Sugar concentration of fruits and their detection via color in the Central American spider monkey (Ateles geoffroyi)

Abstract

Although most arguments explaining the predominance of polymorphic color vision in platyrrhine monkeys are linked to the advantage of trichromacy over dichromacy for foraging for ripe fruits, little information exists on the relationship between nutritional reward and performance in fruit detection with different types of color vision. The principal reward of most fruits is sugar, and thus it seems logical to investigate whether fruit coloration provides a long-distance sensory cue to primates that correlates with sugar content. Here we test the hypothesis that fruit detection performance via trichromatic color vision phenotypes provides better information regarding sugar concentration than dichromatic phenotypes (i.e., is a color vision phenotype with sufficient red-green (RG) differentiation necessary to "reveal" the concentration of major sugars in fruits?). Accordingly, we studied the fruit foraging behavior of Ateles geoffroyi by measuring both the reflectance spectra and the concentrations of major sugars in the consumed fruits. We modeled detection performance with different color phenotypes. Our results provide some support for the hypothesis. The yellow-blue (YB) color signal, which is the only one available to dichromats, was not significantly related to sugar concentration. The RG color vision signal, which is present only in trichromats, was significantly correlated with sugar content, but only when the latter was defined by glucose. There was in fact a consistent negative relationship between fruit detection performance and sucrose concentration, although this was not significant for the 430 nm and 550 nm phenotypes. The regular trichromatic phenotypes (430 nm, 533 nm, and 565 nm) showed higher correlations between fruit performance and glucose concentration than the other two trichromatic phenotypes. Our study documents a trichromatic foraging advantage in terms of fruit quality, and suggests that trichromatic color vision is advantageous over dichromatic color vision for detecting sugar-rich fruits.

A socioecological perspective on primate cognition, past and present

“…Sussman (1991) argues that early primates needed acute powers of visual discrimination and precise coordination to manipulate small seeds and fruits. Trichromatic color vision may help primates distinguish fruit against a background of green leaves (Barton 2000) or may signal the nutritional value of fruit (Riba-Hernández et al 2005), although other possible hypothesized reasons for trichromacy have been suggested (e.g., Dominy 2004). Improved vision would also facilitate the use of long-range landmarks to guide navigation between large, distant fruit trees.…”

Fig Foraging by Dichromatic and Trichromatic Cebus capucinus in a Tropical Dry Forest

“…However, a frugivory hypothesis is a more likely explanation for many Neotropical monkeys, e.g., marmosets and capuchins, because they rarely eat young leaves (Caine and Mundy 2000;Fragaszy et al 2004). Trichromacy could be beneficial for both the detection and selection of desirable fruits because fruits are often conspicuous in color to a trichromat against background leaves (Osorio et al 2004;Smith et al 2003;Sumner and Mollon 2000a), and fruit ripening is often associated with green to reddish (reds, oranges, yellows) color changes that in turn are positively associated with desirable nutrients or fleshiness (Gautier-Hion et al 1985;Riba-Hernandez et al 2005). If this is true, then trichromacy could confer a fitness advantage selecting for heterozygote females and maintaining the opsin polymorphism.…”

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

“…In addition, a statistical analysis of spatial frequencies of natural images suggests that the spatiochromatic properties of the red-green system of human color vision may be optimized for the encoding of any reddish or yellowish objects on a background of foliage at relatively small viewing distances commensurate with a typical grasping distance [24]. Other colorimetric studies incorporating nutritional measurements of primate diets support the trichromat advantage in foraging young leaves or fruits [25]–[29]. Lastly, standardized behavioral experiments demonstrated superior ability of trichromatic to dichromatic primates in detecting reddish objects against greenish background [30]–[32].…”