User:Thochr23
From Comparative Physiology of Vision
Color Vision and Monochromacy
Each individual cone cell expresses just one pigment gene and responds to a specific range of light wavelengths, yet slight overlap does occur between different cone types. Humans have highly developed trichromatic cone receptors that respond to light with spectral peaks in the short wavelength (blue), middle wavelength (green), and long wavelength (yellow) that enables color vision with increased sensitivity and differentiation between colors. Since individual cones do not transmit specific wavelength information and responds to a small spectrum of wavelengths, to distinguish between colors the visual system must compare the differences in a cone cell response with neighboring cone cells of different pigment types. The lack of multiple wavelength sensitive cones renders monochromats unable to perceive differences in color and thus colorblind.
As mentioned previously, the owl monkey (Aotus trigirgatus) is an example of a monochromatic nocturnal mammal. The ratio of rods to cones in the retina is overall about 50:1, while the ratio shifts anywhere from 93:1 in the far periphery down to 14:1 in the central retina. Many species of platyrrhine monkeys have a distinctive cone photopigment polymorphism, which suggests that two separate L and M (long and middle wavelength) genes evolved from a common evolutionary origin through a crossing over mechanism in catarrhine primates sometime after the catarrhine/platyrrhine divergence. Surprisingly, the nocturnal Aotus lacks any evidence of such polymorphism.[1] Evidence suggests that the Aotus have only one class of adaptable cone photopigment that absorbs light between the middle to long wavelengths (between the spectral range of 540-630 nm). Though it can be very difficult to prove that a specific cell type does not exist, researchers have found strong evidence suggesting that Aotus do not have a population of functional short-wavelength sensitive (SWS) cones in the retina. Though DNA hybridization analysis shows the species to contain a pigment gene highly homologous to the human SWS pigment gene, it is still unknown as to why the homologous SWS pigment gene is not translated.[1] The Aotus is also unique among most nocturnal mammals in that (like the aye-aye) it does not have a tapetum lucidum for increased light reflection and absorption by photoreceptor cells in the eye. While monochromacy is commonly found among nocturnal mammals, the aye-aye is a dichromatic mammal, having both M/L- and S-opsin genes still intact. These findings imply that dichromacy may still be advantageous among nocturnal primates for possible reasons such as avoidance and detection of diurnal predators during the day, and the ability to perceive color under moonlit conditions while foraging for food.[1] Though common among Lorisformes, L-cone monochromacy is very rare among terrestrial mammals, implying that the loss of the S-opsin occurred very early in the evolutionary history of this group.[2]
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