Recent studies have uncovered a unique heterozygous advantage in female new world monkeys for color vision. Microspectrophotometric (MSP) measurements of the monkey’s photoreceptors have uncovered similar short-wave photoreceptors yet variances in the medium- to long-wave spectral region of certain new world monkeys (Tovée et al., 1992). The medium- to long-wave region was measured to find three potential max wavelengths, 537, 550, and 565nm. Each individual’s photoreceptors varied; some may access one while others may access two of these pigments, creating dichromatic and trichromatic color vision (Mollen et al., 1984). Although the exact genetic mechanism is debated, it is widely accepted that the X-chromosome holds the specific loci. This is hugely significant as it limits male new world monkeys to mono- or dichromatic color vision, whereas a female heterozygous for the medium/long-wave gene represents the possibility of trichromatic color vision (Kainz et al., 1998). Given the two X-chromosomes of the female, there is potential for both the medium and long wavelength pigments. However, the observation of trichromatic color vision in male Howler monkeys has continued the debate as to how the genetic mechanisms operate.
Early genetic experimentation reasoned the genetic code varied in the long-wave region. The researchers proposed that the long-wave region of the cone pigment was determined by a single gene locus. This locus was to be on the X-chromosome being influenced by three alleles, whereas the short-wave cone pigment was assumed autosomal (Mollen et al., 1984). The discovery of trichromatic male Howler monkeys signifies that a more complex model is necessary. With only one X-chromosome, the male Howler must have complete genes for medium/long wavelengths as well as the transcriptional mechanisms to separate cone populations in order to express trichromatic color vision. It is thought that the promoter regions of the medium/long pigment genes are responsible for separating the two wavelengths into different cone receptors, enabling trichromatic vision (Kainz et al., 1998). Such a discovery is paramount in developing our comprehension of genetic evolutionary processes. Further study of the Howler monkey indicates that a duplication of the opsin gene may be critical to their trichromatic capacity. Further, the duplication of the opsin gene developed independently and more recent than Old World monkey species (Surridge et al., 2003). Perhaps indicating genetic evolution on a scale more rapid than previously thought. To understand the environmental motives for such evolution, behavioral analyses have also conducted.
One of the first approaches to behavioral analyses employed a force-choice discrimination device. In such a device, translucent, touch-sensitive panels illuminated with a desired wavelength. Upon recognition of the illuminated panel, the test subject would make tactile contact with the panel to receive a delightful banana treat (Mollen et al., 1984). This device allowed the researchers to perform tests on the primates’ sensitivity to light and wavelength discrimination in addition to a Rayleigh match. Analysis of the results uncovered substantial individual differences in sensitivity to light between 540-640nm (Mollen et al., 1984). Interestingly, the wavelength discrimination seemed to support the MSP data. Of the eight monkeys tested, only three were able to discriminate wavelengths greater than 540nm. Further, of those three subjects, each individual capacity varied in wavelengths beyond the 540nm threshold (Mollen et al., 1984). Results of the Rayleigh test indicated that five monkeys could make no distinction between red/green/yellow whereas the other five were able to distinguish red/green from yellow, rendering them dichromates (Mollen et al., 1984). Thus, both behavioral and genetic approaches have proved an array of visual spectrum in New World monkeys. The curiosity is now in how and why color vision is developing in these monkeys.
Ecological pressure, well covered by topics such as natural selection, molds the individual and their characters according the available niche space. First and foremost, an advantage in foraging for nutrition could be attained with the use of color vision. This traditional belief holds that a trichromatic monkey will be better equipped to forage for food amongst the forest floor and distinguish ripe vs rotten, i.e. lack of nutrition, in their food choices (Surridge et al., 2003). More recent studies have approached the selection from a sexual bias. In this manner, trichromatic vision is used as in the process of sexual selection. Researchers focused on the common red tone of fur and skin to determine the value of such traits from a trichromatic visual perspective (Fernandez & Morris, 2007). The authors of the study concluded that the red tone of fur and skin developed in response to color vision as the use of such vision favored the sexual selection of those with red hue (Fernandez & Morris, 2007). Thus, intraspecific competition for mates is identified as one of the ecological pressures guiding evolution. In addition to the potentially increased nutrient uptake resulting from an increase in foraging efficiency, a trichromatic visual spectrum seems to be gaining popularity amongst new world monkeys.
Again, similar to the debated genetic mechanisms, the selection pressures are altogether uncertain. This uncertainty is complicated by the wide variety of primate species and habitat span Perhaps trichromacy is advantageous to find the proper fruit, foliage, or mate dependent upon which species is under examination. At any rate, the presence and maintenance of differing phenotypes has been suggested as an example of balancing selection (Surridge et al., 2003). In this case, a variety of alleles are maintained in relative balance with one another. For example, dichromates might have the advantage in detecting predators and prey in different light levels, whereas the trichromatic will excel in a slightly different niche based in visual foraging.
Polymorphism in New World monkeys is a puzzle representing a unique chance to observe the internal mechanisms of evolution at the capacity of contemporary science. Modern techniques enable analyses of the genetic code itself, affording the chance to pinpoint the specific genes responsible for the expression of new evolutionary traits. Further, sophisticated apparatus are tangible concepts viable for very controlled environmental conditions. Manipulating these conditions enables a look at the behavioral variance amongst individuals dependent upon gene expression. Finally, in attempting to sum these factors, a more cohesive comprehension of evolutionary processes and the pressures and mechanisms driving such processes may be discovered.
Kainz, P. M., Neitz, J., & Neitz, M. (1998). Recent evolution of uniform trichromacy in a New World monkey. Vision Research, 38(21), 3315-3320. doi:10.1016/s0042-6989(98)00078-9
Fernandez, A., & Morris, M. (2007). Sexual selection and trichromatic color vision in primates: Statistical support for the preexisting‐bias hypothesis. The American Naturalist, 170(1), 10-20. doi:10.1086/518566
Mollon, J. D., Bowmaker, J. K., & Jacobs, G. H. (1984). Variations of colour vision in a new world primate can be explained by polymorphism of retinal photopigments. Proceedings of the Royal Society B: Biological Sciences, 222(1228), 373-399. doi:10.1098/rspb.1984.0071
Surridge, A. K., Osorio, D., & Mundy, N. I. (2003). Evolution and selection of trichromatic vision in primates. Trends in Ecology & Evolution, 18(4), 198-205. doi:10.1016/s0169-5347(03)00012-0
Tovée, M., Bowmaker, J., & Mollon, J. (1992). The relationship between cone pigments and behavioural sensitivity in a new world monkey (Callithrix jacchus jacchus). Vision Research, 32(5), 867-878. doi:10.1016/0042-6989(92)90029-i