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Is plasticity in mating preferences adapted to perceived
exposure to pathogens?
Pavol Prokop & Markus J. Rantala & Jana Fančovičová
Received: 15 June 2011 / Revised: 10 October 2011 / Accepted: 21 October 2011 / Published online: 25 November 2011
#
Springer-Verlag and ISPA 2011
Abstract Humans are unique among primates due to a lack
of typical thermally insulating fur. The ectoparasite avoid-
ance mediated by the mate choice hypothesis suggests that
the loss of body hair reduces the risk of infection by
ectoparasites and that the movement toward nudity may
have been enforced by parasite-mediated sexual selection.
In this study, we investigated two possible predictions of
this hypothesis: (1) that preferences for hairless bodies
increase with exposure to environmental pathogens and (2)
that disgust sensitivity to the pathogens’ threat predicts the
degree to which a woman will prefer hairless bodies. Using
an experiment comparing the preferences of 88 women for
shaved vs. hairy pictured versions of 20 male torsos, we
found that exposure to the visual cues of pathogens does
not predict preferences for a male chest nor does the
individual disgust sensitivity to disease-rela ted inverte-
brates. Overall, the results suggest that female perception
of male trunk hair is not associated with a risk of
contamination, which questions the salience of the ectopar-
asite avoidance hypothesis in explaining the loss of body
hair in humans.
Keywords Homo sapiens
.
Mate preferences
.
Nakedness
.
Pathogens
.
Trunk hair
Introduction
The loss of body hair in modern humans, about 1.2 Ma ago
(Rogers et al. 2004), has been the target of controversy
since Darwin (for reviews, see Rantala 1999, 2007). It has
been suggested, for example, that bareness was an
adaptation to a hot savannah environment (e.g. Morris
1967; Wheeler 1984; for a detailed review, see Rantala
2007). However, this hypothesis is based on the fallible
idea of bareness as a cooling factor (Amaral 1996; Rantala
2007). At present, the most prominent hypothesis is the
ectoparasite avoidance hypothesis originally proposed by
Belt (1874) and recently rediscovered and elaborated by
Rantala (1999). This hypothesis proposes that the loss of
body hair reduces the risk of infection by ectoparasites, a
number of which have been responsible for pandemics
(Pagel and Bodmer 2 003;Rantala1999, 2007). For
example, in the 1300s, a plague transmitted by fleas was
estimated to have killed between 25% and 50% of the
populations of Europe, Asia and Africa (Gottfried 1983).
This suggests that the selection of hairy individuals as
mates could be risky in terms of transmission of deadly
diseases particularly in a pathogen-rich envir onment.
Parasitic and infectious diseases have had a major impact
on hu man p opulation demography around the wo rld
(Anderson and May 1991; Ewald 1994). They have been
identified as drivers of religious diversity (Fincher and
Thornhill 2008), political stability of countries (Thornhill et
al. 2009), the size (Thomas et al. 2004) and the number of
human offspring (Guégan et al. 2001), parenting (Quinlan
2007) and cooking practises (Sherman and Billing 1999),
marriage structures (Low 1990) and mate preferences
(Gangestad and Buss
1993; Gangestad et al. 20
06;
DeBruine et al. 2010a). Parasite diversity (the number of
kinds) and prevalence (number of cases) in the world vary
P. Prokop
:
J. Fančovičová
Department of Biology, Trnava University,
Trnava, Slovakia
P. Prokop (*)
Institute of Zoology, Slovak Academy of Sciences,
Bratislava, Slovakia
e-mail: pavol.prokop@savba.sk
M. J. Rantala
Department of Biology, University of Turku,
Turku, Finland
acta ethol (2012) 15:135–140
DOI 10.1007/s10211-011-0118-5
greatly; the diversity increases as one proceeds from the
poles to the equator (Guernier et al. 2004), while the
prevalence is influenced by pathogen richness and disease
control efforts (Dunn et al. 2010), leading to socio-
ecological variation in selective pressures across time and
space. This suggests that flexibility in behavioural response
to parasitic threat could have been favoured (Murray and
Schaller 2010; DeBruine et al. 2010a).
Mate choice criteria are thought to reflect the selection of
traits that advertise aspects of mate quality (Andersson
1994). However, judgements of attractiveness, as cues of
mate quality or fertility (e.g. Rhodes et al. 2003; Lie et al.
2010; Prokop and Fedor 2011) may vary depending on the
environment. For example, cues associated with physical
attractiveness, such as masculinity in males, or facial
symmetry, are more desirable in environments with high
pathogen prevalence than in environments with low pathogen
prevalence (Gangestad and Buss 1993; Penton-Voak et al.
2004; Gangestad et al. 2006; Little et al. 2007;DeBruineet
al. 2010a). Laboratory studies have supported these findings:
female participants exposed to visual cues of environmental
pathogens preferred more masculine and more symmetrical
male faces (Little et al. 2011). Finally, women who perceived
themselves as more vulnerable to diseases preferred mascu-
line male faces more than females less vulnerable to diseases
(DeBruine et al. 2010b). This suggests that mate choice
criteria are influenced by environmental and individual
conditions regarding the pathogen threat.
In this study, we used a controlled experiment to
examine whether female mating preferences for male body
hair vary with visual cues of pathogens (the ectoparasite
avoidance mediated by mate choice hypothesis) and with
individual predispositions regarding sensitivity to cues of
pathogens. We predicted that preferences for hairless bodies
increase with exposure and sensitivity to cues of environ-
mental pathogens.
Methods
Participants
The research was carried out in a lecture hall in April 2011
over four occasions (∼20 students per session) at the
University of Trnava, Slovakia. Eighty-eight female stu-
dents (aged between 21 and 23 years, M=21.21, SE=0.15)
took part in the study on a voluntary ba sis. All the
participants were reported to be heterosexuals and earned
an extra credit course for their participation. The partic-
ipants were randomly divided into the two treatments
described below: high pathogen group (n=40) and low
pathogen group (n=48).
Chest hair stimuli
We used the photographs of male participants which
were used by Rantala et al. (2010) in their research on
female preferences for m ale body hair. Briefly, 20 Finnish
males with visible chests aged 20–32 ye ar s participated in
the research. Front views of male torsos were taken under
symmetrical lighting conditions from a fixed distance of
200 cm. Immediately after the photo session, the men
were asked to shave their abdomen with a shaver,
finishing with a razor blade and shaving cream. After
shaving, a new set of pictures was taken with an identical
setup (Fig. 1). More details can be found in Rantala et al.
(2010).
Cues to the high/low pathogen incidence
Images of objects holding a p otential disease threat were
predominantly taken from a published study examining
peoples’ perception of pathogens (Curtis et al. 2004). The
images were pairs depicting, for example, a plate of viscous
liquid
colour-morphed
like bodily fluids (high pathogen) or
a blue chemical dye (low pathogen). For our study, the ten
image pairs which were consistently seen as differing in
disgust perception were extracted from the high-quality
PDF of the stimuli. Specifical ly, six pairs were taken from
Curtis et al. (2004) and four additional pairs were
downloaded from available web sites. These additional
pairs were taken in order to increase the number of
disease-relevant insects that are hypothesized as having
been responsible for body hair loss in humans (Rantala
1999, 2007).
Overall, five out of the ten pictures presented to
each particip ant were i nsects, either disease-relevant
(human flea (Pulex irritans), human louse (Pediculus
humanus), German cockroach ( Blattella germanica),
common tick (Ixodes ricinus ) and mosquito (Anopheles
maculipennis)) (high pat hogen group) or thei r disease-
irrelevant antipoles (ladybird beetle Cocc ninella septem-
punctata, leaf beetle Chrysomela fastuosa,azuredam-
selfly Coenagrion puella,rhinocerosbeetleOryctes
nasicornis and Old World swallowtail Papilio machaon)
(low pa thogen group). Similar invertebrates were used
by Prokop and Fančovičová (2010). The remaining five
pictures were: a plate of viscous liquid (described
above), a male face ( healthy in high pathog en and ill in
low pathogen), Ascaris wo rms (hig h patho gen) or a
wasp (Polistes sp.) (low pathogen), a white cl oth with
either a stain resembling body fluid (high pathogen) or a
stain of blue liquid (low pathogen) , a metro full of
people and one that is empty (high and low pathogen,
respectively).
136 acta ethol (2012) 15:135–140
Procedure
This study followed a similar research design as those used
by Little et al. (2011). The participants were administered a
short questionnaire assessing age and sexual orientation,
followed by the main test. The main test consisted of three
parts: an initial test that assessed the participants’ prefer-
ences for male body hair (the pre-exposure test), a slide
show of either high- or low-pathogen images (the exposure
phase) and a post-exposure test which was identical to the
pre-exposure test. The inclusion of a pre-exposure test is
potentially important as it allowed us to control for possible
preexisting individual differences in female preferences for
male body hair, such as those related to the menstrual cycle,
age or sexual imprinting (see Rantala et al. 2010). Finally,
in ord er to inv estigate (1) whether women perceive
variation in the disease threat and (2) whether female
preferences for male body hair are driven by individual
susceptibility to pathogens, the participants were asked to
rate the disgust of five disease-relevant and five control
(disease-irrelevant) invertebrates on a seven-point Likert
scale (1=not at all, 7=extremely disgusting). Pre vious
research indicated that disgust sensitivity is related to actual
susceptibility to disease (Stevenson et al. 2009; Prokop and
Fančovičová 2011). These ratings were only performed in
the post-exposure phase.
In the pre- and post-exposure phase, the participants
were presented with PowerPoint presentation slides pro-
jected on a screen with forced-choice paired image trials of
20 males with and without trunk hair and consequently
asked to choose between the hairy and the completely bare
(shaved) version of the same body. In order to exclude the
effect of skin irritation caused by shaving, the photographs
were black-and-white similarly as in Rantala et al.’s(2010)
study. The pairs of pictures as well as the presence of a
picture within each pair (to the left of the right side) in each
phase were presented in a random order. In each trial, the
participants were asked to choose the image that they found
sexually most attractive. The participants were not told
whether the males in the pairs were identical or not, or
whether the photographs presented in the pre- and post-
exposure phase were the same. No other information
regarding why the slideshow was presented was provided.
In the exposure phase, participants saw a slideshow of
ten images repeated three times (for a total of 30 images).
The high pathogen group was presented with pictures with
cues of a high incidence of pathogens, while the low
pathogen group was presented with pictures with cues of a
low incidence of pathogens. The images were presented for
3 s each (for a total of 90 s of exposure) with instructions
following Little et al. (2011): ‘Please try and look at these
images carefully’. The image order in the pre- and post-
exposure phase was randomized.
Statistical methods
A generalized linear mixed model (GLMM) with a logit
link function was used to examine how the treatment (high
pathogen/low pathogen) and the phase (pre-exposur e/post-
exposure) influenced preferences for male trunk hair
(dependent variable with binomial distribution, 0 for hairy
and 1 for bare). The participant ID and picture ID were
treated as random factors in order to take into account the
pseudoreplication of the data. The correlation between the
disgust of invertebrates and prefer ence for male trunk hair
was calculated by using the percentage of bodies with trunk
hair chosen out of the 20 pairs of male bodies. These data
could not be included into the GLMM because they were
only gathered in the post-exposure phase.
Results
Figure 2 shows a preference for male trunk hair suggesting
that the probability of choosing a bare body is different
from 0.5. Bare bodies were preferred more than males with
trunk hair in both the pre-exposure and post-exposure tests.
1. Exposure to visual cues of pathogens. The Cronbach
alpha for the pre- and post-exposure phase was high
Fig. 1 Paired photographs of a
male body before (a) and after
(b) the removal of body hair.
The photographs were presented
to women in the forced-choice
trial
acta ethol (2012) 15:135–140 137
(0.80 and 0.82, respectively). The test–retest reliability
was also high (Guttman split-half reliability=0.90),
indicating that environmental exposure to pathogens
has no influence on mate preferences. GLMM resulted
in a nonsignificant model (F
3,3516
=1.19, p=0.31).
Neither the effect of the treatment and the phase nor
the interaction between the variables influenced the
preference for male trunk hair (F
1,3516
=1.793, 1.39 and
0.21, all p>0.18, respectively). This is not due, however,
to the fact that women fail to perceive a variation in the
disease threat. Disease-relevant invertebrates were rated as
more disgusting than the control invertebrates in both high
pathogen (M=25.67±0.75 vs. 10.81±0.65, paired t=
16.55, df=47, p<0.001, respectively) and low pathogen
conditions (M=28.48±0.69 vs. 10.90±0.75, paired t=
19.18, df=39, p<0.001, respectively). Participants in high
pathogen conditions rated disease-relevant invertebrates as
less disgusting than participa nts in low pathogen con-
ditions (M=25.67±0.70 vs. 28.48±0.76, t=−2.71, df=86,
p=0.008, respectively), whereas the control animals were
rated similarly in both conditions (M=10.81±0.67 vs.
10.9±0.73, t=−0.09 , df=86, p=0.93, respectively). This
indicates that visual exposure to pathogens leads to
habituation to disease-relevant stimuli.
2. The influence of individual sensib ility to pathogens for
chest hair preferences. In light of the fact that the perceived
disgust of invertebrates was measured in the post-exposure
phase, only the post-exposure score of preferences for male
trunk hair was included in the subsequent statistical
analyses. There were no correlations betwee n the disgust
ratings of disease-relevant or control invertebrates and
trunk hair preference in the post-exposure test neither in
high pathogen (r =0. 05 and 0.13, bot h p >0.37,
respectively) nor in low pathogen conditions (r=−0.23
and −0.09, both p>0.15, respectively). When participants
in both conditions were divided into two groups with high
andlowdisgustsensitivitytodisease-relevant inverte-
brates according to the median split, we failed to discover
any differences in the preference of male trunk hair in the
post-exposure test between these groups neither in the
high (M=23.13±4.24 vs. 28.33±4.24, t=0.87, df=46,
p=0.39, n
1
=n
2
=24, respectively) nor in the low pathogen
group (M=22.50±4.29 vs. 26.50±4.29, t=0.66, df=38,
p=0.51, n
1
=n
2
=20, respectively).
Discussion
This study examined the salience of ectoparasite avoidance
mediated by the mate choice hypothesis (Pagel and Bodmer
2003; Rantala 1999, 2007) in order to explain the loss of
body hair in humans. Female preferences for the male chest
were quantified based on their recent experience with visual
cues to environmental pathogens. When considering that
hairlessness could have a natur ally selected advantage, it
has been speculated that increasing the reproductive success
of hairless women, who were in their home bases under a
heavier ectoparasite threat than males (Rantala 1999 , 2007),
resulted in stronger preferences of hairless women by men
(Rantala 2007; Pagel and Bodmer 2003).
We predicted that mate choice preferences were flexible
and can quickly respond to subtle exposure to visual stimuli
during adulthood (DeBruine et al. 2010a, b; Little et al.
2011). Our prediction was not supported, however, as
female preferences for hairless male bodies were not
influenced by exposure to visual cues to environmental
pathogens, which contrasts with other research works on
preferences for masculine traits (Gangestad and Buss 1993;
Penton-Voak et al. 2004; DeBruine et al. 2010a,b; Little et
al. 2011). It is possible that the ectoparasite avoidance
mediated by the mate choice hypothesis is not applicable to
the male chest at least in an environment with a low
pathogen threat. This possibility remains open as preference
for the male chest varies across cultures (Dixson et al.
2003, 2007a, b, 2010), and cultures are influenced by
parasites (e.g. Gangestad et al. 2006; Quinlan 2007; Fincher
and Thornhill 2008; Thornhill et al. 2009), suggesting that a
preference for the male chest would be influenced by the
parasite threat. In Slovakia, where our experiment was
performed, the estimated pathogen prevalence and richness
are relatively lower than in countries closer to the equator
(Guernier et al. 2004; Murray and Schaller 2010 ; Prokop et
al. 2010); thus, sensiti vity to pathogens is expected to be
relatively lower. In such an environment which is almost
free of parasites, male hairiness is expected to be stronger
than in environments rich in parasites. Further cross-
cultural research is needed to examine female sensitivity
to pathogens and whether male hairiness correlates with the
parasite threat.
Fig. 2 Differences in preference for naked bodies before (open bars)
and after exposure (grey bars) to high and low pathogen conditions.
Error bars are 95% confidence intervals. NS not statistically significant
138 acta ethol (2012) 15:135–140
The present study indicated that females consistently
preferred the torsos of shaved males more than the torsos of
hairy males, suggesting that the male chest has a negative
effect on female ratings of attractiveness (Dixson et al. 2007a;
Rantala et al. 2010; but see Dixson et al. 2003) and
supporting the idea that the male chest plays a role in mate
choice. This, however, does not seem to result from the
perception of hair as indicative of the pathogen load. If male
chests are cues to parasite threat, then the correlation
between the disgust for disease-relevant insects and the
preference for shaved male bodies is expected, although no
evidence was obtained in the present study. When consider-
ing research on disgust sensitivity, it was determined that
repeated contact with disgusting stimuli obviously results in
habituation (Rozin 2008; Adams et al. 2011), suggesting that
potentially disgusting stimuli (i.e. male bodies with chest
hair) should show the same or a higher preference in the
second exposure. We found that repeated exposure to the
male chest resulted in a similar preference and that females
in the high pathogen treatment rated disease-relevant
invertebrates as less disgusting than females from the low
pathogen group supporting the effect of habituation.
However, the women in our study who were the most
disgusted by disease-relevant invertebrates showed no
stronger preference for shaved male bodies than their less
sensitive counterparts All these evidences suggest that
female perception of male trunk hair is not psychologically
associated with the risk of contamination.
As far as we are aware, this is the first study which
investigated the origin of human hairlessness from an
evolutionary psychol ogical perspective. Our results provide
no evidence for ectoparasite avoidance mediated by the
mate choice hypothesis as the experience with visual cues
to environmental pathogens resulted in similar preferences
for shaved male bodies as in the control group. Moreover, it
seems that sensitivity to disgust does not mediate prefer-
ences for male trunk hair, suggesting that mal e body hair is
not associated with the risk of contamination. Future
research should investigate preference for body hair in
humans with data from larger, more diverse samples and
particularly among cultures that differ in risk of parasite
infection.
Acknowledgements We would like to thank Alexandra Alvergne
and two anonymous referees for their insightful comments on earlier
drafts of this manuscript. David Livingstone improved the English.
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