<P> Therefore, trichromacy alone does not lead to the loss of pheromone communication but rather a combination of environmental factors . Nonetheless research shows a significant negative correlation between the two traits in the majority of trichromatic species . </P> <P> Trichromacy may also be evolutionarily favorable in offspring health (and therefore increasing fitness) through mate choice . M and L cone pigments maximize sensitivities for discriminating blood oxygen saturation through skin reflectance . Therefore, the formation of trichromatic color vision in certain primate species may have been beneficial in modulating health of others, thus increasing the likelihood for trichromatic color vision to dominate a specie's phenotypes as the fitness of offspring increases with parental health . </P> <P> There are two noteworthy genera within the New World monkeys that exhibit how different environments with different selective pressures can affect the type of vision in a population . For example, the night monkeys (Aotus) have lost their S photopigments and polymorphic M / L opsin gene . Because these anthropoids are and were nocturnal, operating most often in a world where color is less important, selection pressure on color vision relaxed . On the opposite side of the spectrum, diurnal howler monkeys (Alouatta) have reinvented routine trichromacy through a relatively recent gene duplication of the M / L gene . This duplication has allowed trichromacy for both sexes; its X chromosome gained two loci to house both the green allele and the red allele . The recurrence and spread of routine trichromacy in howler monkeys suggests that it provides them with an evolutionary advantage . </P> <P> Howler monkeys are perhaps the most folivorous of the New World monkeys . Fruits make up a relatively small portion of their diet, and the type of leaves they consume (young, nutritive, digestible, often reddish in color), are best detected by a red - green signal . Field work exploring the dietary preferences of howler monkeys suggest that routine trichromacy was environmentally selected for as a benefit to folivore foraging . </P>

Why was it an advantage for early primates to evolve rich color vision