Research suggests that the species "Alouatta" does not share the same characteristics of pheromone transduction pathway pseudogenes that humans and Old World monkeys possess and leading howler monkeys to maintain both pheromone communication systems and full trichromatic vision. 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. 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. 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. 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.