A Journal of the Society
for Integrative and
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Impact Protection Potential of Mammalian Hair: Testing the
Pugilism Hypothesis for the Evolution of Human Facial Hair
E. A. Beseris,
S. E. Naleway,
and D. R. Carrier
*Department of Biology, University of Utah, 257 S. 1400 E, Salt Lake City, UT 84112, USA;
Department of Mechanical
Engineering, University of Utah, 100 S. 1495 E, Salt Lake City, UT 84112, USA
Synopsis Because facial hair is one of the most sexually dimorphic features of humans (Homo sapiens) and is often
perceived as an indicator of masculinity and social dominance, human facial hair has been suggested to play a role in
male contest competition. Some authors have proposed that the beard may function similar to the long hair of a lion’s
mane, serving to protect vital areas like the throat and jaw from lethal attacks. This is consistent with the observation
that the mandible, which is superficially covered by the beard, is one of the most commonly fractured facial bones in
interpersonal violence. We hypothesized that beards protect the skin and bones of the face when human males fight by
absorbing and dispersing the energy of a blunt impact. We tested this hypothesis by measuring impact force and energy
absorbed by a fiber epoxy composite, which served as a bone analog, when it was covered with skin that had thick hair
(referred to here as “furred”) versus skin with no hair (referred to here as “sheared” and “plucked”). We covered the
epoxy composite with segments of skin dissected from domestic sheep (Ovis aries), and used a drop weight impact tester
affixed with a load cell to collect force versus time data. Tissue samples were prepared in three conditions: furred
(n¼20), plucked (n¼20), and sheared (n¼20). We found that fully furred samples were capable of absorbing more
energy than plucked and sheared samples. For example, peak force was 16% greater and total energy absorbed was 37%
greater in the furred compared to the plucked samples. These differences were due in part to a longer time frame of force
delivery in the furred samples. These data support the hypothesis that human beards protect vulnerable regions of the
facial skeleton from damaging strikes.
As is the case in other species of great apes, human
males perpetrate the vast majority of violence and
most of these acts of aggression are directed at other
males (Adams 1983;Chagnon 1988;Daly and Wilson
1988;Keeley 1996;Wrangham and Peterson 1996;
Walker 2001;Puts 2010;Ellis et al. 2013;Hill et al.
2016). When human males fight hand-to-hand, the
face is usually the primary target (Carrier and
Morgan 2015). Consequently, it is not surprising
that human males suffer substantially more injuries
to the face from interpersonal violence than do
females. Epidemiology studies of interpersonal vio-
lence indicate that males suffer 68–92% more inju-
ries to the face from fights than do females
(Shepherd et al. 1990;Bostrom 1997;Brink et al.
1998;Simsek et al. 2007;Czerwinski et al. 2008;
Lee 2009;Allareddy et al. 2011;Suh and Kim 2012).
Because sexual dimorphism is often greatest in
those phenotypes that enhance a male’s capacity to
dominate other males (Clutton-Brock and Harvey
2009;Parker 1983;Andersson 1994), it is not sur-
prising that the facial bones which suffer the highest
rates of fracture from interpersonal violence are the
parts of the skull that exhibit the greatest sexual di-
morphism in both modern humans and early hom-
inins (i.e., bipedal apes; Carrier and Morgan 2015).
From the perspective of sexual selection, it is reason-
able to suspect that these dimorphic facial features
emerged as a result of male–male contest competi-
tion, and act to protect the face against damaging
strikes (Puts 2010;Stirrat et al. 2012;Carrier and
Morgan 2015;Puts et al. 2015;Puts 2016).
Consistent with this suggestion is the observations
that masculine facial structure is correlated with
greater upper body strength (Fink et al. 2007;Sell
ßThe Author(s) 2020. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/
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Integrative Organismal Biology
Integrative Organismal Biology, pp. 1–12
doi:10.1093/iob/obaa005 A Journal of the Society for Integrative and Comparative Biology
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et al. 2009;Windhager et al. 2011), aggressive behav-
e and McCormick 2008;Carr
e et al. 2009,
rebicky et al. 2013), social dominance
(Muller and Mazur 1997;Geniole et al. 2015), and
reproductive success (Mazur et al. 1994).
Another trait that exhibits pronounced sexual di-
morphism in humans is facial hair (Darwin 1871;
Dixson et al. 2018). Among our closest relatives,
the African apes (chimps, bonobos, and gorillas),
facial hair is equally prominent in males and females
(Darwin 1871). Relative to the African apes, human
females have significantly reduced facial hair,
whereas at puberty human males develop continu-
ously growing hair that covers the front of the upper
jaw (mustache) and the anterior neck and lower jaw
(beard; Darwin 1871;Dixson et al. 2005;Dixson
et al. 2017). As is true for masculine skeletal features,
men with full beards are reportedly perceived as be-
ing more masculine, socially dominant, and behav-
iorally aggressive in comparison to clean-shaven men
(Neave and Shields 2008;Dixson and Vasey 2012;
Dixson and Brooks 2013;Saxton et al. 2016;
Sherlock et al. 2017;T
rebicky et al. 2019). In addi-
tion, human male facial hair has been shown to pos-
itively impact mating success in highly competitive
environments (Barber 2001;Dixson et al., 2017).
Some authors suggest these relationships are due
to facial hair enhancing the size and appearance of
the sexually dimorphic regions of the face—most
notably the mandible and maxilla (Guthrie 1970;
Muscarella and Cunningham 1996;Neave and
Shields 2008;Dixson et al. 2017,2018). Others
have proposed that the beard actually serves to pro-
tect the throat and jaw during fighting (Blanchard
2010). In this context, the mane of male lions offers
an intriguing possible analogy. Like human beards,
lion manes are specific to males. The very thick hair
of a lion’s mane could provide protection from an
attacker’s teeth or it might make the head, neck, and
chest more difficult for an attacker to grab and hold
with the claws of his forelimbs so that it could de-
liver a damaging bite with his jaws. Indeed, Darwin
(1871) suggested that manes of male lions, Canadian
lynx, baboons, sea lions, bison, and elk provide phys-
ical protection in male–male fights. (In contrast,
when considering humans, Darwin speculated that
the beard evolved as an “ornament” favored by
females.) More recently, Blanchard (2010) has ar-
gued that the manes of lions may “mitigate” the
danger of fights among pride males by making the
existence of multi-male and female groups possible
facilitat protection of prides against take-overs and
infanticide by nomadic males. In contrast, West et al.
(2006) compared patterns of injury, mane
development, and adult mane morphology in
African lions and found no evidence that the mane
conferred effective protection against wounding.
However, they also argue that their results suggest
that “the general mane area is not a target, but hint
that attackers avoid the mane, or that the mane
protects this area from attack.” Thus, the extent to
which the mane of lions is protective remains
The suggestion that human beards may provide
protection in a fight is consistent with the observa-
tions that (1) the mandible, which is superficially
covered by the beard, is one of the most commonly
fractured facial bones in interpersonal violence
(Shepherd et al. 1990;Bostrom 1997;Lee 2009;
Hojjat et al. 2016) and (2) a fractured mandible,
prior to modern surgical methods, likely represented
a relatively grave facial injury. Based on these obser-
vations, we hypothesized that human facial hair pro-
vides physical protection from strikes that would
cause blunt trauma. Specifically, we predicted that
thick facial hair reduces the amount of force that
underlying tissues experience from a strike due to
absorption and dispersal of energy of the strike.
Human bone tissue was modeled using a short fiber
epoxy composite bone analog (manufactured by
Pacific Research Laboratories, Inc., Vashon, WA),
which has material properties similar to human cor-
tical bone (Cuppone et al. 2004;Chong et al. 2007).
Because it was not practical to obtain fully bearded
skin samples from human cadavers, and loose hu-
man hair was anticipated to not distribute the force
of impact the way in situ hair may, we used skin
samples from domestic sheep (Ovis aries) purchased
from a local slaughterhouse. Sheep fleece is not a
perfect analogy for the hair of human beards. The
follicles of sheep fleece average one fourth the diam-
eter of human beard hair (18 lm versus 75 lm;
Bosman 1934;Floyd et al. 2018) and are much
more densely packed (6000 follicles per cm
70 follicles per cm
;Bosman 1934;Maurer et al.
2016). This represents a five-fold greater cross-
sectional area of hair follicles for fleece than beards.
However, the follicles of full human beards are often
more than five-fold longer than the follicles of the
sheep fleece samples that we tested (3.30 61.04 cm,
mean and standard deviation [SD]). Consequently,
the volume of follicles in our fleece samples did ap-
proximate the volume of full beards which is un-
likely to be true for the pelts of most other species.
2E. A. Beseris et al.
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The bone analog was cut into small rectangles
with dimensions 60 mm 65 mm 3 mm and cov-
ered by sheepskin. Skin samples were cut to the same
dimensions as the fiberglass and were soaked in a
saline solution (0.9% NaCl) for at least one hour
prior to testing to ensure the skin had the same
water content as living tissue. Hydration level has
been shown to have significant effects on the mate-
rial properties of organic matter, and therefore must
be standardized for all samples (Lee et al. 2011;Trim
et al. 2011). Care was taken to keep the hair of the
samples dry. The hair of the sheepskin samples was
prepared in three separate conditions: sheared,
plucked, and furred. Sheared samples were trimmed
with manual sheep shears to 0.5 cm in length.
Sheared samples were included to test whether the
presence of hair roots in the skin influenced the
results. Plucked samples had all hair fibers removed,
including the roots. Furred samples were not manip-
ulated in any way, and had an approximate hair
length of 8 cm. Of note, these three conditions result
in different total volumes and masses of hair and
were chosen to best represent states that would occur
in human males (i.e., full beard, trimmed beard, and
All data were obtained by using a drop-weight
impact test on an Instron Dynatup 8250 drop weight
impact tester (Instron Corporation, Norwood, MA;
Fig. 1). All tests were performed in accordance with
ASTM Standard D5420 (ASTM Standard D5420-16
2016). This test involves dropping a blunt striker
(diameter 3 cm, mass ¼4.70 kg), from a known
height toward a material sample mounted on an an-
vil. The anvil had a 55 mm 50 mm hole to allow
free suspension of the sample and to avoid effects of
the contact between the anvil and sample that could
alter the results. The Instron Dynatup Impulse data
acquisition system (Instron Corporation, Norwood,
MA) takes measurements from a 200 kN load cell to
generate a graph of load (kN) versus time (ms). A
velocity detector was also used to measure the in-
stantaneous velocity (m/s) of the striker head at the
time of impact.
Prior to obtaining data to compare across the
three conditions, a standard drop height was deter-
mined. Starting from 5 cm above the anvil, the
striker head was dropped onto a furred sample. If
the sample showed signs of failure, the striker head
was lowered an additional centimeter. If the sample
did not show signs of failure, the striker head was
raised an additional centimeter. Failure was defined
as the point at which the fiberglass sample shows any
cracks, fractures, holes, or dislodged shards. The
striker head mass was not changed during the entire
duration of testing. This process was repeated for 20
samples, following the approach of the ASTM
Standard D5420 (ASTM Standard D5420-16 2016).
From these data, the mean failure height was calcu-
lated by using Equation (1):
h¼mean failure height (mm)
dh¼increment of height (mm)
N¼total number of failures or non-failures,
whichever is smaller
h0¼lowest height at which failure or non-failure
ini, where i¼integer. ni¼number of
events occurring at hi, and hi¼h0þidh
Using this approach, the mean failure height was
determined to be 7.4 cm (Supplementary Table S1),
and the drop tower height was set to this height for
the entire series of experimental tests. Twenty sam-
ples for each condition (shaved, plucked, and furred)
were tested. Using the resultant load data (kN) and
the mass of the striker (4.7 kg), the acceleration of
the striker head (m/s
) was determined using
Newton’s Second Law (F¼ma). The resultant
acceleration dataset was integrated across the impact
time frame to yield the instantaneous velocity (m/s)
Fig. 1 Photograph of the experimental setup using an Instron
Dynatup 8250 drop weight impact tester.
Impact protection from mammalian hair 3
for each time frame, and subsequently, the kinetic
energy (J). The energy absorbed by the sample (J)
was calculated from the amount of kinetic energy
lost by the striker head from the start of impact to
the end of impact. From these data, the peak force in
Newtons (PF), peak energy in Joules (PE), time to
peak force in milliseconds (TPF), and time to peak
energy in milliseconds (TPE) were recorded for each
A series of two-sample, single-tailed, unequal var-
iance t-tests were used to determine statistical signif-
icance between raw PF, PE, TPF, and TPE data. We
assumed the results were significantly different when
the P-value was <0.05. Percent difference was also
calculated for each of the four metrics between con-
ditions, along with mean and SD for each condition.
All data calculations, statistical analyses, and graphs
were performed using Microsoft Excel (Microsoft
Corporation, Redmond, WA).
Statement on human and animal rights
This research did not involve human or animal
The furred samples provided greater protection
against impact than did the plucked or sheared sam-
ples (Table 1). Under the condition of the study in
which the loading was set so that 50% of the
furred samples would fail on impact, all of the
plucked samples, 95% of the sheared samples, and
45% of the furred samples failed.
Example recordings of force and energy absorbed
for impact tests of the furred, plucked, and sheared
skin samples are shown in Fig. 2. As can be seen in
these traces, the average peak force was significantly
lower, energy absorbed was higher, and the time to
peak force and peak energy absorbed was substan-
tially greater in the furred samples than in the
sheared and plucked samples (Fig. 3 and Table 2).
The greatest differences between the furred and
plucked or sheared samples were observed in times
to reach peak force and peak energy absorption
(Fig. 3 and Table 2). The sheared and plucked sam-
ples were loaded more rapidly by impact and more
often than not experienced loads that exceeded their
breaking strength. This suggests that the greatest ad-
vantage offered by the hair is that it distributes the
force of impact over a longer time frame.
The higher variation observed in the furred sam-
ples is largely due to differences in the samples that
failed versus those that did not. Samples that did not
fail (see Fig. 2) had PF, PE, TPF, and TPE values
similar to the mean values for the furred condition
(PF ¼0.67 kN, PE ¼2.46 J, TPF ¼8.24 ms,
TPE ¼10.29 ms). In contrast, samples that did fail
had much higher PF values (0.82 kN) and lower PE
(1.91 J), TPF (5.26 ms), and TPE (5.39 ms) in com-
parison to the mean. The furred condition had a
nearly equal amount of failures and non-failures
(frequency of failure ¼0.45), whereas the plucked
Table 1 Frequency of failure for each condition
Frequency of failure
Fig. 2 Representative graphs of impact force (black line) and
energy (gray line) versus time for (A) a furred sample, (B)a
sheared sample, and (C) a plucked sample.
4E. A. Beseris et al.
and sheared conditions had nearly all failures (fre-
quency of failure ¼1 and 0.95, respectively).
Our results show that on average the furred samples
absorbed nearly 30% more energy than the sheared
and plucked samples. Furred samples experienced
lower peak impact forces and were loaded more
slowly. These factors contributed to a reduced rate
of furred sample failure as compared to sheared and
plucked samples. Thus, the results of this study
indicate that hair is indeed capable of significantly
reducing the force of impact from a blunt strike and
absorbing energy, thereby reducing the incidence of
failure. If the same is true for human facial hair, then
having a full beard may help protect vulnerable
regions of the facial skeleton from damaging strikes,
such as the jaw. Presumably, full beards also reduce
injury, laceration, and contusion, to the skin and
muscle of the face. Although not tested in this study,
it is also likely that the hair of beards helps deflect an
oblique blow by reducing friction between the face
and the object striking it. These protective functions
of beards may provide an advantage in male contest
competition, and therefore be selectively favored.
This may also explain why facial hair is associated
with high masculinity, social dominance, and behav-
ioral aggressiveness, as it may function as a true in-
dicator of level of invulnerablity to facial injury
(Neave and Shields 2008;Dixson and Vasey 2012;
Dixson and Brooks 2013;Saxton et al. 2016;
Sherlock et al. 2017).
No measures were significantly different between
the plucked versus sheared conditions, except for
TPE (P¼0.049). We anticipated this result as the
presence or absence of hair roots in the skin was
expected to have little influence on impact
Among the significant differences between sample
conditions, the time to peak force and time to peak
energy are likely the most salient. Furred samples
absorbed the impact more slowly than the sheared
and plucked samples. We suspect that this is a result
of individual hair fibers taking up part of the load as
the striker head descended toward the skin, slowing
the striker head as it passed by. By loading the hair
fibers in addition to the skin and bone, the force of
impact may also be distributed over a larger surface
area. This is a similar mechanism to how a Kevlar
fiber vest distributes the force of an incoming bullet
(Cheeseman and Bogetti 2003). Regardless, absorp-
tion of energy by the fur must explain why furred
samples were able to absorb 37% more energy than
sheared and plucked ones.
Our results appear to conflict with a recent study
that demonstrated beards do not provide a perfor-
mance advantage in mixed martial arts (MMA)
fights as measured by number wins by knock-out
and decision (Dixson et al. 2018). This carefully con-
trolled and compelling study, compared rates of win-
ning in 600 fights involving 395 fighters, found no
evidence of a performance advantage provided by
facial hair, and concluded that “beards represent dis-
honest signals of formidability that may serve to cur-
tail the escalation of intra-sexual conflict through
Fig. 3 Box and whisker plots showing median, first and third
quartiles, and minimum and maximum values of (A) peak force
(kN), (B) peak energy (J), (C) time to peak force (ms), and (D)
time to peak energy (ms) for each of the three conditions.
Impact protection from mammalian hair 5
intimidation rather than providing advantages in di-
rect combat.” It is sensible to test the protective ef-
fect of beards in MMA fighters because epidemiology
indicates that the most common injuries in MMA
fights are facial lacerations, fractures, and concus-
sions (Lystad et al. 2014;Jensen et al. 2017).
Although this is not the result we would have pre-
dicted based on our observation that thick hair
reduces peak impact force and energy applied to
the structure beneath the hair, the metric used in
their study “number of wins by knockout or techni-
cal knockout” is not a direct measure of the rate of
those injuries that may be reduced by full beards.
Our results provide no evidence that beards provide
protection against being knockedout, rather our
results are presumed to be most relevant to skin
lacerations and facial bone fractures. Finally, as
Dixon and collaborators note, their finding that
beards do not provide a performance advantage
may be more relevant to professional fighters than
While our data are consistent with the hypothesis
that hair can protect bone and skin from the dam-
aging effects of a blunt strike, it should be noted that
this may not be true in every case. Human facial hair
has great variation across populations—individuals
from Middle Eastern and Northern European ances-
try are capable of growing thick, bushy beards,
whereas people of East Asian and American Indian
heritage have relatively little facial hair. The sheep-
skin used in our study was extremely thick and
wooly, and is probably only a good model for a
very full and long human beard. To our knowledge,
no quantitative data exist on how coarseness, den-
sity, and thickness of human facial hair varies across
populations. Future research should incorporate
these measures to determine which types of facial
hair may provide the best protection against impact.
It is unknown why human populations vary in
their developed facial hair. In groups in which thick
facial hair is not present in males, other selective
forces may have acted against facial hair. These
groups may have lower rates of contest competition
between males, thereby negating the advantage of a
beard or they may need to maximize bare skin
surface area for efficient thermoregulation in hot
environments. The fact that facial hair is sexually
dimorphic in humans, with females lacking beards
and mustaches, strongly suggests that there are real
disadvantages to having thick facial hair. If there
were no tangible disadvantages, selection for facial
hair in males would have resulted in beards in
both sexes (Lande 1980).
Additional studies are needed to ascertain the
mechanism by which hair dampens the effects of
impact. We theorized that the hair fibers absorbed
energy from the impactor head as it passed by and
by spreading the force over a larger area. This is
supported by the furred samples having a longer av-
erage time to reach peak force and absorbing more
energy. This could be further substantiated by using
highspeed video to see exactly what the hair fibers
are doing upon impact. This could also be accom-
plished by creating a model of the hair fibers and
The results of this study are consistent with the
suggestion that the sexually dimorphic facial hair of
humans may have evolved in response to selection
on male–male fighting performance. Similarly, al-
though not tested here, our results also support the
suggestion that the mane of male lion’s provides
some level of protection from injury when males
fight (Darwin 1871;West et al. 2006;Blanchard
2010) due to the capacity of hair to slow and expand
the area of energy transfer. As mentioned in the
Introduction, male beards are one of the most sex-
ually dimorphic features of human anatomy (Darwin
1871;Dixson et al. 2018). Men with full beards are
perceived as being more masculine, socially domi-
nant, and behaviorally aggressive in comparison to
clean-shaven men (Neave and Shields 2008;Dixson
and Vasey 2012;Dixson and Brooks 2013;Saxton
et al. 2016;Sherlock et al. 2017;T
rebicky et al.
2019). Additionally, facial hair has been shown to
positively impact mating success in highly competi-
tive environments (Barber 2001;Dixson et al. 2017).
These observations are all consistent with the hy-
pothesis that beards evolved to enhance fighting per-
formance by providing protection to vulnerable
aspects of the face. Indeed, aspects of the anatomy
Table 2 Mean, SD, percent difference, and P-values for furred (F), plucked (P), and sheared (S) conditions
Furred mean6SD Plucked mean6SD Sheared mean6SD F 3P %diff. (P)F3S %diff. (P)P3S %diff. (P)
PF (kN) 0.68 60.16 0.79 60.10 0.77 60.09 15.60 (0.004) 12.79 (0.014) 2.82 (0.23)
PE (J) 2.46 60.43 1.70 60.34 1.80 60.43 36.77 (<0.001) 31.24 (<0.001) 5.69 (0.211)
TPF (ms) 8.24 62.40 4.38 61.29 5.10 61.40 61.17 (<0.001) 47.10 (<0.001) 15.16 (0.049)
TPE (ms) 10.30 64.54 4.57 61.28 5.36 61.70 77.04 (<0.001) 63.14 (<0.001) 15.83 (0.054)
6E. A. Beseris et al.
of the human facial skeleton, and sexual dimorphism
in facial shape, have been suggested to have evolved
as a result of male–male contest competition, and act
to protect the face against damaging strikes (Puts
2010;Stirrat et al. 2012;Carrier and Morgan 2015;
Puts et al. 2015).
More broadly, the results of this study add to a
growing body of evidence suggesting that specializa-
tion for male fighting has played a significant role in
the evolution of the musculoskeletal system of
humans. For example, the short limbs (Carrier
2007), plantigrade foot posture (Carrier and
Cunningham 2017), and bipedal posture of our ear-
liest hominins ancestors (Carrier 2011), and the
force–velocity tuning (Carrier et al. 2011) and size
(Carrier et al. 2015) of the muscles of the human leg
may also be associated with improved fighting per-
formance. Of direct relevance to this study is the
suggestion that the proportions of the human hand
(Morgan and Carrier 2013;Horns et al. 2014), and
human sexual dimorphism in both strength of the
muscles of the arm (Morris et al. 2020) and facial
shape (Carrier and Morgan 2015) are, at some level,
a product of selection on performance during fight-
ing with fists. Many of these anatomical traits dis-
tinguish hominins from the other great apes and all
of them are associated with performance improve-
ments in other non-fighting behaviors. Nevertheless,
the fact that the appearance of hominins in the fossil
record coincides with the appearance of a suite of
anatomical traits that have been demonstrated to
improve performance in behaviors important to hu-
man fighting suggests that specialization for physical
aggression may have played an early and persistent
role in the evolution of our lineage.
This work was funded by National Science
Foundation grant IOS-0817782 (to D.R.C.).
Supplementary data available at IOB online.
Data will be archived on Dryad once the manuscript
is accepted for publication.
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