| The trichromatic color vision of many primates allows an individual to make distinctions among colors such as green, yellow, orange, and red. This capacity is unique among diurnal placental mammals. Most platyrrhine monkeys are characterized by X-linked polymorphic trichromacy in which heterozygous females are trichromatic, while homozygous females and all males are dichromatic and thus unable to make trichromatic color discriminations. This dissertation examines the mechanisms through which natural selection maintains polymorphic trichromacy in a wild population of the platyrrhine monkey Callicebus brunneus in southeastern Peru. Genetic samples from five socially monogamous C. brunneus groups revealed the presence of three X-linked photopigment gene alleles similar to those found previously in other platyrrhines. There was no strong evidence of disassortative mating or differential short-term reproductive success among groups with dichromatic versus trichromatic adult females. This is contrary to the expectation if trichromats enjoy a fitness advantage over dichromats. The long-distance detection of yellow, orange, and red (YOR) food patches is hypothesized to be easier for trichromats than for dichromats. However, an analysis of C. brunneus foraging behavior revealed that most YOR food patches may have been located by means of spatial memory and non-color cues, thereby minimizing any potential trichromatic advantage in encounter rate. For YOR patches unlikely to have been located by means of spatial memory, no difference in the encounter rates of di- and trichromatic females was detected. However, there was a non-significant trend for lactational status to differentially affect the encounter rates of di- and trichromatic females. These results are explained by a new hypothesis for the maintenance of polymorphic trichromacy in C. brunneus populations through temporally variable selection. This hypothesis implicates spatial memory of food resources and female dispersal behavior, as well as temporal fluctuation in food abundance and predation, in the action of selection on color vision. The application of the hypothesis of temporally variable selection for trichromacy to other primates is discussed. Understanding how selection acts on color vision in extant primate populations is the first step in the development of hypotheses to explain the evolution of trichromatic color vision in our primate ancestors. |
