Modeling dichromatic and trichromatic sensitivity to the color properties of fruits eaten by squirrel monkeys (Saimiri sciureus)

Most platyrrhines have a visual polymorphism that is characterized by the presence of multiple alleles of the M/LWS gene on the X chromosome. This polymorphism is probably maintained by selection. There are two possible mechanisms by which this can be explained: First, heterozygous females may have perceptual advantages over dichromats, such that trichromacy would be favored via the existence of different visual pigments. This is known as selection by heterosis. Second, dichromacy may be advantageous in some situations, with polymorphism being maintained by frequency-dependent selection. In this study the reflectance spectra of fruits and flowers eaten by a troop of squirrel monkeys (Saimiri sciureus) in Eastern Amazon were measured using a spectrophotometer. S. sciureus have an SWS cone with a spectral tuning of approximately 430 nm, and three M/LWS alleles with spectral tunings of 535 nm, 550 nm, and 562 nm. Based on the spectral tunings of the different phenotypes and the spectral data obtained from the food items, the responses of the different visual systems to the measured objects were modeled and then compared. The model predicted that trichromatic phenotypes would have an advantage over dichromats in detecting fruits and flowers from background foliage, which suggests that heterosis is the mechanism for maintaining polymorphism in S. sclureus. On the other hand, a large proportion of fruits could not be detected by any of the phenotypes. Additional studies are necessary to determine whether other important aspects of the primates' visual world, such as prey, predator, and conspecific detection, favor tri- or dichromacy.