upon, however, such as rabbits, mice, and deer, have eyes that are located on either side of their head, which allows them a much wider range of vision. This positioning diminishes their binocular vision, but their panoramic field of vision is increased because their eyes work independently to survey their surroundings for predators. A rabbit, for example, has an almost 360 degree field of vision, which means it can spot danger from every direction.
By contrast, a human being looking straight ahead can see about 200 degrees around without moving his or her head. A cats field of vision is slightly narrower – about 185 degrees.
The Predator: A cat has the forward looking eyes of a hunter, with good binocular vision – that is, both eyes focus accurately on prey.
The Prey: As a hunted animal, the mouse needs good all-round vision. It therefore has eyes situated on either side of its head. But, as a result, its binocular vision is very limited.
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