Effect of polymorphic colour vision for fruit detection in the spider monkey Ateles geoffroyi, and its implications for the maintenance of polymorphic colour vision in platyrrhine monkeys

Most platyrrhine monkeys have an X-linked tri-allelic polymorphism for medium and long wavelength (M/L) sensitive cone photopigments. These pigments' sensitivity maxima (lambda(max)) range from 535 to 562 nm. All animals also have an autosomally coded short-wavelength-sensitive (S) cone pigment. In populations with three M/L alleles there are six different colour vision phenotypes. Heterozygous females have trichromatic colour vision, while males and homozygous females are dichromats. The selective basis for this polymorphism is not understood, but is probably affected by the costs and benefits of trichromatic compared to dichromatic colour vision. For example, it has been suggested that trichromats are better equipped than dichromats to detect fruit against a leaf background. To investigate this possibility, we modeled fruit detection by various colour vision phenotypes present in the frugivorous spider monkey, Ateles geoffroyi. Our study population is thought to have three M/L alleles with cone pigment lambda(max) values close to 535, 550 and 562 nm. The model predicted that all trichromat phenotypes had an advantage over dichromats, and the 535/562 run phenotype was best; however, the model predicted that dichromats could detect all of the fruit species consumed by spider monkeys. We conclude that the heterozygote advantage experienced by females may be the most plausible explanation for the maintenance of this polymorphism in A. geoffroyi. Nevertheless, more studies need to evaluate social foraging behaviour and the performance of different phenotypes of other New World monkeys to determine if this is a global explanation for this phenomena or more specific to A. geofforyi.