Consequences of color vision variation on performance and fitness in capuchin monkeys

The origin and maintenance of variation in natural populations are central to the study of evolution. When alternative alleles have obvious effects on phenotype and are common in a population, the maintenance of these alleles requires some form of balancing selection. Understanding how selection maintains multiple phenotypes in a population requires integration of genetic analyses of phenotypic differences with field studies on the performance consequences of these differences within an ecological context. The color vision polymorphism characterizing most diurnal platyrrhine and strepsirrhine primates provides an excellent opportunity to investigate the maintenance of variation in natural populations. The polymorphism leads to multiple forms of color perception coexisting in a population. The mechanisms and behavioral consequences of this polymorphism are still hotly debated. The two main hypotheses for the maintenance are heterosis and some form of negative-frequency dependent selection. My dissertation evaluated the performance and fitness consequences of color vision variation within an ecological context in order to elucidate the mechanism maintaining variation at this locus. In chapter one, I provide an introduction to the subject, as well as a synopsis of the results from my dissertation chapters. In chapters two and three, I examine the performance differences between dichromatic and trichromatic individuals in a highly controlled captive setting using ecologically-relevant detection tasks. My results demonstrate superior performance by trichromatic individuals, especially in low light conditions and amid complex visual tasks. In chapter four, I detail the success of a novel Taqman® probe used to determine opsin genotypes of capuchin monkeys (Sapajus nigritus); use of this probe enabled establishment of genotypes of wild capuchin monkeys sampled noninvasively. In chapter five, I examine the performance differences of dichromatic and trichromatic capuchin monkeys from a wild population when foraging for invertebrates. Trichromatic individuals demonstrated higher success rates than dichromatic individuals for total invertebrate captures and for cryptic invertebrates under all light conditions. There were no differences for non-cryptic prey. In chapter six, I examine fitness consequences of color vision variation in a wild population of capuchin monkeys. Trichromatic females weighed more and had higher birth rates than dichromatic females. Collectively, my research demonstrates clear and consistent advantages to trichromatic females from three distinct perspectives. My results support the heterosis hypothesis for the maintenance of the polymorphic visual system characteristic of New World primates