Human skin-color sexual dimorphism: A test of the sexual selection hypothesis

Article (PDF Available)inAmerican Journal of Physical Anthropology 133(1):779-80; author reply 780-1 · May 2007with 271 Reads
DOI: 10.1002/ajpa.20555 · Source: PubMed
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Notes and Comments
Human Skin-Color Sexual Dimorphism:
A Test of the Sexual Selection Hypothesis
Peter Frost*
De
´partement d’anthropologie, Universite
´Laval, Que
´bec,
Que
´bec G1K 7P4, Canada
Women have lighter skin than men do across a wide
range of populations, even on the unexposed skin of the
upper inner arm, possibly because of sexual selection by
men for lighter-skinned women. If this hypothesis is true,
human skin color should become more sexually dimorphic
with increasing distance from the equator, since sexual
selection for lighter skin in women would be less con-
strained by natural selection for darker skin in both
sexes. Yet when Madrigal and Kelly (2006) analyzed skin
reflectance data from 53 different samples, they found
that the most dimorphic human populations were actually
those of medium skin color at medium latitudes.
Their finding does not necessarily falsify the sexual
selection hypothesis. It may be that this sexual dimor-
phism cannot fully express itself in light-skinned popula-
tions at higher latitudes. Skin reflectance becomes sexu-
ally dimorphic during adolescence, when girls progres-
sively lighten in color, and this lightening may be less
effectively expressed if melanin production is already
low (Frost, 2005; 57). It is perhaps significant that this
sex difference seems to disappear or even reverse itself
when skin reflectance is close to the physiological maxi-
mum, notably in Dutch and Belgian subjects (Leguebe,
1961; van Rijn-Tournel, 1966; Rigters-Aris, 1973). The
authors of these studies attribute the absence of dimor-
phism to their female subjects being more likely to wear
sleeveless shirts. Madrigal and Kelly (2006) cautiously
attribute it to the popularity of tanning among modern
European women. Yet the skin color of Spanish subjects
becomes sexually dimorphic during adolescence even
though young women in post-Franco Spain wear sleeve-
less shirts and sun themselves (Mesa, 1983).
If we eliminate samples near the maximum of skin re-
flectance, can we still tease out the effects, if any, of sex-
ual selection? Doubtful. For one thing, the ‘‘ceiling effect’’
likely begins to manifest itself well before the limit of
maximum skin reflectance. For another, there is probably
too much noise in the data for such a fine-grained analy-
sis. Skin color is less sexually dimorphic at some ages,
particularly childhood (when it is absent) and adoles-
cence (when it is still emerging), and most studies are
poorly controlled for age. Madrigal and Kelly (2006) tried
to minimize this problem by limiting their meta-analysis
to adult men and women: ‘‘By excluding samples which
worked with children and even teenagers, we feel
assured that our sample will not suffer from age-related
heterogeneity.’’ Many of the papers they cite, however,
apply the term ‘‘adults’’ or ‘‘men and women’’ to anyone
past the age of puberty. For example, when Weiner et al.
(1964) studied Black Bushmen at Bagani kraal, the
‘‘adults’’ were defined as those whose third molars had
erupted. Leguebe (1961) studied ‘‘women’’ who ranged in
age from 17 to 23. Banerjee (1984) examined Punjabi
‘‘mothers’’ and ‘‘fathers’’ without specifying their ages, yet
many Punjabi women begin to have children in their
teenage years.
Even after adolescence, age-related noise continues to
muddy the data. Human skin may change more slowly
during adulthood, but it does change. In particular, there
is some evidence that skin color becomes less sexually
dimorphic after 40, with some papers finding that women
become nonsignificantly darker than men (Chamla and
De
´moulin, 1978; Mori and Tokuhashi, 1956; Rigters-Aris,
1973). This is a problem for any meta-analysis because
relatively few skin reflectance studies exclude adults over
40 years of age.
The most serious problem with Madrigal and Kelly’s
meta-analysis, however, is not a methodological one. When
they modeled the selection pressures on human skin color,
they assumed that the only one to vary with latitude was
natural selection for darker skin in both sexes. Yet sexual
selection for lighter skin in women may also have varied
with latitude, especially during the long period of time
when all humans were hunter-gatherers. The intensity of
sexual selection is determined by the operational sex ratio
(i.e., the ratio of men to women among adults available for
mating) and this ratio becomes more female-biased in
hunter-gatherers the further away they are from the equa-
tor: first, polygyny becomes costlier for men because
women cannot gather food in winter and require more food
provisioning; second, more men die at a younger age
because they cover longer distances while hunting. As a
result, in proportion to distance from the equator, fewer
men are available for mating, women compete much more
for mates, and sexual selection of women intensifies. Thus,
sexual selection for lighter skin in women may have varied
with latitude just as natural selection for darker skin
varies with latitude. The two selection pressures are con-
founded. Madrigal and Kelly (2006) allude to this problem
when they deplore the assumption that everywhere, and
to the same extent, men can pick and choose among a
broad range of possible mates.
How, then, can the sexual selection hypothesis be falsi-
fied? Does it make predictions that may be proven true
or false? In my opinion, there are two testable predic-
tions:
1. If sexual selection has shaped this sexual dimorphism,
men and women should differ in all of the visible skin
pigments, and not simply in one of them. This predic-
tion is borne out. Women differ from men in the two
main components of skin color: melanin and hemoglo-
*Correspondence to: Peter Frost, c/o Dr. Saladin d’Anglure,
De
´partement d’anthropologie, Universite
´Laval, Quebec, Quebec
G1K 7P4, Canada. E-mail: peter_frost61z@globetrotter.qc.ca
Received 5 June 2006; accepted 4 December 2006.
DOI 10.1002/ajpa.20555
Published online 26 February 2007 in Wiley InterScience
(www.interscience.wiley.com).
V
V
C2007 WILEY-LISS, INC.
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 133:779–781 (2007)
bin, i.e., women are less brown and less ruddy in com-
plexion (Edwards and Duntley, 1939). To explain this
sex difference in terms of natural selection, one must
postulate two separate selection pressures.
2. If an allele lightens women’s skin more than men’s, and
if we compare different populations with the same level
of solar UV exposure (see Jablonski and Chaplin, 2000;
68), the allele should increase in frequency as we go
from darker- to lighter-skinned populations. In addi-
tion, there should be more alleles that exhibit this kind
of sex linkage. The reason in both cases is that the
lighter-skinned populations would owe some of their
loss of skin pigmentation to sexual selection for lighter-
skinned women, and not simply to natural selection (ei-
ther stronger selection for vitamin D synthesis or
weaker selection for protection against sunburn or skin
cancer). Such sex-linked alleles would thus be over-rep-
resented even if they were less effectively expressed in
fair-skinned individuals. We can test this prediction by
examining alleles that 1) lighten skin color and 2)
appear to have been under positive selection, e.g., the
‘‘redhead’’ MC1R alleles, the derived SLC24A5 and
SLC45A2 alleles, and possibly OCA2, MYO5A, HPS7,
and TYRP1 variants (Sturm, 2006). If some or all of
these alleles affect one sex more than the other, they
should do so in the direction of lightening skin color
more in women than in men.
On a final note, the sexual selection hypothesis does not
presuppose that human skin color became sexually dimor-
phic solely or even mainly via the action of sexual selection.
Some of its proponents argue that women’s lighter skin
first arose through fortuitous causes (the differing effects
of male and female hormones on melanin production) and
that the male mind then came to use this visible female
trait, subconsciously, as a measure of hormonal status and
thus childbearing potential (van den Berghe and Frost,
1986). Others believe that a lighter skin color was first part
of a complex of childlike traits (smoother skin, higher-
pitched voice, more pedomorphic face) that enabled women
to lessen aggressiveness in men and stimulate male provi-
sioning (Guthrie, 1970). Still others think that women first
acquired a lighter skin to facilitate vitamin D synthesis
and thereby ensure more calcium for pregnancy and lacta-
tion (Jablonski and Chaplin, 2000). All three of these
explanations assign sexual selection a secondary, faculta-
tive role. Women’s lighter skin is thought to have first
evolved for other reasons and only later became a criterion
that men could use for mate choice.
More importantly, regardless of the universality of this
mate-choice criterion, the extent to which it actually did
influence mate choice—and hence sexual selection—
would have depended on the availability of mateable
women and on the relative importance of other mate-
choice criteria. We thus come back to one of the flaws in
Madrigal and Kelly’s model: the assumption that sexual
selection for lighter-skinned women was equally intense
in all human populations and was constrained only by
natural selection for dark skin. In fact, the intensity
may have varied considerably in response to a variety of
local conditions.
LITERATURE CITED
Banerjee S. 1984. The inheritance of constitutive and facultative
skin colour. Clin Genet 25:256–258.
Chamla M-C, De
´moulin F. 1978. Re
´flectance de la peau, pigmen-
tation des cheveux et des yeux des Chaouı¨as de Bouzina
(Aure
`s, Alge
´rie). Anthropologie (Paris) 82:61–94.
Edwards EA, Duntley SQ. 1939. The pigments and color of liv-
ing human skin. Am J Anat 65:1–33.
Frost P. 2005. Fair women, dark men. The forgotten roots of
color prejudice. Christchurch (New Zealand): Cybereditions.
Guthrie RD. 1970. Evolution of human threat display organs.
Evol Biol 4:257–302.
Jablonski NG, Chaplin G. 2000. The evolution of human skin
coloration. J Hum Evol 39:57–106.
Leguebe A. 1961. Contribution a
`l’e
´tude de la pigmentation chez
l’homme. Inst R Sci Nat Belg Bull 37:1–29.
Madrigal L, Kelly W. 2006. Human skin-color sexual dimor-
phism: a test of the sexual selection hypothesis. Am J Phys
Anthropol (online pub. www.interscience.wiley.com).
Mesa MS. 1983. Analyse de la variabilite
´de la pigmentation de
la peau durant la croissance. Bull Mem Soc Anthropol Paris,
t. 10 se
´rie. 13:49–60.
Mori O, Tokuhashi M. 1956. Measurement by age group of the
color and gloss of the skin of healthy Japanese. J Anthropol
Soc Nippon 65:1–19.
Rigters-Aris CAE. 1973. A reflectance study of the skin in Dutch
families. J Hum Evol 2:123–136.
Sturm RA. 2006. A golden age of human pigmentation genetics.
Trends Genet 22:464–468.
van den Berghe PL, Frost P. 1986. Skin color preference, sexual
dimorphism and sexual selection: a case of gene-culture co-
evolution? Ethn Racial Stud 9:87–113.
van Rijn-Tournel J. 1966. Pigmentation de la peau de Belges
et d’Africains. Bull Soc R Belge Anthropol Prehist 76:76–96.
Weiner JS, Harrison GA, Singer R, Harris R, Jopp W. 1964.
Skin color in southern Africa. Hum Biol 36:294–307.
Human Skin-Color Sexual Dimorphism:
A Test of the Sexual Selection Hypothesis.
Reply to Frost (2007)
Lorena Madrigal* and William Kelly
Department of Anthropology, University of South Flor-
ida, Tampa, FL 33620
We thank Frost (2007) for his comments. The evolu-
tion of human skin color has not received much atten-
tion in the physical anthropology literature recently, and
we are glad to see it discussed.
Frost’s comments focus on the following points: He
proposes that sexual dimorphism might not be able to be
expressed in light-skinned populations at higher lati-
tudes. Therefore, we should not expect these populations
to be more sexually dimorphic. We are aware of no data
*Correspondence to: Lorena Madrigal, Department of Anthropol-
ogy, University of South Florida, 4202 E. Fowler Avenue, SOC 17,
Tampa, FL 33620. E-mail: madrigal@cas.usf.edu
Received 14 December 2006; accepted 10 January 2007
DOI 10.1002/ajpa.20582
Published online 26 February 2007 in Wiley InterScience
(www.interscience.wiley.com).
780 NOTES AND COMMENTS
American Journal of Physical Anthropology—DOI 10.1002/ajpa
in support of this proposition. Indeed, the only citation
provided by Frost is his own book (which our interlibrary
loan office could not get for this reply to be published
timely, as the book is not held by any library in the State
of Florida system or the Library of Congress). However,
even if it were true that light-skin populations might not
be able to express sexual dimorphism, the other human
populations should follow a correlation with latitude of
increasing dimorphism with increasing distance from the
equator. Moreover, Frost only mentions the Dutch and
Belgian subjects, in which females are darker than
males in support of his point. He fails to mention the
other populations in which this is also the case, all of
which are listed in our Table 1. Most of these popula-
tions are not particularly light skinned, and some of
them are very dark. Lastly, the proposition that the
effects of sexual selection in high latitude populations is
present but not detectable is untestable, and therefore
out of the range of a scientific paper.
We included in our sample of 53 studies, three that
according to Frost should not have been included because
of the age of the subjects. Any researcher who does a
meta-analysis of existing data must make difficult deci-
sions about which studies to include. In this particular
case, the main issue was to increase sample size to
improve statistical power. To argue that three out of 53
papers should not have been included (one because the
subjects had erupted third molars) misses the point about
statistical power. As we clearly acknowledged in the paper,
meta-analyses are done with less than perfect data. In
addition, we note that any osteology textbook will define
the adult human dental formula as one which includes
the third molar because the eruption of this tooth is one
which is considered as a marker of adulthood.
The hypothesis which we tested did not consider that
sexual selection might vary crossculturally. Indeed it did
not. We state:
Simply put, Aoki (2002) and Ihara and Aoki (1999) pro-
posed that there is a universal preference on the side of
males for females with lighter skin color. In the absence
of natural selection for dark skin, sexual selection would
be strongest in areas of low solar radiation. This pro-
posal explains not only the presence of light skin in
areas of low solar radiation, but also human skin color
sexual dimorphism. From this point on, we refer to this
hypothesis as the sexual selection hypothesis.
Therefore, we tested the hypothesis as proposed by
authors other than Frost, who state that the preference
is universal, and who did not consider that there might
be crosscultural differences in such preference. We hope
that Frost will be able to test that the intensity of sexual
selection ‘‘varied considerably in response to a variety of
local conditions.’’ But this is something that we did not
attempt to do because it is not part of the hypothesis as
stated by Aoki (2002) and Ihara and Aoki (1999). We
note that it would have been helpful for further studies
if Frost had provided citations in support of his assertion
that polygyny becomes costlier for men in societies far
away from the equator.
The sexual selection hypothesis does not presuppose
that human skin color became dimorphic solely or
mainly via the action of sexual selection. Different views
of the hypothesis ‘‘...assign sexual selection a secondary,
facultative role.’’ Indeed, the aim of our paper was not to
look at what might have initially caused sexual dimor-
phism but to examine one explanation of what brought it
to its current levels.
Frost then sets an agenda for possible further research,
proposing two predictions. We applaud this endeavor and
hope to see further peer-reviewed research on the matter.
In conclusion, a careful reading of our paper demon-
strates that we tested one specific proposal, namely one
which does not consider crosscultural differences in sexual
selection. In addition, the purpose of our paper was not to
examine the origin of human skin color sexual dimor-
phism. We hope that Frost or others will attempt to do so
in the future, and look forward to seeing those results.
Although our data set is typical of that of meta-analy-
ses, namely, collected by different authors with different
guidelines, we believe that it is better to explore ideas
and different views with the data that are available
rather than not exploring or investigating simply
because the data are not perfect. In addition, we worked
with actual skin reflectance data which can be quantified
and subjected to statistical analysis, as opposed to quali-
tative data on stated sexual preferences.
According to Frost, it is doubtful that we can ‘‘still
tease out the effects, if any, of sexual selection,’’ whether
because data on skin reflectance were collected in people
of different ages, or because sexual selection is more
acute if the subjects evolved in hunter-gatherer societies
far away from the equator. We do not subscribe to a view
of science that claims that something (the sexual selec-
tion hypothesis) should be accepted as valid even if it
cannot be tested. On the contrary, we focused on a spe-
cific proposal (not Frost’s proposal) which could be
tested. We did not incorporate variables which were not
measured in our subjects (such as the operational sex ra-
tio in cold-weather hunters and gatherers) because the
data were not available.
LITERATURE CITED
Aoki K. 2002. Sexual selection as a cause of human skin color
variation: Darwin’s hypothesis revisited. Ann Hum. Biol. 29:
589–608.
Frost P. 2007. Notes and comments. Human skin-color sexual
dimorphism: a test of the sexual selection hypothesis. Am J
Phys Anthropol. (in press).
Ihara Y, Aoki K. 1999. Sexual selection by male choice in
monogamous and polygynous human. Theor Popul Biol 55:
77–93.
781NOTES AND COMMENTS
American Journal of Physical Anthropology—DOI 10.1002/ajpa
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    The evolution of human skin pigmentation must address both the initial evolution of intense epidermal pigmentation in hominins, and its subsequent dilution in modern humans. While many authorities believe that epidermal pigmentation evolved to protect against either ultraviolet B (UV-B) irradiation-induced mutagenesis or folic acid photolysis, we hypothesize that pigmentation augmented the epidermal barriers by shifting the UV-B dose-response curve from toxic to beneficial. Whereas erythemogenic UV-B doses produce apoptosis and cell death, suberythemogenic doses benefit permeability and antimicrobial function. Heavily melanized melanocytes acidify the outer epidermis and emit paracrine signals that augment barrier competence. Modern humans, residing in the cooler, wetter climes of south-central Europe and Asia, initially retained substantial pigmentation. While their outdoor lifestyles still permitted sufficient cutaneous vitamin D3 (VD3) synthesis, their marginal nutritional status, coupled with cold-induced caloric needs, selected for moderate pigment reductions that diverted limited nutritional resources towards more urgent priorities (=metabolic conservation). The further pigment-dilution that evolved as humans reached north-central Europe (i.e., northern France, Germany), likely facilitated cutaneous VD3 synthesis, while also supporting ongoing, nutritional requirements. But at still higher European latitudes where little UV-B breaches the atmosphere (i.e., present-day UK, Scandinavia, Baltic States), pigment dilution alone could not suffice. There, other nonpigment-related mutations evolved to facilitate VD3 production; for example, in the epidermal protein, filaggrin, resulting in reduced levels of its distal metabolite, trans-urocanic acid, a potent UV-B chromophore. Thus, changes in human pigmentation reflect a complex interplay between latitude, climate, diet, lifestyle, and shifting metabolic priorities.
  • Chapter
    Current theories for the development of epidermal pigmentation in hominins are problematic. Skin cancers occur beyond peak reproductive years, and minimal UV-B penetrates to deeper skin layers where folic acid circulates and eccrine glands reside. Generating an optimal permeability barrier was challenging for hominins who employed sweat to thermoregulate as they inhabited African savannas. Epidermal pigmentation provided hominins with a superior barrier. Latitude-dependent pigment dilution to facilitate cutaneous vitamin D3 (VD3) synthesis is also problematic, because: UV-B-irradiated skin, whether pigmented or nonpigmented, generates comparable VD3; extracutaneous storage of VD3 supplies year-round VD3 requirements; adequate dietary sources existed in the Upper Paleolithic; VD3 deficiency is rarely seen in pre-industrial age fossils; and evidence is lacking for mutations facilitating VD3 production/action. Pigment dilution instead largely served in metabolic conservation-the imperative to redirect scarce protein production toward more urgent requirements. Europeans of the far north instead developed loss-of-function filaggrin mutations leading to reduced trans-urocanic acid, the major UV-B absorber in stratum corneum, and to a corresponding increase in circulating VD3 levels.
  • Article
    Among a sample of 115 women and 114 men from Aures, 16 to 57 years of age, there is no significant variation with age or on the site of the upper arm, but the differences are highly significant on the forehead. Skin is still lighter in women than in men: 45% of men and 30% of women have a skin reflectometry on the arm between 55 and 59; 30% of men and 50% of women have a reflectance above 60. Seasonal variations have been studied; tanning is more marked during May and June, than during September and October. The majority of men are brown and dark-haired. Women are darker than men for hair colour. A trend to a lighter hair colour is observed among boys whose definitive pigmentation appears later. Concerning eye colour, men are divided into two groups, one including 60% of the sample with brown and dark brown eyes, the other including 20% of the subjects with green and mixed green eyes. Women have prevailing dark eye colour. Eye colour varies during growth with more pronounced fluctuations among girls than among boys, mainly during the puberty period. Hair and eye colour of Chaouias from Bouzina does not differ between the inhabitants of the different villages studied, in spite of a different origin and a high rate of local endogamy.
  • Article
    This paper is concerned with the results of measurements with the photoerectric color and gloss-meter, of the color and gloss of the skin of eight different parts of the body of 475 (234 male and 241 female) Japanese ranging over the whole life span.The eight parts measured were the forehead, the cheek, the palmar surface, the back of the hand, the inner and outer surface of the upper arm, the breast and the dorsum.Eight periods of age development were established in order to study. The chromatic characteristics belonging to any particular stage of life. There were: newly-born-baby stage, infancy, childhood, elementaly school period, middle-school-period, adolescence, adulthood, and old age.1. These are incontestable differences between individuals in color and gloss, but a certain system is recognizable in the way in which individuals alter as they develop from one stage of life to another.2. Concerning transition by age stages of skin color of Japanese, Fig. 11 and 12 may be consulted.3. Of transition of skin color by age changes, the main points are:a) The color of the inner part of the upper arm is affected least by age transition.b) The inner part of the upper arm and the front of the body undergo less transition of color than the outer parts and the back.4. Of age transition of gloss, the main points are:a) Gloss shows the minimum value at the newly-born-baby stage and reaches maximum at adulthood.b) The part which undergoes least transition of gloss is the inner part of the upper arm.c) The inner part of the upper arm and the front of the body are affected less in transition of gloss than the outer parts and the back.5. Of differences existing between the sexes, the main points are:a) In hue, the male is generally redder that the female though a few parts shows a reverse trend.b) In chromaticity, the male is generally higher than the female.c) In lightness, the female is without exception higher than the male.d) In gloss, the female is generally higher than the male.e) Sex difference grows sharper from childhood on and reach maximum in adolescence.f) Sex differences in gloss are not distinct.6. Each part of the body has its own particular manner of color transition which clearly distinguishes one part from another as evident in Fig. 13.
  • Article
    Full-text available
    L'echantillon, objet de cette etude, est compose de 916 jeunes espagnols des deux sexes, entre 15 et 18 ans. Les resultats indiquent l'existence de variations avec l'âge, surtout par rapport au bras, ou la peau s'eclaircit de plus en plus. Le front presente une tendance plus stable et, dans le sexe masculin, les valeurs de reflectance diminuent avec l'âge. Des differences statistiques significatives ont ete constatees en comparant les deux sexes et le bras au front. Les filles ont une peau plus claire que les garcons ; il en est de meme du bras vis-a-vis du front. Cet echantillon a ete reuni a un autre de la meme population (enfants 6-14 ans) pour faire une revue globale des variations de la pigmentation cutanee pendant la periode de 6 a 18 ans.
  • Article
    In the period February–May 1968 the reflectances of the exposed (forehead) and unexposed (inside upper arm) skin, were measured at nine wavelengths (430, 470, 490, 520, 550, 580, 600, 660 and 685 mμ), using an “EEL” Reflectance Spectrophotometer in 100 Dutch families. The reflectance curves of upper arm and forehead show the shape usually found in white persons. The skin colour of the upper arm is lighter than that of the forehead in males and females. A difference in skin colour between males and females is observed. The forehead reflectances of males are lower than of females, and the upper arm reflectances of females are lower than those of males. A similarly directed sex difference in skin colour at the forehead has been found in other white groups. If no difference in reaction on exposure exists between both sexes, then the skin colour of the male upper arm will be lighter than that of females, although males in general are more exposed to radiation, because male arms are less uncovered than female arms. No change in skin colour at the upper arm is observed during ageing. As judged from the wavelengths concerned the changes in skin colour at the forehead during ageing result for the greater part both from a regular accumulation of melanin, and an increase in the blood flow of the skin, particularly during the years 11–25.
  • Article
    Observations on skin reflectance were made on 309 endogamous Indian families, for constitutive (medial upper arm) and facultative skin colour (forehead) to investigate which of the two colours is under a rigid control of genetic action. An assessment of data by coefficient of correlation "r" show that facultative colour is inherited more strictly than the constitutive colour.
  • Article
    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.