Importance of Achromatic Contrast in Short-Range Fruit Foraging of Primates

Abstract

Trichromatic primates have a ‘red-green’ chromatic channel in addition to luminance and ‘blue-yellow’ channels. It has been argued that the red-green channel evolved in primates as an adaptation for detecting reddish or yellowish objects, such as ripe fruits, against a background of foliage. However, foraging advantages to trichromatic primates remain unverified by behavioral observation of primates in their natural habitats. New World monkeys (platyrrhines) are an excellent model for this evaluation because of the highly polymorphic nature of their color vision due to allelic variation of the L-M opsin gene on the X chromosome. In this study we carried out field observations of a group of wild, frugivorous black-handed spider monkeys (Ateles geoffroyi frontatus, Gray 1842, Platyrrhini), consisting of both dichromats (n = 12) and trichromats (n = 9) in Santa Rosa National Park, Costa Rica. We determined the color vision types of individuals in this group by genotyping their L-M opsin and measured foraging efficiency of each individual for fruits located at a grasping distance. Contrary to the predicted advantage for trichromats, there was no significant difference between dichromats and trichromats in foraging efficiency and we found that the luminance contrast was the main determinant of the variation of foraging efficiency among red-green, blue-yellow and luminance contrasts. Our results suggest that luminance contrast can serve as an important cue in short-range foraging attempts despite other sensory cues that could be available. Additionally, the advantage of red-green color vision in primates may not be as salient as previously thought and needs to be evaluated in further field observations.

Evolution of colour vision in mammals

“…Supporting this conclusion is a recent examination of the efficiency of fruit gathering in polymorphic spider monkeys (Ateles) that also detected no differences between dichromatic and trichromatic animals (Hiramatsu et al 2008). This experiment focused specifically on foraging that is conducted over very short range (within an arms length) and the physical feature of the target fruits that best predicted foraging efficiency was not colour, but rather luminance contrast, a cue that should be equally available to trichromatic and dichromatic viewers (Hiramatsu et al 2008). It may be noted that short-range foraging such as this also allows for the exploitation of various non-visual cues (e.g.…”

“…For instance, several sets of observations made on monkeys feeding in natural circumstances found no causal relationships between colour vision status and efficiency in foraging (Dominy et al 2003;Smith et al 2003;Vogel et al 2007). Supporting this conclusion is a recent examination of the efficiency of fruit gathering in polymorphic spider monkeys (Ateles) that also detected no differences between dichromatic and trichromatic animals (Hiramatsu et al 2008). This experiment focused specifically on foraging that is conducted over very short range (within an arms length) and the physical feature of the target fruits that best predicted foraging efficiency was not colour, but rather luminance contrast, a cue that should be equally available to trichromatic and dichromatic viewers (Hiramatsu et al 2008).…”

Reliabilities of identifying positive selection by the branch-site and the site-prediction methods

“…Therefore, even if some gene experiences a functional change, it may not necessarily affect the fitness of the individual. It is interesting to note that even the selective advantage of trichromatic color vision over the dichromatic vision has been disputed in New World monkeys (39,40). It is important not to be overenthusiastic about statistical signatures of positive selection without biological confirmation.…”

A foraging advantage for dichromatic marmosets ( Callithrix geoffroyi ) at low light intensity

“…The polymorphism is maintained by balancing selection, but the mechanisms acting are the subject of considerable debate (e.g. Mollon et al 1984;Surridge & Mundy 2002;Surridge et al 2003;Hiramatsu et al 2008). Trichromats have improved discrimination in the red -green part of the spectrum and, although the spectral separation of the three pigments in callitrichids (i.e.…”