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.
Highlights
The selective pressures that led to the evolution of the unique form of trichromatic colour vision in primates from ancestral mammalian dichromacy have been debated for decades [1,2,3,4]
Our results indicate that the separation of cones is an important factor for discriminating face colour variation in the context of detecting reproductive status of female rhesus macaques
Our results suggest that both of these phenotypes enable the extraction of meaningful skin colour variation, it is known that other various factors, including olfactory, haptic, behavioural and auditory signals, may play important roles in sexual behaviour in female platyrrhines [47,48,49]
Summary
The selective pressures that led to the evolution of the unique form of trichromatic colour vision in primates from ancestral mammalian dichromacy have been debated for decades [1,2,3,4]. One puzzling feature of primate colour vision is that peak sensitivities of photopigments to different wavelengths are not evenly distributed across the visual spectrum, unlike the more regular distribution that is known to occur in many other trichromatic (and tetrachromatic) animals, including bees, hawkmoths, reptiles and passerine birds [5,6,7,8,9]. Various explanations of the narrow tuning window in primates have been proposed, including non-adaptive ones: (i) the shared origin of primate M and L cones and phylogenetic constraint [12], and/or (ii) a minimizing of the cost to luminance vision via chromatic aberration because M and L signals are pooled to generate a luminance signal [5,13,14,15]
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