Most tests on the different drugs and treatments designed for the human eye are performed using laboratory rats because of one reason; their eyes are very similar to that of a human. Both eyes have two different types of light receptors, namely, cones and rods. Cones are most sensitive to the different colors in the light spectrum while the rods are most capable of identifying the intensity of light. Although the retinas of a human's eye differ from that of a rat, in terms of the mechanisms as to how it identifies color, they are technically just the same.
Retinal Color Vision
The main difference between a human's eye and a rat's eye is the number of cones presents in their retinas -- this also sets the limitation of the kinds of colors that each of them can see. Rats are dichromatic, which means that they have two types of color cones, while humans are trichromatic implying that we have three color cones. Rats can only see the short "blue-IV" light wave and the middle-length "green" light wave; human can see long (red), middle-lengthen (green) and short (blue) light waves. Nonetheless, despite their obvious disadvantage when it comes to seeing a lot of colors, rat vision is still "more advanced" than that of human's because of one thing: they can see ultraviolet light waves and some of the other colors that the human eye can't see.
How Rats Perceive Colors?
Two facts need to be clear when studying rat color vision, that the rat's retina is extra sensitive:
1. To green light waves
2. To blue-ultraviolet rays
Having known these, we can now deduce that rats can indeed "see" colors. But how do rats distinguish between these colors? For some time in the past, people believed that rats are completely colour-blind, it was only in 2001 that scientists were able to find out that the rat's retina can distinguish between the visible light (or that which humans can see) and the ultra-violet rays (which humans can't see).
While it is almost impossible for humans to imagine how a rat's vision looks like, with the aid of computers and simulation software utilities, scientists are able to conclude that rats see from a "neutral" point area that is most sensitive to the blue-green hue; because of their visual limitations, rats will not be able to distinguish between blue and green.
Nonetheless, the limitations of a rat's vision caused by its lack of cones are heavily compensated by its rods. 99% of their retina is composed of rods; this is the reason why rats are extra sensitive to lighting intensity -- something which serves the rats interests best. The unique foraging behaviour of rats requires them to be sensitive to even the slightest change in the light intensity around them. This fact is further supported by rat trainers who can attest that training rats to identify between colors is much more difficult that teaching them to differentiate brightness.
Why Do Rats Need Ultraviolet Vision?
Since humans do not see ultraviolet light, it is not easy for us to point out how the ability to see it can help the rats. Some of the possible reasons might be:
1. Tracing of Urine Marks - Scientists are most inclined to take this possibility more seriously. Because rats are highly territorial animals, it is really of great help if they can "see" the marks that their fellows leave behind to identify their territorial boundaries. However, one thing is problematic about this possibility -- predators also see urine marks, thus, it is still questionable if rats would really intentionally leave marks which their predators can follow.
2. Identifying body parts - It was found out that some of the rat's body parts reflect more ultra-violet light than the rest. Thus, rats would be able to "see" their fellow rats better because of their ultraviolet-vision capability.
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