Experimental evidence that primate trichromacy is well suited for detecting primate social colour signals

Abstract

Primate trichromatic colour vision has been hypothesized to be well tuned for detecting variation in facial coloration, which could be due to selection on either signal wavelengths or the sensitivities of the photoreceptors themselves. We provide one of the first empirical tests of this idea by asking whether, when compared with other visual systems, the information obtained through primate trichromatic vision confers an improved ability to detect the changes in facial colour that female macaque monkeys exhibit when they are proceptive. We presented pairs of digital images of faces of the same monkey to human observers and asked them to select the proceptive face. We tested images that simulated what would be seen by common catarrhine trichromatic vision, two additional trichromatic conditions and three dichromatic conditions. Performance under conditions of common catarrhine trichromacy, and trichromacy with narrowly separated LM cone pigments (common in female platyrrhines), was better than for evenly spaced trichromacy or for any of the dichromatic conditions. These results suggest that primate trichromatic colour vision confers excellent ability to detect meaningful variation in primate face colour. This is consistent with the hypothesis that social information detection has acted on either primate signal spectral reflectance or photoreceptor spectral tuning, or both.

Keywords: colour vision; face colour variation; primate; reproductive state; social signal; trichromacy.

Figure 1.

An example of face pair stimuli for common (unmanipulated) trichromacy and simulated colour visions. Numbers below each stimulus indicate the combinations of cone peak wavelength sensitivities (λ_max) used to create the test images. Schematic drawings of cone sensitivities are depicted for each condition.

Figure 2.

Transition of percentage accuracy and RT in each condition. The change of mean accuracy across participants through sessions for (a) training pairs and for (b) test pairs. The change of mean RT across participants through sessions for (c) training pairs and for (d) test pairs. Note that test pairs were not included in the training session. Error bars indicate 95% CI of the means, truncated at 100% for the accuracy variable. The sample size included in each plot is 10 individuals. Circle, common trichromacy; diamond, LM-half trichromacy; square, LMS-even trichromacy; triangle, protanopia; inverted triangle, deuteranopia; arrowhead, tritanopia. (Online version in colour.)