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Bare skin, blood and the evolution of primate colour vision

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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.
Biol. Lett. (2006) 2, 217–221
Published online 7 February 2006
Bare skin, blood and the
evolution of primate
colour vision
Mark A. Changizi
*, Qiong Zhang
and Shinsuke Shimojo
Sloan-Swartz Center for Theoretical Neurobiology, California Institute
of Biology, MC 139-74, Caltech, Pasadena CA 91125, USA
Division of Biology, Computation and Neural Systems, MC 139-74,
Caltech, Pasadena CA 91125, USA
JST ERATO Shimojo Implicit Brain Function Project, California
Institute of Technology, MC 139-75, Pasadena CA 91125, USA
*Author for correspondence (
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 discrimi-
nating variations in blood oxygen saturation,
one of the two blood-related dimensions deter-
mining skin reflectance. We also show that,
consistent with the hypothesis, trichromat pri-
mates tend to be bare faced.
Keywords: colour; primate; evolution; skin; blood;
The primate face and rump undergo colour modu-
lations (such as blushing or blanching on the human
face, or socio-sexual signalling on the chimpanzee
rump), some which may be selected for signalling and
some which may be an inevitable consequence of
underlying physiological modulations. Because for
highly social animals like most primates, one of the
most important kinds of object to be competent at
perceiving and discriminating is other members of
one’s own species, we investigated the hypothesis that
primate colour vision has been selected for discrimi-
nating the spectral modulations on the skin of
conspecifics, these modulations providing useful
information about the current state or mood of
another conspecific.
A first prediction of our hypothesis is that the
space of skin colour modulations should be ade-
quately spanned by the two chromatic mechanisms
available to trichromats, but not to the single chro-
matic mechanism available to dichromats. This is
indeed the case, as we now explain. The reflectance
spectra for human skin possess a characteristic signa-
ture (figure 1a), including a ‘W’ feature near 550 nm
(see electronic supplementary material, figure 2).
This feature is due to the absorption spectrum of
oxygenated haemoglobin in the blood (figure 1b), and
is found in spectra of primate skin as well (figure 1a).
What is important for our hypothesis is not the
baseline reflectance spectrum of skin, which will differ
across human phenotypes ( Jablonski & Chaplin
2000) and across primates (Sumner & Mollon 2003),
but the manner in which the skin reflectance is
modulated in the short term, something that is
universal across primates. There are two dimensions
of skin reflectance modulation, (i) haemoglobin oxy-
gen saturation and (ii) haemoglobin skin concen-
tration (Zonios et al. 2001). Changes in these two
parameters lead to predictable changes in the reflec-
tance of skin (figure 1c). Greater oxygen saturation
leads to a more-defined ‘W’ feature with a larger
difference between its troughs and centre peak, raising
the L cone activation (which is at the peak of the ‘W’)
relative to the M cone (which is near the first trough
of the ‘W’), leading to redder; lower oxygen saturation
does the opposite, leading to greener (figure 1d). For
example, skin with veins underlying it, possesses a
high concentration of deoxygenated blood and is
greenish blue (Kienle et al. 1996), and skin with
blood accumulation after administering a tourniquet
possesses a high concentration of relatively oxyge-
nated blood and is reddish-blue (i.e. purplish); these
two skin conditions differ primarily in regards to
oxygen saturation (because they both have high-
haemoglobin concentration), and their colour differ-
ence is primarily a red–green one (figure 1e). Greater
haemoglobin concentration in the skin, on the other
hand, leads to an overall lowering of the entire ‘W’
feature in the filtered spectrum (but not much change
in the difference between the troughs and the peak of
the ‘W’), lowering the M/L activation relative to the S
activation, which leads to bluer; lower haemoglobin
concentration does the opposite, leading to yellower
(figure 1d). For example, bloodless skin is relatively
yellow, whereas skin with greater amounts of blood is
bluer, e.g. green-blue for veins and reddish-blue
after application of a tourniquet (figure 1e). Dichro-
mat primates have only one chromatic dimension, not
two, and will be able to capture only one of the two
blood-related dimensions of skin colour variation,
namely the haemoglobin concentration dimension.
If trichromacy was selected for discerning the
colour modulations in skin, then trichromats should
not merely be sensitive to the oxygen saturation
dimension. Rather, the hypothesis predicts that the M
and L maximum wavelength sensitivities should be
near optimal for discriminating this dimension. To
see that this is the case, note first that varying the
oxygen saturation of haemoglobin in the skin modu-
lates the ‘W’ feature, turning it from a ‘W’ when
oxygen saturation is high to a ‘U’ when oxygen
saturation is low (figure 1band c). Supposing that the
M and L sensitivities must jointly serve the ancestral
role of the single M/L cone in dichromatic primates
(which has maximal sensitivity at 543 nm), it follows
that the maximal sensitivities for M, L and their sum
must be near 543 nm. With this constraint, M and L
wavelength sensitivities would be optimized for
detecting oxygenation variation if they were at
approximately 540 and 560 nm, respectively
(figure 2a). This prediction fairs well among the
The electronic supplementary material is available at http://dx.doi.
org/10.1098/rsbl.2006.0440 or via
Received 5 December 2005
Accepted 3 January 2006
217 q2006 The Royal Society
400 450 500 550 600 650 700
wavelength (nm)
Caucasion (dark)
Caucasion (light)
African American
East Indian (light)
East Indian (dark)
mandrill (red nose)
mandrill (purple rump)
0.1 0.04
0.1 0.1
400 450 500 550 600 650 700
wavelength (nm)
molar extinction coeff. e (l cm–1 mmol–1)
high blood
low blood
blood space
Figure 1. (a) Reflectance spectra from a variety of human skin (data from NCSU spectral database), and from one male
primate, namely Mandrillus sphinx (Sumner & Mollon 2003). Also shown here and in (b) are the maximal sensitivities for the
M and L cones for routine trichromats. (b) Absorption spectrum for oxygenated and deoxygenated haemoglobin (from Scott
Prahl, Oregon Medical Laser Center, (c) Blood space for skin spectral modulation, showing the two
principle variables that affect skin colour in the short term: haemoglobin oxygen saturation (x-axis), and haemoglobin skin
concentration (y-axis). ‘High’, ‘baseline’ and ‘low’ values for these two variables were chosen, and the figure shows the nine
skin spectra for all pairs of these parameter settings. Colours code the approximate direction of colour shift from baseline
(centre). (d) Relative change from baseline for LKM and SK(LCM) for the nine model skin spectra varying over blood
space from (c). Shown now are the filtered skin spectra actually reaching the retina. (e) Example skin colour modulations
from modulations of blood variables. Data points show positions in this colour space for RGB values of skin under these
conditions, along with the average values. See electronic supplementary material for the extended legend for this figure.
218 M. A. Changizi and others Evolution of primate colour vision
Biol. Lett. (2006)
routine trichromats, who have M and L maximal
sensitivities (Jacobs & Deegan 1999) of approximately
535 and 562 nm, respectively (figure 2a), providing
near-optimal sensitivity to oxygenation modulation
(figure 2b). Among polymorphic trichromats, most of
the Cebid (e.g. Callicebus,Ateles,Lagothrix,Cebus and
Saimiri ) trichromat phenotypes possess significant
sensitivity to oxygen saturation, although not all
phenotypes are near-optimal (figure 2b). Among the
Callitrichidae (e.g. Saguinus,Leontopithecus,Callimico,
Cebuella and Callithrix) trichromat phenotypes, two of
the three possess near optimal sensitivity to oxygen
saturation, and the third (approximately 556/562)
possesses little or no sensitivity (figure 2b). It is
interesting to note in this regard that the M/L cone
with maximal sensitivity at 556 nm occurs dispropor-
tionately rarely in the population (Rowe & Jacobs
2004), only 19.7%, perhaps because the 556/562
phenotype is insensitive to oxygen saturation vari-
ation. Our hypothesis predicts sensitivity to skin
colour variation not just for the early visual mechan-
isms (i.e., cone sensitivities and opponency), but in
500 510 520 530 540 550 560 570 580 590 600
wavelength (nm)
routine trichromatic
polymorphic trichromatic
(a) (b)
sensitivity to
difference between He and HeO2
molor extinction coefficient (liters/cm/mole)
Figure 2. (a) The curve shows the difference between the absorption spectrum for oxygenated and deoxygenated
haemoglobin. The difference between oxygenated and deoxygenated skin spectra lead to qualitatively identical curves, no
matter the specific skin constants (i.e. same peak and valley wavelengths). Shown also are peak M/L cone sensitivities for the
primate genera shown (reviewed in Surridge et al. (2003)). (Height in the plot for these is only to separate the data.) The
vertical lines are at wavelengths where we would expect the maximal sensitivities of M and L cones to be, respectively, if they
are optimally sensitive for oxygenation variation, and subject to the constraint that M and L jointly function as the single
dichromat M/L cone. For Tarsius,Surridge et al. (2003) give two different values for the single M/L cone, and an ‘x’ point is
shown for the longer wavelength one. For Cebuella, an allele at 543 nm has been added because Surridge & Mundy (2002,
p. 2164) believe it probably exists but was not measured due to low sample size. (b) A plot of sensitivity of M/L cones to
oxygenation variation, for each primate genus, where 100% would occur if the maximal sensitivities of M and L occurred at
the optimum for oxygenation sensitivity. For polymorphic trichromatic primates, points are placed for each of the possible
pairs of M/L cones. In several cases, Surridge et al. (2003) do not mention all three M/L cones, and we utilized the value
from other genera in the same family. The line shows the average sensitivity for all M and L pairs centred around 543 nm
(the typical dichromat maximal sensitivity wavelength), where M ranges as low as 500 nm.
Evolution of primate colour vision M. A. Changizi and others 219
Biol. Lett. (2006)
perception as well, and evidence supports this (see
electronic supplementary material, §2). And related
to this, our hypothesis predicts that dichromats
should be perceptually handicapped at discriminating
skin colour modulations, and they are, as predicted,
notoriously poor at such discriminations (see elec-
tronic supplementary material, table 1).
Because skin spectral variations cannot be per-
ceived on a face without bare skin, the hypothesis
predicts that trichromatic primates should have
bare faces (or at least some other body region with
bare skin, such as a bare rump, something widely
known to be true among Old World Primates,
Wickler (1967)). A cursory look at photographs of
97 species from 35 primate genera demonstrates
that this is a strong regularity (see electronic
supplementary material, figure 1). Estimates of the
average bareness on the face are shown in elec-
tronic supplementary material, figure 1e, and one
can see that monochromats and dichromats tend to
have furry faces, whereas polymorphic and routine
trichromats tend to have bare faces. Note that
among the polymorphic trichromats are two prosi-
mians (prosimians who in other known cases are
monochromatic or dichromatic), Varecia and Pro-
pithecus (the top two photographs in electronic
supplementary material, figure 1efor the poly-
morphic trichromats), and they each have substan-
tial bare spots on their face. This connection
important in understanding why humans are the
‘naked ape’: for primates with colour vision, skin
modulations may serve as signalling on any body
part that can be seen (e.g. a chimpanzee rump),
and for apes that walk upright, more parts of the
body are potentially visible and amenable to colour
signalling. (See §3 of the electronic supplementary
material for further discussion of face bareness and
also see §4 and figures 2 and 3, for a discussion of
evidence of the visibility of skin colour
We should emphasize that the idea that colour
vision is important for colour signalling is not new
(e.g. Hingston 1933;Wickler 1967;Regan et al.
2001;Liman & Innan 2003;Waitt et al. 2003;
Zhang & Webb 2003), except that typically it is
assumed that colour vision was originally selected for
some other reason. One of the main contributions
we make here is the argument that colour vision is
near-optimal for discriminating skin colour modu-
lations, something that increases the prima facie
plausibility of the hypothesis that trichromacy was
originally selected for the perception of skin colour
signalling. Other adaptive explanations have been put
forth to explain primate colour vision, including
advantages for frugivory (Allen 1879;Mollon 1989;
Osorio & Vorobyev 1996;Regan et al.2001;
Surridge & Mundy 2002), and for folivory (Lucas
et al. 2003). Our discussion here provides no answer
as to which of these may more likely have been the
original selection pressure for trichromacy, or
whether all these hypotheses may be important
contributors (Regan et al. 2001). One advantage of
the skin colour-signalling hypothesis is that, whereas
there is a wide variety of trichromat frugivory and
folivory behaviour, skin colour modulation is due to
fundamental properties of blood shared by all pri-
mates, and this could be key in understanding the
universal M and L cone sensitivities of routine
trichromats. There are other phenomena that colour
signalling can explain but these others cannot,
including the high degree of perceptual discrimin-
ability and colour-uncategorizability of skin tones
(see electronic supplementary material, §2), the bare-
ness of trichromat faces, and the close affinity of
colour to blood, skin colour and emotional
states (see electronic supplementary material, §5and
figures 4, 5 and 6).
We wish to thank two helpful referees for their comments.
Support for this research was given by 5F32EY015370-02,
NIH (to M.A.C) and JST.ERATO, Japan (to S.S.).
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Color carries gender information (e.g., red–female/blue–male). This study explored whether red could bias sex categorization of human bodies. Visual stimuli were created from body silhouettes that varied along the waist-to-hip ratio from female to male perception, combined with the red, green, and gray colors that were used as body color (Exp. 1) and background color (Exp. 2). Participants were instructed to categorize the sex of body stimulus as male or female by pressing one of two labelled keys. Results showed that red body color induced a female-body bias, while red background color induced a male-body bias, compared with green and gray colors. Thus, red plays a role in body-sex processing, and the color positioning affects this red effect. Those results suggest that there are different levels of activation of color–sex associations in the body-sex perception.
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Primates are apparently unique amongst the mammals in possessing trichromatic colour vision. However, not all primates are trichromatic. Amongst the haplorhine (higher) primates, the catarrhines possess uniformly trichromatic colour vision, whereas most of the platyrrhine species exhibit polymorphic colour vision, with a variety of dichromatic and trichromatic phenotypes within the population. It has been suggested that trichromacy in primates and the reflectance functions of certain tropical fruits are aspects of a coevolved seed–dispersal system: primate colour vision has been shaped by the need to find coloured fruits amongst foliage, and the fruits themselves have evolved to be salient to primates and so secure dissemination of their seeds. We review the evidence for and against this hypothesis and we report an empirical test: we show that the spectral positioning of the cone pigments found in trichromatic South American primates is well matched to the task of detecting fruits against a background of leaves. We further report that particular trichromatic platyrrhine phenotypes may be better suited than others to foraging for particular fruits under particular conditions of illumination; and we discuss possible explanations for the maintenance of polymorphic colour vision amongst the platyrrhines.
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Abstract Evolution of the red-green visual subsystem in trichromatic primates has been linked to foraging advantages, namely the detection of either ripe fruits or young leaves amid mature foliage. We tested competing hypotheses globally for eight primate taxa: five with routine trichromatic vision, three without. Routinely trichromatic species ingested leaves that were “red shifted—compared to background foliage more frequently than species lacking this trait. Observed choices were not the reddest possible, suggesting a preference for optimal nutritive gain. There were no similar differences for fruits although red-greenness may sometimes be important in close-range fruit selection. These results suggest that routine trichromacy evolved in a context in which leaf consumption was critical.
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Trichromatic colour vision is of considerable importance to primates but is absent in other eutherian mammals. Primate colour vision is traditionally believed to have evolved for finding food in the forest. Recent work has tested the ecological importance of trichromacy to primates, both by measuring the spectral and chemical properties of food eaten in the wild, and by testing the relative foraging abilities of dichromatic and trichromatic primates. Molecular studies have revealed the genetic mechanisms of the evolution of trichromacy, and are providing new insight into visual pigment gene expression and colour vision defects. By drawing together work from these different fields, we can gain a better understanding of how natural selection has shaped the evolution of trichromatic colour vision in primates and also about mechanisms of gene duplication, heterozygote advantage and balancing selection.
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We investigate why vessels that contain blood, which has a red or a dark red color, may look bluish in human tissue. A CCD camera was used to make images of diffusely reflected light at different wavelengths. Measurements of reflectance that are due to model blood vessels in scattering media and of human skin containing a prominent vein are presented. Monte Carlo simulations were used to calculate the spatially resolved diffuse reflectance for both situations. We show that the color of blood vessels is determined by the following factors: (i) the scattering and absorption characteristics of skin at different wavelengths, (ii) the oxygenation state of blood, which affects its absorption properties, (iii) the diameter and the depth of the vessels, and (iv) the visual perception process.
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The disabilities experienced by colour-blind people show us the biological advantages of colour vision in detecting targets, in segregating the visual field and in identifying particular objects or states. Human dichromats have especial difficulty in detecting coloured fruit against dappled foliage that varies randomly in luminosity; it is suggested that yellow and orange tropical fruits have co-evolved with the trichromatic colour vision of Old World monkeys. It is argued that the colour vision of man and of the Old World monkeys depends on two subsystems that remain parallel and independent at early stages of the visual pathway. The primordial subsystem, which is shared with most mammals, depends on a comparison of the rates of quantum catch in the short- and middle-wave cones; this system exists almost exclusively for colour vision, although the chromatic signals carry with them a local sign that allows them to sustain several of the functions of spatiochromatic vision. The second subsystem arose from the phylogenetically recent duplication of a gene on the X-chromosome, and depends on a comparison of the rates of quantum catch in the long- and middle-wave receptors. At the early stages of the visual pathway, this chromatic information is carried by a channel that is also sensitive to spatial contrast. The New World monkeys have taken a different route to trichromacy: in species that are basically dichromatic, heterozygous females gain trichromacy as a result of X-chromosome inactivation, which ensures that different photopigments are expressed in two subsets of retinal photoreceptor.
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Most mammals possess two classes of cone, sensitive to short and to long wavelengths of light, but Old World primates (Catarrhini) have distinct medium and long wavelength sensitive classes. The sensitivities of these cones photopigments are alike in all catarrhines with peaks at about 440 nm ('blue'), 533 nm ('green') and 565 nm ('red'). One possible reason for the evolution and conservatism of catarrhine trichromacy is that colour vision is a specialization for finding food. A model of retinal coding of natural spectra, based on discrimination thresholds, is used to examine the usefulness of dichromatic and trichromatic vision for finding fruit, and for identifying fruit and leaves by colour. For identification tasks the dichromat's eye is almost as good as a trichromat's, but the trichromat has an advantage for detecting fruit against a background of leaves.
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It is often assumed that all Old World monkeys share the same trichromatic colour vision, but the evidence in support of this conclusion is sparse as only a small fraction of all Old World monkey species have been tested. To address this issue, spectral sensitivity functions were measured in animals from eight species of Old World monkey (five cercopithecine species and three colobine species) using a non-invasive electrophysiological technique. Each of the 25 animals examined had spectrally well-separated middle- and long-wavelength cone pigments. Cone pigments maximally sensitive to short wavelengths were also detected, implying the presence of trichromatic colour vision. Direct comparisons of the spectral sensitivity functions of Old World monkeys suggest there are no significant variations in the spectral positions of the cone pigments underlying the trichromatic colour vision of Old World monkeys.
Many New World (NW) primates possess a remarkable polymorphism in an X-linked locus, which encodes for the visual pigments (opsins) used for colour vision. Females that are heterozygous for opsin alleles of different spectral sensitivity at this locus have trichromatic colour vision, whereas homozygous females and males are dichromatic, with poor colour discrimination in the red–green range. Here we describe an extensive survey of allelic variation in both exons and introns at this locus within and among species of the Callitrichines (marmosets and tamarins). All five genera of Callitrichines have the X-linked polymorphism, and only the three functional allelic classes described previously (with maximum wavelength sensitivities at about 543 nm, 556 nm and 563 nm) were found among the 16 species and 233 or more X-chromosomes sampled. In spite of the homogenizing effects of gene conversion, phylogenetic analyses provide direct evidence for trans-specific evolution of alleles over time periods of at least 5–6 million years, and up to 14 million years (estimated from independent phylogenies). These conclusions are supported by the distribution of insertions and deletions in introns. The maintenance of polymorphism over these time periods requires an adaptive explanation, which must involve a heterozygote advantage for trichromats. The lack of detection of alleles that are recombinant for spectral sensitivity suggests that such alleles are suboptimal. The two main hypotheses for the selective advantage of trichromacy in primates are frugivory for ripe fruits and folivory for young leaves. The latter can be discounted in Callitrichines, as they are not folivorous.
Skin color is one of the most conspicuous ways in which humans vary and has been widely used to define human races. Here we present new evidence indicating that variations in skin color are adaptive, and are related to the regulation of ultraviolet (UV) radiation penetration in the integument and its direct and indirect effects on fitness. Using remotely sensed data on UV radiation levels, hypotheses concerning the distribution of the skin colors of indigenous peoples relative to UV levels were tested quantitatively in this study for the first time.