RESEARCH ARTICLE
Can Persistence Hunting Signal Male Quality?
A Test Considering Digit Ratio in Endurance
Athletes
Daniel Longman1*, Jonathan C. K. Wells2, Jay T. Stock1
1 Department of Archaeology and Anthropology, University of Cambridge, Cambridge, United Kingdom,
2 Childhood Nutrition Research Centre, UCL Institute of Child Health, London, United Kingdom
* dannylongman@gmail.com
Abstract
Various theories have been posed to explain the fitness payoffs of hunting success among
hunter-gatherers. ‘Having’ theories refer to the acquisition of resources, and include the di-
rect provisioning hypothesis. In contrast, ‘getting’ theories concern the signalling of male re-
sourcefulness and other desirable traits, such as athleticism and intelligence, via hunting
prowess. We investigated the association between androgenisation and endurance running
ability as a potential signalling mechanism, whereby running prowess, vital for persistence
hunting, might be used as a reliable signal of male reproductive fitness by females. Digit
ratio (2D:4D) was used as a proxy for prenatal androgenisation in 439 males and 103 fe-
males, while a half marathon race (21km), representing a distance/duration comparable
with that of persistence hunting, was used to assess running ability. Digit ratio was signifi-
cantly and positively correlated with half-marathon time in males (right hand: r = 0.45,
p<0.001; left hand: r = 0.42, p<0.001) and females (right hand: r = 0.26, p<0.01; left hand:
r = 0.23, p = 0.02). Sex-interaction analysis showed that this correlation was significantly
stronger in males than females, suggesting that androgenisation may have experienced
stronger selective pressure from endurance running in males. As digit ratio has previously
been shown to predict reproductive success, our results are consistent with the hypothesis
that endurance running ability may signal reproductive potential in males, through its associ-
ation with prenatal androgen exposure. However, further work is required to establish
whether and how females respond to this signalling for fitness.
Introduction
Hunting and reproductive success
The high value placed by females on male ability to acquire resources has been well docu-
mented [1]. This is evident in pre-industrial human societies, where males exhibit a positive re-
lationship between status and number of surviving offspring. Such observations have been
made across the continents of Africa, South America and Asia [2–4]. It has been suggested that
PLOS ONE | DOI:10.1371/journal.pone.0121560
April 8, 2015
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a11111
OPEN ACCESS
Citation: Longman D, Wells JCK, Stock JT (2015)
Can Persistence Hunting Signal Male Quality? A Test
Considering Digit Ratio in Endurance Athletes. PLoS
ONE 10(4): e0121560. doi:10.1371/journal.
pone.0121560
Academic Editor: Bernhard Fink, University of
Goettingen, GERMANY
Received: May 17, 2014
Accepted: February 11, 2015
Published: April 8, 2015
Copyright: © 2015 Longman et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are
credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information files.
Funding: These authors have no support or funding
to report.
Competing Interests: The authors have declared
that no competing interests exist.
the same is true of contemporary Western society; increasing income has been shown to have a
significant effect on male reproductive success and desirability as a marriage partner [1,5–7].
Prior to agriculture, hunting may have represented an important means by which male re-
sourcefulness could be demonstrated. Food is inexorably linked to status in many cultures
around the world [8], and there is evidence that superior hunters enjoy social prestige within
the community [9]. Indeed, successful hunters have been shown to enjoy higher reproductive
success [10,11]. Hunting may therefore be motivated by male-male competition [12]. Smith
[4] has reviewed quantitative links between hunting ability and fitness-related factors such as
fertility, offspring survivorship and number of mates, and qualitative links between hunting
success and knowledge of hunting ability within communities. However, it is not clear whether
it is the high social status of hunters (which reflects their ability to acquire resources), or the
actual resources they obtain, that provides the mechanism linking hunting success with
reproductive fitness.
Theories posed to explain the fitness payoffs of hunting success include the "direct provi-
sioning" hypothesis, which asserts that successful hunters are more able to share food with
their mate and children, thereby enhancing fertility and offspring survivorship through physio-
logical mechanisms. In accordance with this theory, the offspring of successful hunters may in-
deed be better nourished [13]. Recent endocrine data from subsistence populations has
revealed that while testosterone levels increase upon a successful kill, this increase is not associ-
ated with the number and size of kills or the presence of an audience beyond the hunter's fami-
ly [14]. These findings are consistent with the direct provisioning model; meat provisioning
enhances reproductive success either directly [11], or indirectly through political alliances and
other benefits stemming from the community's desire to retain a successful hunter as a neigh-
bour [15–18]. Although this theory has intrinsic appeal, the egalitarian organisation of many
forager societies ensures that this nepotistic distribution of food is not consistently observed
[3]. While some male hunters are more effective than others, meat is widely distributed
throughout the group. As a result most of the food consumed is caught by a man outside one's
own nuclear family, such that the hunter's own family may receive no more meat than anyone
else [3]. Such sharing has been attributed to a means of maintaining social identity, and has
been considered a defining characteristic of hunter-gatherer behaviour [19]. Food sharing also
serves as a mechanism to deal with fluctuations in food availability [20]. A hunter may ex-
change a short-term food surplus for receipt of food in the future should his own hunting ef-
forts fail; a phenomenon known as the reciprocal altruism [21]. When food resources are
scarce and hunting success unpredictable, food sharing is prevalent as a type of culinary insur-
ance policy, as seen in the Inuit of Akalivik [22]. However, food donations may not always be
reciprocated [23,24]. Smith [4] is doubtful as to the efficacy of this proposal due to the lack of a
system of conditional reciprocity linked to meat provisioning. Thus, perhaps counter-
intuitively, there is very little clear evidence that the meat produced by hunting is the primary
mechanism underpinning the reproductive fitness of hunters. The 'having' theories do not pro-
vide adequate explanation.
A third hypothesis, based on Zahavi's "handicap principle" [25], is that hunting success acts
as a reliable signal of enviable underlying traits such as athleticism, intelligence or altruism.
Successful hunters benefit the community through the provisioning of public goods, an act
which enhances their reputation for generosity. The pursuant social standing thereby attained
may be attractive to women due to benefits of association such as protection [26]. As such,
hunting returns may be exchanged for the future fitness-enhancing benefits of increased social
capital or status [27–29]. This theory differs from those previously mentioned in that it need
not necessarily be the resource acquisition itself which promotes the reproductive fitness ('hav-
ing'), but the signal such resourcefulness conveys of underlying male quality ('getting'). The
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distribution of acquired meat beyond one's own family unit would also ensure that any such
signal was more widely observed [30]. Indeed, hunters are aware that others will know who ac-
quired the meat [31,32]. Male hunting may not then be motivated purely by consequent meat
consumption [12,33]. Such signalling would be reliable if it were costly to the signaller, such
that an individual lacking in a certain trait would be unable to afford to mimic the signal.
The hypothesis that hunting may act as a signal has been addressed by several authors
[30,34,35], but it is as yet unclear what the underlying mechanism may be. In other words,
what makes the signal a reliable index of fitness. We first review the physical traits that contrib-
ute to hunting success, and then consider potential mechanisms whereby these traits might sig-
nal reproductive potential.
Hunting success and physical skills
The hunting of mid-sized animals using hunting technologies such as spears favours a method
of disadvantaging or weakening the animal before approaching for the kill. This is because the
killing range of spears is very limited, and being close enough to kill a mid-sized animal would
be to risk injury to the hunter. Persistence hunting offers such a method, and may be a very ef-
fective means of food acquisition under certain conditions. Persistence hunting is a technique
by which hunters track and chase prey to the point of prey exhaustion or hyperthermia, often
during the hottest part of the day. This technique has been observed in the Kalahari in Africa,
and the Tarahumara tribe of Northern Mexico, who have been reported chasing deer until they
collapse before strangling them by hand [36,37]. Other animals targeted in this way include
steenbok, gemsbok, duiker, caracal, cheetah, kudu and eland [38,39]. During the hunt, chases
of up to 35km have been documented. Persistence hunting is useful from the point of view of
the early hunter for three reasons; it is relatively low risk, easy for a fit human with animal
tracking skills, and requires low metabolic cost relative to potential pay-off [40].
Endurance running may therefore represent a valuable component of hunting success. The
evolution of human endurance running ability has attracted attention previously [41,42]. This
followed observations that, in comparison with other cursorial animals, humans perform very
well over long distances. We are unique amongst primates in possessing the ability to run dis-
tances of several kilometres using aerobic metabolism [41], with many amateur human runners
able to sustain speeds of 5m/s [40]. This is fast compared with specialised quadrupedal cursors;
a dog with a similar mass to a human (65kg) has a trot-gallop transition speed of 3.8m/s, and
can then only sustain a gallop for a maximum of 15 minutes under ideal conditions [43]. The
same is true of horses, which is surprising as they have been selectively bred for running ability
[44]. Human runners can easily cover distances exceeding 10km a day; comparable with hunt-
ing dogs and hyenas which run to scavenge and hunt [40]. Consequently, the physical capacity
for endurance running appears have been selected for in our genus. Hunting may well have
provided the primary selective pressure; evidence for hominin carnivory dates back to approxi-
mately 2.5Ma [45]. The ability to run long distances to either hunt or scavenge may have im-
proved the chances of acquiring meat [40,46]. But as described above, if meat was widely
shared, how might hunting ability translate into fitness payoffs?
Endurance running might benefit male fitness if it acted as a reliable signal of reproductive
potential. Since testosterone is widely associated with reproductive success [47,48], an associa-
tion between testosterone and endurance running would make running prowess a reliable sig-
nal of male reproductive potential. Recent work has reported associations between sporting
ability and a marker for foetal testosterone exposure, the 2D:4D ratio [49,50]. This develop-
mental association between testosterone and physical abilities makes it a viable candidate as a
signalling mechanism.
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The ontogenetic development of digit ratio variability, and significance
for reproductive success
The sexually dimorphic digit ratio (2D:4D), first noted by Baker [51], is linked to prenatal
androgenisation [52]. The presence of significant sexual dimorphism in the digit ratio of de-
ceased human foetuses suggests 2D:4D ratio is established early in prenatal development [53].
Analysis of amniotic fluid provided direct evidence for a relationship between foetal hormones
and digit ratio [54]. This followed indirect reports stemming from homeobox genes [55,56]
and congenital adrenal hyperplasia [57,58]. Although the relationship between high digit ratio
and prenatal testosterone exposure has been questioned [59], and may be confounded by sexu-
al dimorphism in body size [60], the majority of evidence suggests that digit ratio is a suitable
instrument for investigating effects of prenatal androgen exposure on subsequent phenotype
[52]. As a result digit ratio is increasingly used as a proxy for foetal hormone environment—to
investigate early life predictors of later phenotype, and sex differences therein.
Digit ratio predicts reproductive success in both men and women once the effects of age and
population have been removed [61]. Mechanistically, low digit ratios are associated with higher
sperm counts and testosterone levels in men, and, conversely, high digit ratios are linked with
high oestrogen and luteinising hormone in women [62]. Furthermore, males exhibit a negative
relationship between digit ratio and other correlates of reproductive success such as preferred
number of children, strength of sex-drive, ease of achieving sexual excitement [47] and sperm
numbers per ejaculate [62]. Thus, digit ratio variability emerging early in life has significant im-
plications for fitness.
Digit ratio is negatively correlated with physical prowess across a range of sporting disci-
plines from slalom skiing [63] to football [64]. Digit ratio has also been tentatively correlated to
middle-distance running ability, albeit with a small sample size at distances of less than 4 miles
[65]. This study by Manning et al was the first to discuss the relationship between digit ratio
and running within the context of persistence hunting. Digit ratio was found to explain a larger
portion of the variance in endurance running than expected (up to 25%); explaining more vari-
ation than other sports [66] and running over shorter distances [67,68]. As such, the literature
supports the view that while 2D:4D does predict running speed, the predictive power increases
from sprinting events (1–2%) to events of up to 4 miles in length (20–25%).
We therefore sought to test the hypothesis that physical prowess at endurance running is as-
sociated with this putative marker of testosterone exposure. This was performed with a larger
sample and distance than previously reported. A female sample was included, as the relation-
ship between female digit ratio and running ability is as yet unknown. Since men undertake the
majority of hunting [69], we further predicted that the association between digit ratio and en-
durance running prowess would be stronger in males than females. Such a sex-difference
would suggest that the selective pressure of endurance running has shaped running prowess to
be a stronger signal of reproductive potential in males than females. Through such preferences,
males would then convert their physical prowess into fitness gains. Our study therefore tests a
potential mechanism whereby ‘getting’ meat could translate into male fitness returns.
Materials and Methods
Athletes (N = 542; m = 439, f = 103) were recruited at the 2013 Robin Hood Half Marathon
(21km), held on September 29th. Athletes received an email explaining the study prior to race
day, and were invited to take part following the race. Weather conditions were clear skies,
with a mean day-time temperature 14°C. The race is a high-profile event, with course records
of 61:38 and 73:32 for men and women respectively. Participants ranged between the ages of
19 and 35, and were all Caucasian. The half-marathon distance was chosen due to its
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appropriateness to evolutionary hunting-associated running [36]. Race time over 21km was
the performance indicator. While this is a measure of speed, the half-marathon distance is
considered to be very different from pure speed events such as the 100m sprint in terms of the
energy systems utilised. While the 100m sprint utilises the anaerobic system to supply ATP at
high rates, prolonged endurance races such as the half-marathon are dependent upon the
more sustainable aerobic energy system and lipid metabolism [70–74]. Half-marathons are
often used to investigate the effects of endurance exercise on physiology [75–78], so we are
confident the event may be used to consider endurance running ability.
All participants participating in the same race, ensuring standardisation of not only dis-
tance, but also of time-affecting factors such as weather conditions, time of day and race-course
elevation profile. All competitors wore small electronic chips which uniquely identified them
as they crossed electronic mats at the start and finish lines, guaranteeing accurate race timings.
Photocopies were taken of athletes' hands upon finishing the race, and measurements of
digit ratio were made at a later date. It is appreciated that ratios obtained from photocopies and
from direct measurements should not be combined within one study [79], so all measurements
were taken from photocopies, with the same machine being used for all copies. This method
was chosen due to ease of use and speed in facilitating a large sample size. Digit ratio was mea-
sured using Mitutoyo electronic callipers, reading to 0.01mm. Measurements were taken twice
from each photocopy to check repeatability. Digit ratios from the first measurement were
strongly correlated with the lengths recorded from the second measurement for all individual
participants (all r > 0.95). In addition, the means of individual right hand and left hand digit
ratios were significantly correlated (male r = 0.88, P < 0.001; female r = 0.762, P < 0.001). The
precision of each measurement was found using the method of Bland & Altman [80]; male
right 2D = 2.52; male right 4D = 2.21; male left 2D = 2.36; male left 4D = 2.07; female right
2D = 2.24; female right 4D = 2.04; female left 2D = 1.93; female left 4D = 2.42, all measure-
ments to 2 decimal places. These associations gave confidence in the reliability of the hand-spe-
cific measurements of digit ratio, and therefore in the conclusions subsequently drawn.
Ethical approval for the project was granted by the University of Cambridge Human Biology
Ethics Committee.
Statistics
Crude associations between digit ratio and race time were explored in each sex using correla-
tion analysis. The difference between the male and female correlation coefficients was analysed
using the Fisher r-to-z transformation and subsequent comparison of z-scores [81,82]. Multi-
ple linear regression was used to analyse the sex difference in the relationship between digit
ratio and race time. Sex was coded as 0 (female) versus 1 (male), and an ‘Interaction’ term was
constructed by multiplying sex code by digit ratio. Each of sex, ‘interaction’ and digit ratio were
then used as predictors of race time.
Results
Descriptive statistics
A description of the male and females samples is given in Table 1. Males were older, and had
significantly lower right and left digit ratios than females.
Among the male subsample there was a significant positive correlation between right and
left hand 2D:4D ratio and half-marathon time (right r = 0.45, p < 0.001; left r = 0.42,
p < 0.001). When age was controlled for the correlation strengthened (right r = 0.47,
p < 0.001; left r = 0.43, p < 0.001). The same was true among the female subsample (right
hand r = 0.26, p < 0.01; left hand r = 0.23, p = 0.02), with the correlation strengthening when
Persistence Hunting and the Signalling of Male Quality
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age was controlled for (right r = 0.29, p < 0.01; left r = 0.26, p < 0.01). These positive correla-
tions can be seen below in Fig. 1. Note the steeper regression line in the male subsample com-
pared to the female subsample.
Regression analysis revealed that men performed better than women, higher levels of per-
formance were associated with lower digit ratio, and digit ratio increases performance more
significantly in men than women (p<0.05). Age was not a significant contributor in the regres-
sion model, so was removed. See Table 2 for a summary of the results.
Discussion
It was hypothesised that 2D:4D ratio correlates with endurance running performance over the
half-marathon distance. This hypothesis has been supported. The male effect sizes are similar
to those of Manning et al. [65], which is consistent with the theory that digit ratio explains
more variation in endurance running than it does in shorter running events or other sports.
The link between digit ratio and maximal oxygen uptake may well relate to these observations
[83].
A marker of testosterone exposure is therefore associated with running ability, which eth-
nographic evidence has shown to be an important attribute for hunting [37]. As testosterone
(including the 2D:4D ratio) has been consistently associated with reproductive success [47],
this relationship between endurance running ability and digit ratio provides mechanistic evi-
dence in support of the hypothesis that running prowess could act as a reliable signal for male
reproductive potential. A conceptual diagram outlining this potential mechanism by which
Table 1. Descriptive characteristics of the samples.
Males (n = 439)
Females (n = 103)
Mean
SD
Mean
SD
Age (y)
31.7
4.93
28.8
4.58
2D:4D ratio right
0.97
0.033
0.98
0.028
2D:4D ratio left
0.94
0.036
1.01
0.035
Race time (s)
6946
1313
7002
926
doi:10.1371/journal.pone.0121560.t001
Fig 1. Scatter plot of male and female right hand 2D:4D ratio versus half marathon performance (s). The steeper male gradient is visible.
doi:10.1371/journal.pone.0121560.g001
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hunting success and running performance could act as a signal of reproductive fitness is given
below in Fig. 2.
As a positive relationship between hunting ability and male reproductive success has been
reported [2,3], it may be that women are attracted to men with the capacity to ‘get’, rather than
those who ‘have’. This may be a consequence of the egalitarian organisation of many forager
Table 2. Regression of half-marathon time in seconds on digit ratio (right in model 1, left in model 2),
sex and sex-ratio interaction (right in model 1, left in model 2).
Step
Predictor
β
t
p
Model 1
1
Age
-.057
-1.317
.188
Δ R2 = .003, F Change (1,540) = 1.736, p = .188
2
Age
-.007
-.186
.853
Sex Code
-2.692
-2.058
.040
Right Interaction
2.725
2.104
.036
Right ratio
.249
2.410
.016
Δ R2 = .203, F Change (3,537) = 45.692, p < .001
Model summary: F(4,537) = 34.810, p < 0.001, R2 = .206
Model 2
1
Age
-.057
-1.317
.188
Δ R2 = .003, F Change (1,540) = 1.736, p = .188
2
Age
-.030
-.734
.463
Sex Code
-2.419
-2.174
.030
Left Interaction
2.561
2.427
.016
Left ratio
.248
2.201
.028
Δ R2 = .172, F Change (3,537) = 37.239, p < .001
Model summary: F(4,537) = 28.451, p < 0.001, R2 = .175
doi:10.1371/journal.pone.0121560.t002
Fig 2. Conceptual diagram outlining a potential mechanism by which hunting success and running
performance act as a signal of reproductive fitness.
doi:10.1371/journal.pone.0121560.g002
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societies, which ensures that meat is widely distributed throughout the group [3]. It is acknowl-
edged, however, that this study has not demonstrated that women are in fact differentially at-
tracted to faster male runners, or that this is the only mechanism potentially linking hunting
success with reproductive success. Of course, hunting ability and reproductive success may
both be correlated with another independent variable which is itself a cause of higher reproduc-
tive success, independent of the male's hunting abilities [4].
Although both sexes exhibited a statistically significant positive correlation between right
and left hand 2D:4D ratio and time taken to complete a half-marathon running race, the rela-
tionship exhibited by males was significantly stronger than that of females. As such, the sec-
ondary hypothesis was also supported, suggesting that the ability of running ability to signal
reproductive potential has been under stronger selection in males than in females.
Testosterone is not only a link between digit ratio and endurance running ability, but also
an important mediator of the male reproductive effort. Behaviourally, testosterone plays an im-
portant role in producing sex drive [84] and mediating confidence and assertiveness in social
situations [85] – qualities deemed beneficial in the male mating effort. Physiologically, testos-
terone serves to promote muscle growth [86,87], providing an advantage in male-male combat
situations [48,88]. Muscularity is also a sexually attractive trait; more muscular men to report a
greater number of sexual partners and younger age at first conception [89,90] and more off-
spring sired [91].
Muscularity per se, however, could be disadvantageous as a signal of male fitness in a hot
environment, as the link between muscle mass and metabolic rate means that a larger muscle
mass could induce overheating. Additionally, the high metabolic cost of muscle tissue would
not be well suited to environments prone to famine. Running ability may therefore represent
a signal of reproductive fitness that is better suited to a hot stochastic environment, where
it is furthermore compatible with immediate benefits, given that hunting promotes
food acquisition.
Comparative analyses of Olympic winning times and world records has lead some authors
to conclude that women will outcompete men in the marathon by the end of the 20th century
and over 100m in the mid 22nd century [92,93]. However, caution is required as the initial
greater rate of improvement in female performances is most likely due to historical consider-
ations such as the later social acceptance and inclusion of female distance running in major
events such as the Olympic Games [94]. The gender difference is now believed to have
reached a plateau [95], with the remaining disparity explicable by biological differences be-
tween males and females [96]. While it has been suggested that the gender difference in per-
formance disappears as distances increase beyond that of the marathon [97,98], several
studies have reported that such distances exhibit no change in relative ability [99]. Indeed,
performances between the sexes may even diverge [100,101]. Although it is possible that fe-
males improve relative to males with distance, such considerations are not deemed to be rele-
vant to this study as ultra-marathon distances are not applicable to the evolutionary
pressures applied by persistence hunting.
To conclude, this investigation has shown that both male and female digit ratios are signifi-
cantly and negatively correlated to ability in endurance running of a distance/duration compa-
rable with that of persistence hunting (a positive correlation between digit ratio and half
marathon time). The relationship was stronger for males than females, suggesting that males
faced greater evolutionary pressure to develop endurance running capabilities. We suggest that
hunting ability might therefore act as a reliable signal of male fitness, in addition to its function
of calorie acquisition. Consequently, the ability to 'get' meat may contribute to the positive rela-
tionship between hunting ability and male reproductive success. Our work has used sporting
Persistence Hunting and the Signalling of Male Quality
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ability as a proxy for hunting prowess; further work is now required to test this hypothesis in
hunting societies.
Author Contributions
Conceived and designed the experiments: DL JTS JCKW. Performed the experiments: DL. An-
alyzed the data: DL JTS JCKW. Contributed reagents/materials/analysis tools: DL JTS JCKW.
Wrote the paper: DL JTS JCKW.
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Can persistence hunting signal male quality? A test considering digit ratio in endurance athletes.
Longman, Daniel,Wells, Jonathan C K,Stock, Jay T