Bare skin, blood and the evolution of primate colour vision. Biology Letters, 2, 217-221

California Institute of Biology, Sloan-Swartz Center for Theoretical Neurobiology, MC 139-74, Caltech, Pasadena, CA 91125, USA.
Biology letters (Impact Factor: 3.25). 07/2006; 2(2):217-21. DOI: 10.1098/rsbl.2006.0440
Source: PubMed


We investigate the hypothesis that colour vision in primates was selected for discriminating the spectral modulations on the skin of conspecifics, presumably for the purpose of discriminating emotional states, socio-sexual signals and threat displays. Here we show that, consistent with this hypothesis, there are two dimensions of skin spectral modulations, and trichromats but not dichromats are sensitive to each. Furthermore, the M and L cone maximum sensitivities for routine trichromats are optimized for discriminating variations in blood oxygen saturation, one of the two blood-related dimensions determining skin reflectance. We also show that, consistent with the hypothesis, trichromat primates tend to be bare faced.

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    • "This trait may be potentially informative to conspecifics of both sexes in a sexual context, providing information about both the condition and the competitive ability of the signaler. Indeed, skin color is influenced by blood oxygenation and flow and is thus closely linked to underlying physiology and condition (Changizi et al. 2006; Bradley and Mundy 2008). Furthermore, according to the immunohandicap hypothesis (Folstad and Karter 1992), because the hormone testosterone is an immunosuppressant, only individuals in good condition may be able to exhibit the most intense coloration. "
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    ABSTRACT: The effects of intrasexual and intersexual selection on male trait evolution can be difficult to disentangle, especially based on observational data. Male–male competition can limit an observer's ability to identify the effect of female mate choice independently from sexual coercion. Here, we use an experimental approach to explore whether an ornament, the red facial skin exhibited by male rhe-sus macaques (Macaca mulatta), might be involved in both female mate choice and male–male competition. We used a noninvasive experimental approach based on the looking time paradigm in a free-ranging setting, showing images of differently colored male faces to both adult females (N = 91) and males (N = 77), as well as to juveniles (N = 94) as a control. Results show that both adult females and males looked longer at dark red faces compared with pale pink ones. However, when considering the proportion of subjects that looked longer at dark red faces regardless of preference strength, only females showed a significant dark red bias. In contrast, juveniles did not show any preferences between stimuli, suggesting that the adult bias is not a consequence of the experimental design or related to a general sensory bias for red coloration among all age–sex classes. Collectively, these results support the role the ornament plays in female mate choice in this species and provide the first evidence that this ornament may play a role in male–male competition as well, despite a general lack of observational evidence for the latter effect to date.
    Behavioral Ecology 08/2015; DOI:10.1093/beheco/arv117 · 3.18 Impact Factor
    • "The adaptive significance of such bright colors is suggested to relate to the emergence of trichromatic vision in primates, where trichromacy was selected to detect these biologically driven, socially significant color cues [Changizi et al., 2006]. "
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    ABSTRACT: Human eye morphology is considered unique among the primates in that humans possess larger width/height ratios (WHR), expose a greater amount of visible sclera (SSI; width of exposed eyeball/width of visible iris), and critically, have a white sclera due to a lack of pigmentation. White sclera in humans amplifies gaze direction, whereas the all-dark eyes of apes are hypothesized to conceal gaze from others. This study examines WHR and SSI in humans (N = 13) and gorillas (N = 85) engaged in direct and averted gazes and introduces a qualitative assessment of sclera color to evaluate variations in sclera pigmentation. The results confirm previous findings that humans possess a larger WHR than gorillas but indicate that humans and gorillas display similar amounts of visible sclera. Additionally, 72% (N = 124) of gorilla eyes in this sample deviated from the assumed all-dark eye condition. This questions whether gaze camouflage is the primary function of darkened sclera in non-human primates or whether other functional roles can be ascribed to the sclera, light or dark. We argue that white sclera evolved to amplify direct gazes in humans, which would have played a significant role in the development of ostensive communication, which is communication that both shows something and shows the intention to show something. We conclude that the horizontal elongation of the human eye, rather than sclera color, more reliably distinguishes human from great ape eyes, represented here by gorillas. Am. J. Primatol. © 2015 Wiley Periodicals, Inc. © 2015 Wiley Periodicals, Inc.
    American Journal of Primatology 04/2015; 77(8). DOI:10.1002/ajp.22411 · 2.44 Impact Factor
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    • "Among primates, colorful skin patches are known to act as signals of competitive ability, quality, reproductive status, or fecundity [see reviews in Bradley & Mundy, 2008; Dixson, 2012, Dubuc et al., 2014; Gerald, 2003]. Skin color may be a particularly sensitive and more honest signal of shortterm health and quality because skin is more immediately influenced by physiological changes in hormones, hydration, or nutrition [Caro, 2005], as well as mood [Changizi et al., 2006]. Male mandrills have vivid skin color that is status-dependent [Setchell & Dixson, 2001], and reddened males are preferred by females [Setchell, 2005]. "
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    ABSTRACT: Old World Monkeys (Cercopithecoidea) are unusual among primates for the high percentage of species exhibiting circumperineal coloration, as well as the large percentage of highly terrestrial species. Kingdon [1974, 1980] suggested that circumperineal skin coloration is functionally related to terrestriality but this hypothesis has not been tested. From the literature we collected data on habitat use (terrestrial/arboreal) and circumperineal coloration (present/absent) for 78 species. Indeed, among the 78 species surveyed here, 75% of them fall into either the category of colored circumperineals with terrestrial lifestyle, or of uncolored circumperineals with arboreal lifestyle (Χ2(1)=19.550, P<0.001). However, conventional statistical procedures assume all taxa are equally related—which is not usually the case in multispecies analyses—leading to higher rates of both Type I and II statistical errors. We performed Bayesian trait co-evolution analyses that show that models of dependent trait evolution are not significantly better than models assuming independent evolution of the two traits (log-likelihood ratio test P=0.396, BayesFactor=1). Bayesian nodal reconstructions of the cercopithecoid phylogeny indicate that relatively few trait transitions are needed to account for the distributions of the two traits. Further, chi-squared distributional tests show that sub-family affiliation (i.e. Cercopithecinae, Colobinae) is an accurate predictor of trait status. The discordance of the analyses may represent the results of a few different evolutionary scenarios, but ultimately circumperineal coloration seems weakly linked to terrestrial ecology.
    American Journal of Primatology 01/2015; 77(5). DOI:10.1002/ajp.22374 · 2.44 Impact Factor
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