Gorillas in Our Midst? Human Sexual
Dimorphism and Contest Competition
, D.H. Bailey
and D.A. Puts
University of Washington, Seattle, WA, United States;
University of California, Irvine, CA, United States;
The Pennsylvania State University,
University Park, PA, United States
Around 500 BCE, the Carthaginian explorer Hanno the
Navigator sailed south with a ﬂeet of 60 ships along
the northwest coast of Africa, encountering an island people
he reported were comprised mostly of women. The men
managed to escape, but Hanno’s expedition captured three
women who fought back so violently that they were
executed by their abductors, their skins carried back to
Carthage. Apparently oblivious to the hypocrisy, Hanno
referred to these people as savages. His interpreters called
them Gorillai,“tribe of hairy women”(Hanno, 1832). Over
two millennia later, in 1847, Americans Thomas Savage and
Jeffries Wyman borrowed this term when ﬁrst describing the
western gorilla (Conniff, 2009). Savage, Wyman, and the
actions of Hanno’s expedition were products of their times
and easily strike the 21st century reader as racist and vile.
We will revisit the human tendencies to classify others as
in-group versus out-group, potentially regard out-group
members as subhuman, and aggress against them, in our
discussion of coalitional aggression in a later section.
Returning for the moment to gorillas, we note that
research on these intelligent, elusive apes has revealed a
wealth of information about their ecology and behavior.
One of the most conspicuous aspects of gorilla social
structure is that males aggressively defend groups of
females from other males using their massive size, long
canine teeth, and threat displays (Fossey, 1983; Maple and
Hoff, 1982). Such traits are central to the behavioral
repertoire of male gorillas, who experience what Charles
Darwin termed sexual selection, a type of natural selection
engendered by competition over mates (Andersson, 1994).
Since the publication of Darwin’sThe Descent of Man
in 1871, a vast literature has accumulated demonstrating the
salience of sexual selection in shaping the phenotypes of
sexually reproducing organisms, including gorillas and
humans. Sometimes referred to as “Darwin’s other idea,”
sexual selection favors traits helpful in winning mating
opportunities and is effectuated to varying degrees across
species by a handful of mechanisms (Andersson, 1994).
These mechanisms include mate choice, which favors traits
desired by the opposite sex; sexual coercion, the use of
force or threat against mates; sperm competition, the post-
copulatory struggle for fertilization of ova that favors traits
such as increased sperm production and motility; and
contest competition, favoring traits that help win mates
through same-sex physical contests and/or threatening
displays directed at rivals.
Much research has focused on mate choice, though
recent years have seen a reevaluation of sexual selection in
humans with an increase in attention to contest competition,
whose inﬂuence now seems to have been underestimated
(eg, Carrier and Morgan, 2015; Hill et al., 2013; Puts, 2010,
2016; Puts et al., 2015; Scott et al., 2012). In this chapter, we
consider data bearing on the inﬂuence of contest competition
on men’s phenotypes, but ﬁrst we evaluate evidence for the
overall intensity of sexual selection among ancestral men.
STRENGTH OF SEXUAL SELECTION IN
A fundamental reality of reproduction in many sexually
reproducing species is an asymmetry in parental
On Human Nature. http://dx.doi.org/10.1016/B978-0-12-420190-3.00015-6
Copyright ©2017 Elsevier Inc. All rights reserved.
investment: one sex, usually females, invests more in pro-
ducing and caring for offspring (Trivers, 1972). Women,
for example, require approximately nine months for
gestation, followed by a long period of lactation and
childrearing, while a contrastinglysmallparentaleffortis
required for men to reproduce (Eibl-Eibesfeldt, 1989;
Geary, 2000). As a result, men have a higher potential
reproductive rate (Clutton-Brock and Vincent, 1991), and
in natural fertility populations, there are at all times more
women (Trivers, 1972). This skews the operational sex
ratio (OSR), the number of sexually active males per
fecund female, in a way that tends across species to force
males into competition for the relatively few available
females (Clutton-Brock and Vincent, 1991; Emlen and
The physiological OSR, which includes all individuals
capable of reproducing, can be distinguished from the
behavioral OSR, which includes only those engaged in
mating (Marlowe and Berbesque, 2012). This may be an
important distinction because the amount of time in-
dividuals are typically eligible as mating partners may exert
a greater effect on sexual selection than does the amount of
time individuals are physiologically capable of reproducing
(Kokko and Jennions, 2008; Kokko et al., 2012). The hu-
man behavioral OSR has been estimated to be 8.6, and the
physiological OSR has been estimated to be 11.7 (Marlowe
and Berbesque, 2012). Regardless of which measure is
used, however, the human OSR falls below orangutans
(OSR ¼55.0) and gorillas (OSR ¼83.8), which display
the highest degrees of sexual dimorphism in the primate
order, and above promiscuously mating chimpanzees
(OSR ¼4.5), which also exhibit substantial individual and
coalitional contest competition, as well as a high degree of
sperm competition (Mitani et al., 1996). Hence the human
OSR indicates the opportunity for moderately strong sexual
selection in men.
The intensity of sexual selection is also inﬂuenced by
the spatial distribution of males and females, which affects
whether mates can be monopolized. Across species, female
reproduction is typically limited by access to food, whereas
male reproduction is limited by access to mates. Hence,
females generally distribute themselves according to
resource availability, as well as predation risk, and males
then distribute themselves according to the distribution of
females (Lindenfors et al., 2004). If females are social or
otherwise spatially clustered, then one or a few males
may be capable of monopolizing multiple females, and
competition to be one of the few breeding males will tend
to be intense (Emlen and Oring, 1977). However, if female
group sizes are large, then it may be infeasible for a smaller
number of males to defend them from other males, and
larger multimale/multifemale groups will emerge. Male
mating competition is often intense in such species,
frequently taking the form of male dominance hierarchies
in which dominant males gain more mating opportunities,
especially with estrous females. However, in primates,
canine size dimorphism and body size dimorphism tend to
be less extreme in multimale/multifemale species than in
species with single-male polygyny (Clutton-Brock and
Harvey, 1984), probably in part reﬂecting less intense
Although humans form large multimale/multifemale
groups, the human mating system is not typical of such
primates, as concealed ovulation prohibits males from
attempting to monopolize estrous females (Gangestad and
Thornhill, 2008; Puts et al., 2013). Perhaps as a conse-
quence, within these larger groups are embedded smaller
groups of individual males mated in relatively stable and
exclusive relationships to one or more females. These
mateships exhibit moderate polygyny. For example, on
average, 21% of married women are married polygynously
across forager societies, although the degree of polygyny
varies substantially across cultures and ecological contexts
(Marlowe and Berbesque, 2012). As in hamadryas baboons
(Schreier and Swedell, 2009), males’proprietariness over
their mates helps maintain these embedded “harems”(Daly
and Wilson, 1988).
Males’ability to monopolize females may also be
contingent on the dimensionality of the mating environ-
ment (Puts, 2010). Males may more effectively monopolize
females in environments that are one-dimensional (eg,
burrows or tunnels) or two-dimensional (eg, land or ﬂoors
of bodies of water), as opposed to three-dimensional
environments, such as air, open water, or trees, where the
region that must be defended is larger, and there are many
in-routes for sexually interloping males. As a result,
compared to three-dimensional mating environments,
terrestrial environments may favor greater physical
competition for mates, as in terrestrially breeding seals
(Stirling, 1975) and turtles (Berry and Shine, 1980) relative
to aquatically breeding species. Hominins have evolved in
a two-dimensional environment at least since the
emergence of habitual bipedalism in Australopithecus
anamensis roughly four million years ago (Harcourt-Smith,
2007). This shift in the human paleoenvironment likely
played an inﬂuential role in shaping mating behavior in our
lineage. In particular, the new terrestrial environment may
have enabled ancestral males to restrict sexual access to
mates to a degree that was infeasible in their more arboreal
Male mating competition also tends to increase with
reproductive variance among males compared to females
(Bateman, 1948). In humans, male reproductive variance
exceeds that of females but this difference is highly variable
across populations (Brown et al., 2009). For example,
reproductive maxima have been reported as 8 and 14 for
women and 23 and 43 for men among two populations of
lowland South American Indians (Chagnon, 1992; Salzano
et al., 1967), an approximately three-fold increase among
236 PART | I Biological Basis of Human Diversity
males. However, the concentrations of wealth and power
afforded by state-level societies have produced staggering
reproductive variances. According to the Guinness Book of
World Records, the largest number of children known to
have been produced by a single woman is 69, while that for
a man exceeds 1000 (Glenday, 2013). Men who are able to
compete successfully for mates have the potential to pro-
duce a large number of offspring, whereas others may not
reproduce at all. Women, by contrast, accrue less additional
reproductive success from acquiring multiple matesdat
most, they can reproduce approximately once per year, but
interbirth intervals span several years in traditional societies
(Blurton Jones, 1987).
It is thus unsurprising that societies allowing polygy-
nous marriage are far more common in the ethnographic
record than are those with only monogamous marriages
(Murdock, 1967). Yet, even in societies with exclusively
monogamous marriage, the mating system may neverthe-
less tend toward polygyny; men remarry (Buckle et al.,
1996) and reproduce (Jokela et al., 2010) after divorce at
higher rates than women (though see Borgerhoff Mulder,
2009), with reproductive variances comparable to those
seen in societies with polygynous marriage (Brown et al.,
2009). This pattern is not restricted to the West, the
industrialized world, or the Global Northdmen exhibit
greater reproductive variance than do women among some
serially monogamous hunter-gatherers, as well (Hill and
The previously mentioned evidence suggests a robust
potential for sexual selection in shaping men’s anatomy and
reproductive behavior. Women provide greater levels of
parental investment through producing and rearing
offspring, removing themselves from the mating pool for
longer periods of time with each reproductive event. The
consequent imbalance in the OSR predicts increased
competition among males for mating opportunities. Men’s
potential to monopolize mates is reduced by women’s ag-
gregation into large groups but promoted by their terres-
triality, which enables some men to defend multiple mates.
As a result, men display greater reproductive variance than
do women. These lines of evidence are consistent with a
strong potential for sexual selection to shape men’s phe-
notypes. However, sexual selection also requires
nonrandom mating among men, such that men’s pheno-
types affect their mating opportunities. As we will now
discuss, ancestral men appear to have won mates
by developing traits that contributed to success in contest
SELECTION FOR MALE CONTESTS
When evaluating the inﬂuence of past sexual selection on a
trait, it is useful to consider several types of evidence
(Table 15.1). First, high levels of sexual dimorphism
suggest past sexual selection (Andersson, 1994; Darwin,
1871; Lande, 1980). Second, traits that function in mating
competition are often costly to produce and maintain, and
so often develop or increase in their expression at sexual
maturity when they can begin to compensate for these
costs. Third, if a trait affects success in one or more
mechanisms of sexual selection (eg, by attracting mates or
winning ﬁghts with same-sex competitors), then this
suggests that the trait did so over its evolution. This may be
demonstrated experimentally (eg, Andersson, 1982), or by
providing correlational evidence that the trait inﬂuences
success under a mechanism of sexual selection
(eg, Schwagmeyer and Woontner, 1986). Fourth, if a trait
affects mating success, then this indicates that it did so over
its evolution. Again, experimental manipulations provide
stronger evidence of causation (eg, Andersson, 1982), but
correlational data can also provide evidence of a trait’s
inﬂuence on mating success and may have the advantage of
increased ecological validity.
None of these lines of evidence is sufﬁcient by itself.
For example, different ecological selection pressures such
as niche partitioning can produce sexual dimorphisms,
although this appears to be relatively rare (Andersson,
1994), and sexual selection may sometimes act similarly in
the two sexes producing low sexual dimorphism (Hooper
and Miller, 2008). In addition, if relevant aspects of the
environment have changed, then a trait may currently
inﬂuence mating success differently than it did ancestrally.
However, in combination, these lines can provide strong
evidence for past sexual selection. If a trait develops at
sexual maturity, is sexually dimorphic, and appears to
inﬂuence mating success through one or more mechanisms
of sexual selection, then it is likely that the trait was shaped
by sexual selection through these mechanisms.
Research exploring the inﬂuence of sexual selection on
men’s traits often emphasizes the role of female mate
choice, but this emphasis may be based partly on
misperceptions regarding the freedom with which women
chose mates ancestrally (see, eg, Puts, 2010). Given the
latitude with which women seem to choose mates in
contemporary Western societies, it may surprise some
readers that ancestral women probably experienced far less
autonomy. For example, more than two-thirds of extant
hunter-gatherer groups in a large sample had parentally
arranged marriages (Apostolou, 2007), and while women
may sometimes circumvent kin limitations on partner
TABLE 15.1 Evidence of Past Sexual Selection on a
lDevelops or increases in expression at sexual maturity
lAffects success in one or more mechanisms of sexual
lAffects mating success
Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 237
choice (eg, via “mock bride theft”;Ayres, 1974),
phylogenetic analyses suggest that family members
inﬂuenced mating among the earliest members of our
species (Walker et al., 2011). Moreover, constraints on
female choice are imposed not only by kin but also by
unrelated men via sexual coercion (Puts, 2016; Puts et al.,
2015a), and especially by men’s exclusion of male rivals by
force or threat (eg, Hill et al., 2013).
Indeed, the weight of evidence suggests that many
aspects of the human male phenotype are best explained by
an evolutionary history of contest competition. Contests
tend to produce several types of traits that aid in excluding
same-sex competitors by force or threat, including
behaviors such as same-sex aggression and threat displays,
and anatomical traits such as large body size and anatom-
ical weapons (Table 15.2). If any of these traits is present,
then it is a good candidate to examine for evidence of
having been shaped by sexual selection via contests. As we
will see, men tend to exhibit all of the hallmarks of a
species that has experienced an evolutionary history of
male contest competition.
Across cultures, men and boys are more physically
aggressive than are girls and women (Archer, 2004; Ellis
et al., 2008), and men’s aggression peaks at the age of
greatest mating competition (Archer, 2009). Murder in
particular is overwhelmingly a male phenomenon. Men
greatly outnumber women in both killing and being killed
everywhere that homicide has been studied (Daly and
Wilson, 1988), including both subsistence societies
(Beckerman et al., 2009; Chagnon, 1988; Walker and
Bailey, 2013) as well as industrialized Western nations
(Daly and Wilson, 1990). For example, in the Yanomamo,
horticulturalists living in parts of Brazil and Venezuela,
Chagnon (1988) estimated violence to account for nearly
one-third of male deaths, while Walker and Bailey (2013)
found the mean percentage of violent deaths among 11
traditional lowland South American societies to be 30%.
Males, moreover, accounted for 69% of all such deaths,
with comparable ﬁgures seen across the societies in the
sample. From a wide array of human populations, Daly and
Wilson (1988) report 95% of same-sex killings to be
perpetrated by men, even when war-related homicides are
excluded from analysis. This sex difference holds across
populations even as the number of homicides changes
dramatically from one to another (Daly and Wilson, 1990).
As Wright (1995, p. 72) has noted, “From an evolutionary
point of view, the leading cause of violence is maleness.”
Of course, a male bias in physical aggression and
violence, especially a bias in male-on-male aggression and
violence, is predicted from the hypothesis that ancestral
men competed for mates via contest competition. Physical
aggression may have helped men obtain or defend mates
directly, for example, by killing or injuring a competitor
attempting to win the same mate (Marlowe, 2004), or
attempting to steal one’s mate (Chagnon, 1992), but these
may have more frequently been accomplished indirectly
through the threat of physical harm (see later in this
Another possible set of behavioral adaptations that may
allow males to remain physically aggressive during contests
relates to pain thresholds and tolerance. Physical pain sig-
nals actual or potential tissue damage and can motivate
withdrawal from a damaging situation and protection of a
damaged body part (Lynn, 1984). However, withdrawing
from injurious behavior imposes costs if the behavior is
potentially ﬁtness enhancing. In such situations, organisms
face a tradeoff between avoiding tissue damage and
missing opportunities to increase ﬁtness. The relative
weights of the associated costs and beneﬁts will vary be-
tween individuals and across contexts. Given stronger
sexual selection and greater reproductive variance among
men compared to women, men are generally expected to
engage in costlier, more injurious behaviors in contexts that
can augment mating success, such as contest competition.
Ancestral men who disengaged from or avoided contests
may have suffered fewer injuries but left fewer offspring.
The experience of less pain for a given stimulus may
represent a proximate mechanism shaped by sexual selec-
tion to facilitate continued engagement in contests despite
injury. If so, then men would be expected to exhibit less
pain sensitivity and greater pain tolerance, perhaps espe-
cially in competitive contexts.
These predictions have been borne out. Relative to
women, men can undergo more intense stimulation such as
physical pressure on the body for longer periods of time
before experiencing pain, they are able to tolerate more
pain, and these effect sizes are moderate to large (Fillingim
et al., 2009; Riley et al., 1998). Moreover, in one study,
men experienced analgesia after competition against a
same-sex competitor regardless of exercise, whereas
women experienced analgesia only after exercise (Sternberg
et al., 2001).
Anatomical Adaptations for Male Contests
While observational studies of aggression among primates
provide the most direct evidence of contest competition,
another relevant line of evidence concerns body size sexual
TABLE 15.2 Traits Favored by Contest Competition
lLarge body size
lBehavioral displays of formidability
lAnatomical threat displays
238 PART | I Biological Basis of Human Diversity
dimorphism. In a sample of 18 species of anthropoids
representing 12 genera, Mitani et al. (1996) conﬁrmed the
relationship between body size dimorphism and OSR ex-
pected from sexual selection theory. In the human lineage,
sexual size dimorphism exhibits great antiquity revealed
through a large, albeit fragmentary, hominin fossil record
dating back millions of years (Plavcan, 2012). Yet, research
on levels of sexual dimorphism in earlier hominins is
equivocal. For example, the best-represented fossil homi-
nin, Australopithecus afarensis, which lived roughly 3e
4 million years ago, has been alleged by some researchers
(Gordon et al., 2008; Lockwood et al., 1996; McHenry,
1991), but not others (Reno and Lovejoy, 2015; Reno et al.,
2010; Reno et al., 2003), to have exhibited a level of sexual
dimorphism commensurate with that of extant gorillas and
By the time of Homo erectus, whose existence covers
most of the last 2 million years, sexual dimorphism had
reached the approximate levels of modern Homo sapiens
(Antón, 2003). Among modern humans, skeletal
dimorphism is consistent with a primate species in which
males are 45%e50% larger (Gordon et al., 2008).
Compared to women, men possess 31%e43% more
fat-free body mass (Lassek and Gaulin, 2009; Wells, 2012),
61% more muscle mass overall, and 75% more upper-body
muscle mass (Abe et al., 2003; Lassek and Gaulin, 2009).
As a result, the average man is stronger than 99.9% of
women (Lassek and Gaulin, 2009).
Men with a masculine, muscular body shape have
more sex partners (Frederick and Haselton, 2007; Hill et al.,
2013; Lassek and Gaulin, 2009), particularly in short-term
relationships (Rhodes et al., 2005), and begin having sex
at an earlier age (Hughes and Gallup, 2003) than do less
masculine men. Larger men, both in terms of height
(Frederick and Jenkins, 2015; Mueller and Mazur, 2001)
and body mass (Frederick and Jenkins, 2015), also report
more sex partners. Although mating success does
not necessarily translate into reproductive success among
natural fertility populations, positive relationships have
been observed between men’s reproductive success and
both height (Mueller and Mazur, 2001; Pawlowski et al.,
2000) and physical prowess (Chagnon, 1988; Smith
et al., 2003).
Men also appear designed to weather bodily insults,
particularly to the head, which is disproportionately
targeted (Shepherd et al., 1988) and injured (Carrier and
Morgan, 2015)inﬁghts. Brink et al. (1998) examined 2432
bodily injuries in 1156 men and 325 women in Denmark
for a one-year period in the mid-1990s, reporting 69% to
have been craniofacial, with injuries in both sexes tending
to be produced by blunt force at close range. Indeed, sexual
dimorphism in cranial robusticity may partly be attributable
to physical violence among men, as features such as more
robust mandibles and brow ridges may protect against
catastrophic facial fractures (Carrier and Morgan, 2015;
Puts, 2010). Women generally do not experience
commensurate levels of physical aggression (Campbell,
2013; Daly and Wilson, 1988) and display less cranial
robusticity and fewer cranial injuries (Carrier and Morgan,
2015; Shepherd et al., 1988).
Contest competition often favors the evolution of
anatomical weapons, such as antlers, horns, and, in pri-
mates, large canine teeth. Yet, compared with our closest
living relatives, both men and women possess relatively
small canines, and we lack substantial canine-size sexual
dimorphism (Wood et al., 1991). Diminution in canine size
and a departure from the CP
honing complex, a typical ape
feature, begins with one of the earliest hominin candidates
(7 to 6 Ma), Sahelanthropus tchadensis (Brunet et al.,
2002), and continues through the Pliocene genera Ardipi-
thecus (Suwa et al., 2009) and Australopithecus (White
et al., 2000)toHomo after 2.5 Ma (Suwa et al., 2009).
One hypothesis for reduction in canines and other
skeletal features related to biting is that canine weaponry
was supplanted by handheld weapons and forelimbs freed
by bipedal locomotion (Carrier, 2011; Darwin, 1871;
McHenry, 1991). Clubs, spears, and hurled stones may
have obviated biting by keeping enemies at a distance in the
way that antlers appear to have replaced large maxillary
canines in several deer species (Barrette, 1977). In addition,
our shift to habitual bipedalism and its associated
orthograde posture enhanced the injuriousness of physical
blows (Carrier, 2011).
Male chimpanzees make and use tools, including using
branches in dominance displays (but not as offensive
weapons; van Lawick-Goodall, 1968), suggesting that tools
have been used since the last common ancestor of Pan and
Homo. While the earliest evidence of a weapon-inﬂicted
wound has been dated to roughly 100,000 years ago
(Pickering et al., 2000), the emergence of manufactured
stone tools by at least 3.4 million years ago (Harmand et al.,
2015; McPherron et al., 2010) suggests that handheld
weapons were used far earlier. Indeed, it is difﬁcult to
imagine an ancestral species with the mental capacity to
shape stones for use as cutting tools, and with males
ﬁghting over mates, in which males would not also utilize
branches, bones, antlers, and other materials in their
environment as weapons.
The use of projectile weapons such as hurled rocks and
spears may have contributed to the very large male advan-
tage in throwing velocity (3.5 standard deviations by age 12
years; Thomas and French, 1985), as well as men’s1.5
standard deviation advantage in targeting and avoiding
projectiles (Watson and Kimura, 1991). This targeting dif-
ference remains large after controlling for experience
(Watson and Kimura, 1991) and appears to depend
developmentally on early androgen exposure (Hines et al.,
Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 239
2003). Across societies, the manufacture and use of weapons
against same-sex rivals is ubiquitous among men and rare
among women (Archer, 2004; Ellis et al., 2008; Smith and
Smith, 1995; Warner et al., 2005).
The development of handheld weapons represented a
watershed moment in hominin evolution, imbuing physical
contests among men with a previously unknown degree of
lethality. Moreover, for the ﬁrst time in primate evolution,
individuals were able to aggress from a distance, reducing
the advantage of anatomical weaponry and possibly
spurring the diminution of sexual dimorphism in body, and
especially canine, size. This inference has important
implications: if the use of handheld weapons is responsible
for reduction in the typical trappings of primate intrasexual
selection, then an examination of body size dimorphism
and canine size dimorphism may lead us to underestimate
the intensity of contest competition over the evolution of
Traits such as physical aggression, pain tolerance,
size, strength, facial robusticity, and weapons use may have
served to make men more competitive in physical
contests. However, physical violence is costly energetically
and in terms of risk of injury or death, as well as risk of
retribution (eg, Beckerman et al., 2009; Daly and Wilson,
1988). Across species, male contests frequently involve
displays and mutual assessment of formidability, often
ending when one rival submits before either is injured (see
Smith and Parker, 1976 for a discussion of asymmetric
contests). While the costs of submission in terms of reduced
social status and mating opportunities may be high, the
costs of defeat may be higher. As a result, ancestral
men capable of accurately assessing rivals’physical
formidability likely obtained a selective advantage (Sell
et al., 2009, 2010; Wolff and Puts, 2010). Because
maleemale aggression is culturally ubiquitous (Daly and
Wilson, 1988) and characterizes all extant apes (Puts,
2010), we can be conﬁdent that there was selective pressure
for such acuity ancestrally.
Men are therefore expected to attend closely to
the formidability and volatility of their same-sex competi-
tors, to exercise caution accordingly, and to use nonviolent
means such as threats and negotiation to obtain status
and valued resources. Men appear to utilize cues such
as facial appearance (Carrier and Morgan, 2015; Sell et al.,
2014; Sell et al., 2009; Zilioli et al., 2014), muscularity
(Hill et al., 2013), and height (Stulp et al., 2015)to
assess one another’s formidability. However, some traits
that inﬂuence dominance perceptions may have been
shaped by selection speciﬁcally to signal formidability, as
we now discuss.
Behavioral Displays of Formidability
Men may avoid potentially deadly conﬂict by displaying
their formidability to rivals in diverse ways, including
greater risk-taking behavior when peers are present
(Ginsburg and Miller, 1982; Morrongiello and Dawber,
2004) and when those peers are male (Ermer et al., 2008).
In support of this, research has associated dangerous
risk-taking with perceptions of physical formidability
(Fessler et al., 2014), suggesting that risk-taking and acuity
to risk-taking may have evolved for success in contests.
Among the Yanomamo, contests often involve “shout-
ing matches, chest pounding duels, side slapping duels,
club ﬁghts, ﬁghts with axes and machetes, and shooting
with bows and arrows with the intent to kill”(Chagnon,
1988, p. 986). Among the Meriam of Australia, the hunting
of large sea turtles is physically demanding, potentially
injurious, and may function to signal formidability to other
men (Bliege Bird et al., 2001; Smith et al., 2003). Optimal
foraging and reciprocal altruism are unlikely to explain
turtle hunting behavior. Turtle hunting is inefﬁcient as a
means of procuring food, hunters typically give away the
meat at feasts, and this altruism tends not to be recipro-
cated. In addition, women do not report greater attraction to
turtle hunters, so turtle hunting does not appear to function
in mate attraction either (Smith et al., 2003). However,
turtle hunting is respected by men, and turtle hunters report
earlier onset of sexual behavior and larger numbers of sex
partners than nonhunters, as well as 2.4 times greater life-
time reproductive success, with an even larger difference
for hunt leaders (Smith et al., 2003).
Men’s greater average interest in playing and observing
sports, both in the contemporary United States (Deaner
et al., 2012) and across traditional societies (Deaner and
Smith, 2013), may also reﬂect selection for displays of
formidabilitydas well as for physical aggression, interest
in competition, and the predisposition to engage
in activities that build strength and hone skills useful in
contests. The male bias in sports participation is striking:
in a sample of 50 societies taken from the Human Relations
Area Files, males participated in 95% of all sports, females
in only 20%, with men predominating especially in
combat-related sports (Deaner and Smith, 2013).
Anatomical Threat Displays
Humans are among the most visually sexually dimorphic
primates (Dixson et al., 2005), and recent research suggests
that at least some conspicuous traits may function to
increase men’s appearance of formidability. For example,
beards and eyebrow hair grow at puberty in males and may
signal formidability through associations with physical
maturity and testosterone levels and by increasing the
apparent size of the jaw and brow (Guthrie, 1970;
240 PART | I Biological Basis of Human Diversity
Muscarella and Cunningham, 1996; Neave and Shields,
2008). Male faces with beards are rated as more dominant
but not more attractive than the same faces clean-shaven
(Dixson and Vasey, 2012; Muscarella and Cunningham,
1996; Neave and Shields, 2008).
Likewise, both correlational (Hodges-Simeon et al.,
2010) and experimental (Feinberg et al., 2005; Puts et al.,
2006, 2007; Wolff and Puts, 2010) research shows positive
relationships between vocal masculinity, such as low pitch
and vocal timbre, and perceptions of men’s dominance.
Men’s vocal tracts are 15% longer, and their vocal folds
60% longer, than women’s(Fant, 1960; Titze, 2000),
several times the 7%e8% expected from the sex difference
in stature (Gaulin and Boster, 1985). Elevated testosterone
levels at puberty cause males’vocal folds to grow longer
and thicker than those of females, both absolutely and
relative to overall body growth (Harries et al., 1997;
Hollien et al., 1994). Men’s larger vocal folds consequently
vibrate at a fundamental frequency approximately half that
of females during phonation, which we perceive as a lower
pitch. Similarly, males’larynges descend a full vertebra
lower than females’at puberty (Fitch and Giedd, 1999),
producing a longer vocal tract and resulting in lower, more
closely spaced formant frequencies and a deeper, richer-
In a cross-cultural sample of voice recordings, men
accurately assessed physical strength from the voice even
when listening to unfamiliar languages (Sell et al., 2010).
Although pitch and timbre track body size within-sex only
modestly (González, 2004; Lass and Brown, 1978; Pisanski
et al., 2014; Rendall et al., 2005), masculine voices have
also been associated with physical aggressiveness, testos-
terone levels, and peer evaluations of ﬁghting ability (Hill
et al., 2013; Puts et al., 2012a). It may be the case that
masculine voices are reliable signals of dominance even
while masculine voices are only modestly associated with
any particular correlate of dominance.
Dominance, Mating, and Reproductive
Displays of formidability, whether behavioral or anatom-
ical, may have contributed to mating success among
ancestral men by increasing dominance (coerced social
status) and prestige (freely conferred deference; Henrich
and Gil-White, 2001). Displays of formidability may in-
ﬂuence prestige because dominant men can make strong
leaders and powerful allies and may also possess skills
worthy of emulation. Indeed, experimental evidence sug-
gests that social status is conferred upon dominant men in
proportion to their being viewed as likely to generate
beneﬁts for the group via within-group enforcement and
between-group representation (Lukaszewski et al., 2015).
Both dominance and prestige can thus aid in social
competition over all contested resources, including food
and territory, as well as mates (West-Eberhard, 1983).
Indeed, success in competition with other males has been
shown to increase men’s preferences for feminine female
mates (Welling et al., 2013), suggesting that success in
maleemale competition increases access to desirable sex
In non-Western samples, both dominance and prestige
have been associated with increased mating and reproduc-
tive success in men (Chagnon, 1988; Smith et al., 2003;
von Rueden et al., 2011). However, this does not imply that
sexual selection has favored high levels of unrestrained
male belligerence. As noted previously, physical aggres-
sion is costly and should be dependent upon context,
including the likelihood of defeat and the threat of retri-
bution. For example, among the extremely bellicose
Waorani of Ecuador, men who participated in the most
raids of other villages did not have more wives or offspring
(Beckerman et al., 2009). Although failure to avenge ho-
micides may be perceived as a sign of weakness among the
Waorani, raiding also brought immediate retribution
against the raider’s village. It is thus possible that a mod-
erate level of raiding represented the optimal balance be-
tween the costs of retribution and the costs of appearing
Among Western undergraduate students, a component
of mating successdnumber of sex partners in the past
yeardwas positively related to self-rated ﬁghting ability in
two samples (Wolff and Puts, 2010), and male acquain-
tances’ratings of ﬁghting ability, as well as size and
muscularity, in another sample (Hill et al., 2013). Displays
of physical competitive ability such as sports performance
(Faurie et al., 2004; Honekopp et al., 2007) and gang
membership (Palmer and Tilley, 1995) have also been
positively related to mating success.
In addition, traits that inﬂuence perceptions of domi-
nance predict mating and reproductive success. For
example, a masculine, dominant-sounding voice has been
associated with greater mating success in samples of US
undergraduates (Hill et al., 2013; Hodges-Simeon et al.,
2011; Puts, 2005), as well as with greater reproductive
success in a sample of Tanzanian foragers (Apicella et al.,
2007). Likewise, dominant facial appearance has been
found to predict eventual military rank and
reproductive success among military cadets (Mueller and
Males are more likely than females to kill and be killed by
conspeciﬁcs among our closest living relatives, chimpan-
zees (Wrangham et al., 2006), who, like humans, engage in
Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 241
coalitional aggression. Wrangham and Glowacki (2012,
p. 20) argue that humans generally conform to the pattern
seen in chimps: “consistent intergroup hostility, safe
killing, and beneﬁts from intergroup dominance.”Thus,
aggressive behavior among allied groups of males, which
presents early via boyhood competition (Geary et al.,
2003), has likely not only long been a feature of human life
(eg, Bamforth, 1994; Frayer, 1997), but may also be a
more primitive feature of our primate heritage. There are,
however, important distinctions that set humans apart.
Notably, hunter-gatherer groups display an ability for
peacemaking involving protracted periods of nonviolence
toward rival groups that is uncharacteristic of chimpanzees.
As Wrangham (1999, p. 18) writes, “Peace is the normal
human condition, in the sense that most human groups, for
most of the time, are not at war.”
A further difference is that chimpanzees engage in far
more overt aggression overall, but human aggression is
more often lethal, so that chimps and humans living in
subsistence societies exhibit similar levels of lethal
aggression (Wrangham et al., 2006). For example, the
Arnhem Land people of Australia are characterized by an
unusually high rate of physical aggression among human
populations (Wrangham et al., 2006) but nevertheless
display a rate of physical attack two orders of magnitude
below that of chimpanzees. In light of this, a comparison
with chimps seems apt only to a point in informing
our understanding of the possible evolutionary history of
human violence. Perhaps the lethality of human weapons
elevates the importance of threats, deference, and peace-
making in relation to physical attacks when negotiating
intragroup dominance hierarchies and intergroup conﬂict.
In addition, the substantial death tolls attributable to
violence recorded among traditional human societies occur
at a level of social complexity greater than physical contests
between two males. Of importance is the presence of
“organized and sanctioned group violence that involves
armed conﬂict, including confrontations that combatants
recognize may result in deliberate killing,”as Webster
(1998, pp. 313e314) has deﬁned warfare. Conﬂicts,
according to Webster, are perpetrated “with the intent of
maintaining the status quo or bringing about a shift of
power relations, usually the latter.”This deﬁnition is
similar to understandings of coalitional violence in
chimpanzees, which Wrangham’s (1999) imbalance-
of-power hypothesis argues is contingent on (1) hostility
and (2) power asymmetries among groups. The psycho-
logical traits that might be favored in the service of
coalitional aggression include, as Wrangham (1999, p. 23)
suggests, “a tendency to classify others as in-group or out-
group, to regard members of out-groups as potential prey,
to be alert to (or search for) power asymmetries between in-
group and out-group parties, and to be ruthless in attacking
out-group parties when the perceived power asymmetry is
While there are no doubt myriad proximate motivations
for organized group violence in our species, such as a
desire for slaves, territory, political control, revenge,
resolution of economic disagreements, and more fruitful
environments (eg, Keeley, 1996), selection ultimately
favors traits that contribute to reproduction. Hence, it is at
least parsimonious to hypothesize that reproduction lies at
the root of coalitional violence, as well. Even when a desire
for resources or political control is the immediate cause,
these desires may themselves have been forged in the ﬁres
of mating competition.
By way of raiding, men are able to forcibly procure
female mates through “bride theft,”which appears to be a
species-typical behavioral trait (Ayres, 1974). Across 10
traditional Amazonian societies, women were captured
during 26% of raids occurring within a language family and
54% of raids occurring across language families (Walker
and Bailey, 2013). Intergroup aggression among both
chimpanzees (Mitani et al., 2010) and humans (Bollig,
1990; Mathew and Boyd, 2014) may additionally enable
males to obtain territory and resources that contribute to
their mating success. Among human subsistence societies,
men appear well aware of what is in the reproductive
balance. As Chagnon (1988) reports of the Yanomamo, a
desire for women is the main impetus for engaging in
warfare, and, importantly, this is the top reason given by
Yanomamo, a ﬁnding not unique to that particular society.
While reasons for warfare and lesser forms of coalitional
violence are undoubtedly complex, the desire for mates is
acknowledged as a nearly ubiquitous motivation for
preindustrial warfare, even among scholars generally
unsympathetic to sociobiological theories of behavioral
evolution (eg, Keeley, 1996).
Male coalitional violence, regardless of its most
immediate cause, has produced an archaeological record
riddled with evidence of violent, often lethal, physical
aggression among men stretching back beyond the advent
of agriculture (Lahr et al., 2016). In one North American
paleoindian burial site, roughly 16% of skeletal remains
indicate violent death, 5% showing evidence of having
been scalped, and 4% decapitated (Milner et al., 1991).
Moreover, males account for a higher percentage of victims
of violence among all individuals of known sex. This is far
from an aberrant ﬁnding, with other burial sites yielding
similar results. Andrushko et al. (2005) estimated from a
burial site of 59 males and 86 females that at least 20% of
males, but only 2% of females, experienced a violent death,
likely the result of warfare, as evinced by perimortem
amputation. The men, furthermore, tended to be young
adults, which is the age range of ﬁercest competition for
242 PART | I Biological Basis of Human Diversity
Of course, the relationship between male coalitional
violence and reproductive success is not always linear and
positive. As noted earlier, among the Waorani of Ecuador,
Beckerman et al. (2009) report poorer reproductive success
as well as exceptionally high mortality rates among the
most ardent warriors. Among humans everywhere, there are
great costs associated with aggression (Chagnon, 1988).
This may result in a curvilinear relationship between
aggression and reproductive success, with a maximum that
is likely contingent on numerous aspects of the social
environment. There would have thus been great beneﬁt
ancestrally associated with correctly assessing the potential
costs and likelihood of success in a raid, just as there would
have been great beneﬁt in correctly assessing the physical
formidability of a single male rival. Just as there would be
costs to stealing another man’s mate, there are costs to
participating in a raid to steal the mates of many men. The
costs and beneﬁts associated with intragroup aggression
may have selected for high levels of intragroup cooperation
speciﬁcally in the context of warfare. Indeed, experimental
research has shown males to exhibit greater group-level
contribution in the face of competition from other groups
(Van Vugt et al., 2007).
ALTERNATIVES TO MALE CONTESTS
We have reviewed evidence that men’s phenotypes are
partly products of ancestral contest competition for mates.
Men exhibit each of the traits typical of species with male
contests, and these traits appear to have been shaped by
sexual selection; they are sexually dimorphic and predict
men’s mating success as well as success in contest
competition. Many of these traits (eg, deep voices, beards,
muscularity) also emerge at sexual maturity. The excep-
tions are behaviors that require years of practice to hone
relevant skills: ﬁghting, weapons use, behavioral displays
of formidability, and coalition formation (eg, Pellis and
Pellis, 2007; Thomas and French, 1985)dall of which
exhibit prepubertal sex differences, although the sexes may
further diverge at puberty. However, we have not yet
considered alternative hypotheses: whether some of the
previously mentioned traits were shaped by other selective
pressures, or arose as byproducts of selection on develop-
mentally correlated traits.
For example, some human sexual dimorphisms may
partly be products of a sexual division of labor that is
essentially ubiquitous across forager societies: men spend
more time hunting, especially larger game, and women
spend more time gathering or hunting smaller game
(Murdock, 1967). Thus, ecological selection may have
contributed to men’s greater size, strength, and weapons
proﬁciency to the extent that these contributed to hunting
success ancestrally (Kaplan et al., 2000). However, other of
men’s traits, such as beards, deep voices, more robust faces,
and high levels of same-sex aggression, are not easily
understood as adaptations for hunting. In addition, given
that male contests and sexual size dimorphism probably
characterized the common ancestor of the great apes,
contest competition likely predates specialized hunting and
the human sexual division of labor by several million years.
Hence, it is more likely that the sexual division of labor is
partly a consequence rather than the initial cause of these
anatomical dimorphisms, although hunting likely imposed
additional selection pressures on these male traits.
Some aspects of men’s phenotypes may also have been
produced via female choice, or through a combination of
contests and female choice. Because traits that evolve in
contest competition are often costly to produce and main-
tain and are constantly tested by competitors, such traits
may represent honest indicators of heritable ﬁtness, and
females may consequently evolve preferences for them
(Berglund et al., 1996). On the one hand, masculine bodies,
faces, and voices in men have indeed been found to
increase attractiveness to women (Frederick and Haselton,
2007; Puts et al., 2012b). On the other hand, the inﬂuence
of facial and vocal masculinity on ratings of dominance
is considerably larger and more consistently positive than
the effects on attractiveness (Puts et al., 2012b). Recent
work suggests that male facial masculinity may not be
universally preferred by women across human societies,
whereas it much more consistently conveys the impression
of aggressiveness (Scott et al., 2014). Likewise, beards
reliably increase perceptions of age, aggression, dominance,
and social status across societies, but generally decrease
attractiveness to women (Dixson and Vasey, 2012;
Muscarella and Cunningham, 1996; Neave and Shields,
2008). Furthermore, in samples spanning Western (Hill
et al., 2013), traditional agricultural (Llaurens et al., 2009),
and preindustrial (Smith et al., 2003) societies, men’smating
success has been found to relate more strongly to dominance
among men and the traits that contribute to dominance than
to attractiveness to women. Across the suite of male
secondary sexual characteristics, then, selection for success
in physical contests may have been either attenuated or
augmented by selection for attractiveness to females. In
general, however, men’s traits function far more effectively
in the context of male contests than in mate attraction, and
thus they do not appear to have evolved primarily as sexual
ornaments to attract women.
Another possibility is that some of men’s traits
represent developmental byproducts of male body size or
testosterone levels. Strength increases with body mass and
height (Balogun et al., 1991), for example, although other
male traits are not known to relate to body size (eg, beards)
or relate only weakly (eg, voice pitch; Pisanski et al., 2014).
However, even traits that are correlated with size are far
more sexually dimorphic than would be predicted from sex
differences in size alone (Puts et al., 2012b). Similarly,
Gorillas in Our Midst? Human Sexual Dimorphism and Contest Competition in Men Chapter | 15 243
androgens such as testosterone play important roles in the
development of male-typical traits, so one might conjecture
that these traits are merely developmental side-effects of
androgens. This viewpoint confuses proximate and ultimate
explanation, leaving unresolved the question of why
humans have evolved to respond to testosterone by
growing facial hair and longer-thicker vocal folds, for
example. Why instead do we not respond to testosterone by
growing antlers, as in red deer (Suttie et al., 1995), or
canines, as in many other primates (Van Wagenen and
Hurme, 1950)? Why does testosterone not increase paternal
investment, as in the California mouse (Peromyscus
californicus)(Gleason and Marler, 2013), rather than
having the opposite effect, as it does in many vertebrates,
apparently including humans (Kuzawa et al., 2009; Puts
et al., 2015b)? Clearly, different species, even closely
related ones, can evolve quite different responses to the
same hormones. An evolutionary history of male contests
parsimoniously explains why, in humans, a particular
constellation of sexually dimorphic traits including large
size may be developmentally linked to testosterone and to
Aﬁnal alternative to contest competition in our
hominin ancestors is phylogenetic inertiadtheideathat
we have inherited our traits from ancestral species rather
than experiencing selection for these traits in our own
species. Fossil and comparative evidence indicate that we
did indeed inherit traits such as greater male size and
aggression from an ancient hominin ancestor. However,
for other traits such as deep voices (Puts et al., 2016),
beards, and the use of handheld weapons, this appears
not to have been the case. Even for traits such as greater
male size and aggression that were likely sexually
dimorphic in our common ancestor with chimpanzees, we
would expect considerable reduction in modern humans
if these traits were not functional over recent hominin
evolution, given their substantial costs. And yet, as dis-
cussed earlier, men’s physical aggression is equally lethal
to that of male chimpanzees, and we are more sexually
dimorphic than chimpanzees in both skeletal size and fat-
Despite the comparatively strong overall evidence for
the importance of contest competition over men’s evolu-
tion, each of these alternative factors may have played a
role. These are not mutually exclusive alternatives; any
aspect of the phenotype can experience multiple selection
pressures, as well as responding to selection on other traits
with which it is developmentally correlated.
We have reviewed multiple converging lines of evidence
supporting a role for contest competition in shaping the
human male phenotype. Sex differences in parental in-
vestment, reproductive rates, and reproductive variance;
the OSR; and patterns of mating and marriagedall indicate
a history of moderately strong sexual selection among our
Contest competition in particular tends to favor size,
strength, aggression, weapons, and threat displays, and
men display all of these features. Traits that point to an
evolutionary past in which our male ancestors competed
for mates through force and threat include a proclivity
for same-sex violence including coalitional aggression,
higher pain threshold and tolerance compared to women,
increased body size and strength, facial robusticity,
fashioning and use of weapons, beards, deep voices, and
behavioral displays of formidability. This evidence is
taken from research across ﬁelds ranging from human
anatomy and physiology to psychology, ethnography,
paleoanthropology, animal behavior, and archaeology.
The alternative hypotheses that men’s traits were shaped
by selection for hunting ability, female mate choice, or
selection operating on developmentally correlated traitsd
or that men’s traits are consequences of phylogenetic
inertiadcan help account for some of the above aspects of
men’s phenotypes, but not others. The success of any
hypothesis is contingent on a parsimonious explanation of
the totality of evidence, and only contest competition
accomplishes this. In some ways, we may be more
gorilla-like, or chimp-like, than we prefer to suppose.
It is important to bear in mind, however, that while
human nature includes a propensity for violence in both
individual and coalitional forms, we are also capable of
negotiation, compromise, and restraint. For a species
currently numbering in the billions that now possesses
weapons capable of bringing about its own annihilation, the
importance of understanding our capacity for violence is
more than academicdit can potentially illuminate and
suggest solutions to problems of pressing societal concern.
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