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Sight – by Ferdie Muller

 

There are two main types of photoreceptors in a mammal’s retina, they are called rods and cones. Rods are responsible for seeing things in low light.  This is also called scotopic vision. They do not act as a go-between colour vision, and have a low spatial perception.

Cones are active at higher light levels and are often referred to as photopic vision and are capable of colour vision and are responsible for high spatial perception. The central fovea ( located in the center of the macula region of the retina ) is populated exclusively by cones. There are 3 types of cones, short-wavelength sensitive cones, middle-wavelength sensitive cones and the long-wavelength sensitive cones.

 

Some animals, usually prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view such as most prey animals. In such animals, the eyes often move independently to increase the field of view. Even without moving their eyes, some birds have a 360-degree field of view.

 

Other animals, usually predatory animals, have their two eyes positioned on the front of their heads. Because of this they have what we call binocular vision. Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes bring forth at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a wider field of view. For example, a human has a horizontal field of view of approximately 200 degrees with two eyes but only 160 degrees with one. Third, it gives binocular abridgment in which the ability to detect faint objects is enhanced. Fourth it can give stereopsis. This is the process in visual perception leading to the sensation of depth from the two slightly different projections or angels of the world onto the retinas of the two eyes. Such binocular vision is usually accompanied by singleness of vision or binocular fusion, in which a single image is seen despite each eye's having its own image of any object.

 

Some predator animals, such as sperm whales, have their two eyes positioned on opposite sides of their heads. Other animals that are not necessarily predators, such as fruit bats and some primates also have forward facing eyes. These are usually animals that need fine depth discrimination, for example, to pick fruit or to find a branch.

 

In animals with forward-facing eyes, the eyes usually move together. When the eyes move laterally, in the same direction, this is called a version. When the eyes move in opposite directions, to an object closer than where the eyes are pointing or farther than where the eyes are pointing, this is called a vergence. When the eyes move in, it is a convergence eye movement; when the eyes move out, it is a divergence eye movement.

 

Some animals use both of the above strategies. A starling, for example, has laterally placed eyes to cover a wide field of view, but can also move them together to point to the front so their fields overlap giving stereopsis. A remarkable example is the chameleon, whose eyes appear to be mounted on turrets, each moving independently of the other, up or down, left or right. Nevertheless, the chameleon can bring both of its eyes to bear on a single object when it is hunting, showing vergence and stereopsis

 

Cats are generally nocturnal in habit. Their large eyes are especially adapted for seeing at night. The retina has a layer of guanine called the tapetum lucidum, which reflects light and causes the eyes to shine at night when illuminated. Cats have good senses of sight and hearing, but their sense of smell is not as developed as that of the canids, a fact suggested by the cat’s short snout.

 

This then brings another question, can animals really see colour.  Is has been proven that domestic dos and cats can see colour, the same with pigeons and horses.  We therefore assume that our cats and dogs relatives in the wild can also then see colour.  The question is if they really use it.  It is more important for a predator in the wild to see depth and movement in order to catch it’s prey and therefore would not worry much about the colour of the animal.  There is no doubt that the colours will help them to see the prey better but whether it has an imperative function remains a question.  For a parrot and primates colour again is vitally important since the selection of their food depend on.  Birds also use it for attracting the opposite sex and this is also the case in various fish such as Trout and Salmon.  It is therefore that normally, animals that can see colour will have overall more cones than rods in their eyes.