Resource Ramblings 2007 - 11
Vision at Night
Ever wonder why some animals have excellent night vision, while we poor humans stumble over the most obvious items? There are basically three reasons.
First, nocturnal animals (animals active at night) tend to have proportionally bigger eyes than people do. With this, they also tend to have pupils that will open wider in low light conditions. Thus, they can gather more light.
Second, after the light passes through the front of the eye (pupil), it is focused by the lens onto the retina. The retina lies at the back of the eye. It has a very complex structure and is connected to the brain by the optic nerve.
The retina contains two different types of light receptor cells, commonly referred to as rods and cones. Most diurnal animals (those active during the day) have more specialized cones that work in bright light and allow the recognition of detail. This is how a hawk can see well camouflaged prey far below it on the ground. Nocturnal species have specialized rods that have adapted to recognize motion and minor detail. Some species have become so specialized that they have few or no cones at all, as exemplified by some bats, nocturnal snakes and lizards.
The third reason is that many nocturnal species have developed a layer in the eye that actually amplifies the amount of light reaching the retina. This is called the tapetum lucidum and acts to collect and reflect light back to re-stimulate the rods in the retina, allowing animals to see much better in low light conditions. This layer of cells is present in nearly all carnivores and many other animals. Light that is not absorbed through this reflection passes out of the eye. Thus, spotlights or other light sources will be reflected from the eyes of animals in different colors according to the color of the tapetum. In fact, some species of wildlife can be identified by the color of the reflected light from their eyes (i.e., elk and deer are yellow, bison and some rabbits are red, and black-footed ferrets and cats are green). Cameras have been specially developed that reduce the amount of reflection, especially in humans, calling this “red-eye reduction”.
Because the rods are adapted to detecting motion and not detail, nocturnal predators are adept at detecting movement that crosses their field of vision. For example a predator such as a ferret is able to focus on a short range of vision. Anything that passes across that range is highly detectable, while something moving towards or away from the ferret is not as noticeable. This explains why a spotlight can captivate a ferret allowing the researcher to approach to a relatively short distance. As long as the researcher moves in a straight line toward the ferret, the ferret detects no observable threat until a short distance is reached.
One problem of having light sensitive eyes is that they must be adequately protected intensive amounts of light during the day. Some animals have developed a retractable eye flap, while others have retained light management through the pupil. Because many predators are nocturnal, their eyes have adapted the ability to restrict the amount of light entering the eye by constricting their pupil. For example, some feline species (cats) possess pupils that can be narrowed to a thin slit, thus restricting the light entering the eye and allowing them to see in bright light conditions. Large cats (lions) hunt during the day and have retained circular pupils that constrict into small circles (much like the human eye). Hence, their night vision is not as acute as their nocturnal cousins. Most circular pupils are not very efficient at closing rapidly because of how the muscles have to bunch on contraction. As a result, round eyes are not well adapted to night vision. Pupils with slits are more efficient, as they can close like a sliding door. Many nocturnal species have slit pupils and these can be vertical, horizontal, or diagonal.
Some snakes, such as pit vipers, merge sight and the ability to detect heat to aid in hunting at night. Pit vipers have holes, between the eye and the nostril, which are able to detect heat given off from another body. The pits work together in stereoscope, much like the eyes. Heat and visual data are sent to the snake’s brain through the optic nerve, where the two sets of data are merged into a single image.
Many bats, such as fruit bats, have folded retinas, which provide additional room for rods. Fruit bats rely primarily on sight for navigation, unlike most other bats, which rely on echolocation.
Some species of owls have such acute night vision that they can detect movement more than 100 meters away on moonless nights. Owls have an altered eye shape that allows for a broadened retinal surface. They have an iris that can dilate almost completely. They also have a special lens that allows for visual enlargement like a telescope.
(Information from: http://www.pbs.org/wgbh/nova and
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