Light is a very interesting phenomenon. Different animals use it in different ways. Colobus monkeys tend to target young reddish not green leaves. Plant species tend to have young leaves that are red and are more nutritious. They are simply taking advantage of their sense of full color vision. This is the ability to distinguish certain shades of colors. Therefore this ability helps them with their survival. Non-primate animals lack full color vision. More specifically apes and Old World monkeys have trichromatic vision. This means they see across the visible spectrum from blue to green to red. Most animals are dichromatic which means they only see blue and yellow. Therefore in the case of the red leaves or the green leaves, dichromatic animals do not have the ability to see the red leaves.
This ability of animals to see light and distinguish color is a physiological response of the eye to light. The human eye usually has only three types of color receptors and each one is stimulated by light of different ranges of the color spectrum. Light of certain wavelengths strikes visual pigments in the retina. The cells in the retina are known as photoreceptors. There are two types: rods and cones. Rods are light sensitive regions and most useful at night under low light but they do not have the ability to distinguish different wavelengths and therefore humans are color blind at night. Cones are better in bright light and they are responsible for color vision. It is in these rods and cones that the visual pigment is found.
This visual pigment is made of the protein known as opsin and a small molecule called a chromophore. In humans this chromophore is a derivative of vitamin A and for this reason humans are encouraged to eat carrots, spinach and other source of this vitamin A. The visual pigments’ ability to respond to light by in the eye is determined by the sequence of the opsin protein and how the chromophore interacts with it. Therefore each visual pigment is tuned to a certain wavelength of light based on the opsin and chromophore.
Humans have three different opsin genes that are sensitive to short, medium and long wavelengths. They are known as SWS, MWS and LWS opsins. These respond to light of 417nm (SWS-blue), 530nm (MWS-green) and 560nm (LWS-red) and therefore humans see light within those ranges. Humans cannot see ultraviolet light or infrared because is out of their range because UV has a wavelength less than 400nm, while infrared light has a wavelength more than 700nm. Humans, chimpanzees and apes all have 3 opsin genes (trichromats) while most mammals have 2 (dichromats). Birds and fish have four or more opsins. This explains why brids can see UV light.
Interestingly, the opsin genes for green and red light are encoded by two genes on the X chromosome. Color blindness refers to the inability to distinguish between one color and another. Due to the fact that human males only have one X chromosome, they are at a greater risk of color blindness. Less than 1 % of females are colorblind while about 8 % males are.
References
The Making of the Fittest: DNA and the ultimate forensic record of evolution. S.B. Carroll (W.W. Norton & Company, Inc.) 91-110.
