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

Abstract

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. sciureus. 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.

Color Vision Variation and Foraging Behavior in Wild Neotropical Titi Monkeys (Callicebus brunneus): Possible Mediating Roles for Spatial Memory and Reproductive Status

“…These results accord well with reflectance and modeling studies investigating the detectability of naturally occurring food items to dichromatic and trichromatic primates (de Araujo et al 2006;Hiramatsu et al 2008;Lucas et al 2003;Regan et al 2001;Riba-Hernandez et al 2004;Snodderly 1979;Stoner et al 2005;Sumner and Mollon 2000a), most of which report that trichromats may be better able to detect many, if not most, ingested fruits and leaves, or exploited fruit and leaf species, against a mature leaf background. Importantly, however, our results suggest that such observations may lead to overestimates of the number of instances in which trichromacy would provide an advantage over dichromacy to a foraging primate.…”

“…Several modeling studies have suggested that the trichromatic vision of both catarrhines and heterozygous platyrrhines is superior to that of dichromats for detecting both fruits (de Araujo et al 2006;Osorio et al 2004;Regan et al 2001;Riba-Hernandez et al 2004;Stoner et al 2005;Sumner and Mollon 2000a) and young leaves Lucas et al 2003;Sumner and Mollon 2000a) against a background of mature green leaves. In accord with these predictions, in a study of captive marmosets (Callithrix), designed to mimic natural detection tasks at maximum distances of 6 m, Caine and Mundy (2000) found a modest trichromatic advantage over dichromats in distinguishing orange targets against a green/brown background.…”

“…Relying even partially on human color classifications to represent animal vision can be inappropriate, especially when an animal's visual perception is known to be very different from that of humans (Bennett et al 1994). However, with few exceptions (Regan et al 2001), the set of colored food items that a normal human trichromat can distinguish from background leaves should match or encompass the set distinguishable by any polymorphic platyrrhine phenotype (de Araujo et al 2006;Osorio et al 2004;Riba-Hernandez et al 2004;Stoner et al 2005). We classified food patches containing brightly colored immature fruits, e.g., Stylogyne cauliflora (Myrsinaceae: Bunce 2009, Fig.…”

Detection of fruit by theCerrado's marmoset (Callithrix penicillata): modeling color signals for different background scenarios and ambient light intensities

“…One somewhat interesting leaf adaptation found in Cerrado species that certainly affects background spectra is the presence of hair, especially at lower leaf surfaces, that provide a pale aspect to certain leaves. Similarly from what is usually shown for rainforest species (Sumner and Mollon, 2000b;Regan et al, 2001;Osorio et al, 2004;De Araújo et al, 2006), the upper leaf surfaces from Cerrado trees appear darker and greener than lower leaf surfaces. According to our results, however, subtle differences in background spectra only seem to minimally interfere in target detection by dichromats and trichromats.…”

“…Recent investigations (Sumner and Mollon, 2000a;Regan et al, 2001;Osorio et al, 2004;Riba-Hernandez et al, 2004;Stoner et al, 2005;De Araújo et al, 2006) seemed to succeed in predicting how different animal phenotypes see their fruit diets against a background of leaves. However, in a natural condition, targets are perceived against a variety of leaf backgrounds and light conditions , which has not yet been extensively considered.…”