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No Association between 2D:4D Ratio and Hunting Success among Hadza Hunters


Abstract and Figures

The ratio of index- and ring-finger lengths (2D:4D ratio) is thought to be related to prenatal androgen exposure, and in many, though not all, populations, men have a lower average digit ratio than do women. In many studies an inverse relationship has been observed, among both men and women, between 2D:4D ratio and measures of athletic ability. It has been further suggested that, in hunter-gatherer populations, 2D:4D ratio might also be negatively correlated with hunting ability, itself assumed to be contingent on athleticism. This hypothesis has been tested using endurance running performance among runners from a Western, educated, and industrialized population as a proximate measure of hunting ability. However, it has not previously been tested among actual hunter-gatherers using more ecologically valid measures of hunting ability and success. The current study addresses this question among Tanzanian Hadza hunter-gatherers. I employ a novel method of assessing hunting reputation that, unlike previous methods, allows granular distinctions to be made between hunters at all levels of perceived ability. I find no statistically significant relationship between digit ratio and either hunting reputation or two important hunting skills. I confirm that Hadza men have higher mean 2D:4D ratios than men in many Western populations. I discuss the notion that 2D:4D ratio may be the consequence of an allometric scaling relationship between relative and absolute finger lengths. Although it is difficult to draw clear conclusions from these results, the current study provides no support for the theorized relationship between 2D:4D ratio and hunting skill.
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No Association between 2D:4D Ratio and Hunting
Success among Hadza Hunters
Duncan N. E. Stibbard-Hawkes
#Springer Science+Business Media, LLC, part of Springer Nature 2019
The ratio of index- and ring-finger lengths (2D:4D ratio) is thought to be
related to prenatal androgen exposure, and in many, though not all, populations,
men have a lower average digit ratio than do women. In many studies an
inverse relationship has been observed, among both men and women, between
2D:4D ratio and measures of athletic ability. It has been further suggested that,
in hunter-gatherer populations, 2D:4D ratio might also be negatively correlated
with hunting ability, itself assumed to be contingent on athleticism. This
hypothesis has been tested using endurance running performance among runners
from a Western, educated, and industrialized population as a proximate measure
of hunting ability. However, it has not previously been tested among actual
hunter-gatherers using more ecologically valid measures of hunting ability and
success. The current study addresses this question among Tanzanian Hadza
hunter-gatherers. I employ a novel method of assessing hunting reputation that,
unlike previous methods, allows granular distinctions to be made between
hunters at all levels of perceived ability. I find no statistically significant
relationship between digit ratio and either hunting reputation or two important
hunting skills. I confirm that Hadza men have higher mean 2D:4D ratios than
men in many Western populations. I discuss the notion that 2D:4D ratio may be
the consequence of an allometric scaling relationship between relative and
absolute finger lengths. Although it is difficult to draw clear conclusions from
these results, the current study provides no support for the theorized relation-
ship between 2D:4D ratio and hunting skill.
Keywords 2D:4D .Digit ratio .Hadza .Hunting reputation .Foragers .Hunter-gatherers
Human Nature
Electronic supplementary material The online version of this article (
09359-z) contains supplementary material, which is available to authorized users.
*Duncan N. E. Stibbard-Hawkes
Department of Anthropology, Durham University, Durham DH1 3LE, UK
Digit [2D:4D] ratio is the length of the second digit of the hand (index finger) over the
length of the fourth digit (ring finger). In many (Manning 2002; Manning and Fink
2008; Manning et al. 1998,2000; Rammsayer and Troche 2007; Trivers et al. 2006),
though not all (Apicella et al. 2016;Brosnan2008; Manning 2002) human populations,
digit ratios show considerable sexual dimorphism; men generally have lower digit
ratios than women. The same pattern of sexual dimorphism in digit ratio has also been
observed in several nonhuman species (Brown et al. 2002a;BurleyandFoster2004;
Cain et al. 2013; Direnzo and Stynoski 2012; Tobler et al. 2011). Several authors (Galis
et al. 2010; Manning and Fink 2008) have argued that sexual dimorphism in digit ratio
is a product of sex differences in fetal androgen exposure. Studies of early sex
differentiation in the digit ratios of human fetuses (Galis et al. 2010), as well as negative
associations between digit ratio and congenital adrenal hyperplasia (Brown et al.
2002b;Oświecimska et al. 2012), appear to confirm this supposition.
The negative associations observed between androgen exposure and digit ratio have
led some authors to infer that digit ratios may act as a good measure of both
developmental and adult testosterone levels. For this reason, authors have looked for,
and in many cases found, negative associations between digit ratio and other measures,
including athleticism, fertility, and fecundity, also thought to be associated with high
testosterone: Among 153,000 participants in a BBC online survey which employed
self-measurement (Manning and Fink 2008), lower male digit ratios were associated
with higher lifetime fertility (number of children corrected for age), sensation-seeking
behaviors and also dominance(i.e., responses to 10 questions taken from the
International Personality Item Pool ( concerning the desire to outdo,
surpass, or control peers). Manning (2002) found a negative correlation between digit
ratio and sperm count. Hönekopp et al. (2006) identified a negative relationship
between digit ratio and number of sex partners in heterosexual German
undergraduates. Finally, in a North American and British population, Wlodarski et al.
(2015) observed that low digit ratios occurred slightly more often in more promiscuous
men. However, Manning and Fink (2008) found no such trend. Further, Falcon (2016)
was unable to replicate Wlodarski et al.s findings and criticized their method of
analysis (though see Dunbar and Wlodarski 2016 for reply).
Digit ratio has also been negatively associated, in many studies, with mea-
sures of athleticism and sporting success in both sexes (Lombardo et al. 2018;
Manning and Taylor 2001; Manning et al. 2007). Male professional footballers
have lower mean digit ratios than non-professional controls (Manning and
Tay lor 2001). In a study of 607 British women, digit ratio was found to be
significantly negatively associated with lifetime sporting achievement (Paul
et al. 2006). Two independent studies found significant negative associations
between digit ratio and endurance running performance (Longman et al. 2015;
Manning et al. 2007); Manning and Hill (2009)foundasimilarnegative
relationship between digit ratio and sprinting ability. Digit ratio has been
associated with rowing performance (Longman et al. 2011), skiing speeds
(Manning 2002), and rugby ability (Bennett et al. 2010), as well as fencing
ability among female, but not male fencers (Voracek et al. 2010). It has been
suggested, for this reason, that digit ratio is in some way connected to
intrasexual competition in humans (Manning and Taylor 2001), as well as
nonhuman primates (Howlett et al. 2015;Nelsonetal.2010).
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Related to these findings, the persistence hunting hypothesis has previously been
invoked to highlight a potential link between digit ratio and hunting success among
forager populations (Longman et al. 2015). Proponents of the persistence hunting
hypothesis posit that humans are well-adapted endurance runners. Lieberman et al.
(2007a) have suggested that endurance running is an important evolved trait that has
allowed human hunters to capture their prey by persistence hunting; following an
animal, keeping it above its trot-gallop transition for several hours, driving the animal
into hyperthermia so that it can be killed safely at close range(2007a:290). Drawing
on persistence hunting theory, Longman et al. (2015:9) cited endurance running
performance in a half-marathon as a proxy for hunting success. They found that
endurance running performance and digit ratio were inversely correlated and concluded
that hunting ability might therefore act as a reliable signal of male fitness(2015:8).
Although complex, Longman et al.s(2015) argument is based on the following
clearly-set-out assumptions: (a) Hunting, as others have suggested (e.g., Hawkes et al.
2018, though see Stibbard-Hawkes 2019), is a form of male intrasexual competition
and acts as an honest signal (Hawkes and Bird 2002), perhaps of athletic ability. (b)
Athletic ability is positively related to testosterone. (c) Adult testosterone is positively
related to reproductive fitness, including measures of fertility. (d) Adult testosterone is,
furthermore, positively related to developmental androgen exposure. (e) The 2D:4D
ratio is also related to developmental androgen exposure.
Although the argument is clear, there are some complications. First, Longman et al.s
argument rests on a causally complex chain of assumptions. Therefore, although each
of these assumptions appears evidenced and reasonable, the model lacks parsimony.
Second, the ecological validity of the persistence hunting hypothesis has been the
subject of some debate (Lieberman et al. 2007b; Pickering and Bunn 2007). In
ethnographic and living hunter-gatherer populations, persistence hunting is rare. Even
among the Kua San, one of only two modern forager groups known to employ
endurance hunting, the practice is uncommon, employed far less frequently than
ambush hunting (Pickering and Bunn 2007). Hunters in most ethnographically record-
ed and living hunter-gatherer groups more often either make use of netting and ambush
tactics (Wilkie and Curran 1991) or, in the majority of cases, hunt using long-range,
mechanically projected weapons and weapon poisons (Lee 1979; Marlowe 2010).
Lieberman et al. (2007b) have, however, argued that too great an emphasis is placed
on the ethnographic record, especially given that mechanically projected weaponry and
weapon poisons appear to be relatively recent inventions (Brown et al. 2012; Wadley
et al. 2015). This debate has not been resolved to the satisfaction of all parties
(Lieberman et al. 2007b). It is apparent, however, that marathon performance,
Longman et al.s chosen proxy for hunting success, is at the very least not an
ecologically valid reflection of hunting practices in most extant forager groups and
perhaps not in the majority of past populations either (Pickering and Bunn 2007). There
is a call to test this hypothesis using measures that are more reflective of real foraging
patterns in living populations.
Third, Longman et al.s sample of Western, educated participants in a Cambridge-
shire fun-run are likely to be unrepresentative of foragers in several respects, including
diet, activity budget, and pathogen exposure. For most of our evolutionary history,
humans have lived in small-scale societies without schools, hospitals, or supermarkets
and with greater exposure to hunger, pain, chronic diseases, and lethal dangers
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(Henrich et al. 2010:80). For this reason, Western populations may often be the worst
population[s] from which to make generalizations(Henrich et al. 2010:79), and results
typical of studies from Western populations often do not conform to patterns observed
elsewhere (Henrich et al. 2010). Indeed, the pattern of sexual dimorphism in digit ratio
frequently observed in Western populations (e.g., Manning et al. 2000) does not hold
true for the Hadza (Apicella et al. 2016). Despite these concerns, Longman et al. (2015)
set out a reasoned precedent for expecting an inverse relationship between digit ratio
and hunting success.
The Hadza of northern Tanzania are one of very few populations who continue to
subsist through hunting and gathering. Endurance running does not feature in the
Hadza hunting repertoire. However, general fitness and athletic ability, elsewhere
associated with digit ratio, are certainly important; Hunting reputation has been asso-
ciated with several indices of athletic ability (Apicella 2014; Stibbard-Hawkes et al.
2018) and many foraging activities, including climbing for honey, carrying food and
other resources, as well as drawing/aiming bows, require a significant degree of
strength and physical fitness (Apicella 2014; Blurton Jones et al. 2002).
Digit ratio has been previously measured among the Hadza. A recent sum-
mary of published data on Hadza digit ratios (Apicella et al. 2016) showed an
absence of sexual dimorphism in left-hand measurements (0.9730.997 for men
vs. 0.9651.01 for women). In right-hand measurements, contrary to the pattern
commonly observed elsewhere, Hadza women had a lower mean digit ratio than
Hadza men (0.9820.994 for men vs. 0.9650.980 for women) (Apicella et al.
2016). Butovskaya et al. (2010,2015) did find that Hadza men had lower digit
ratios than Hadza women. Butovskaya et al. (2010) report apparent sexual
dimorphism in both hands (0.95 in both male left- and right-hand mean digit
ratios, respectively), although Butovskaya et al. (2015) report smaller sex
differences (mens digit ratios had a mean of 0.98 in left-hand measures and
0.97 in right-hand measures, compared with left and right means of 0.99 and
0.98, respectively, for women). Furthermore, Hadza male digit ratios were, in
most cases (Apicella et al. 2016; Butovskaya et al. 2015; though not
Butovskaya et al. 2010), higher than or equivalent to mean digit ratios observed
among women in US samples (e.g., Puts et al. 2004). Although there is no
conclusive explanation for this pattern, Apicella et al. (2016) have proposed
that the Hadza may have different hormonal profiles than people in Western
populations, perhaps because of greater exposure to energetic stress.
Given these results, it is clearly important to test for relationships between 2D:4D
and hunting success in an actual forager population who live in a subsistence environ-
ment more representative of pre-agricultural populations than do participants drawn
from Western, industrialized societies. Indeed, Longman et al. concluded that further
work is now required to test [for a relationship between 2D:4D and hunting success] in
hunting societies(2015:9). Despite this, no other studies have, to date, searched for an
association between digit ratio and hunting success among either the Hadza or, to the
authors knowledge, any other hunter-gatherer group. Stibbard-Hawkes et al. (2018)
recently introduced a novel measure of hunting reputation which acted as a viable
proxy of hunting skills. I use this metric, as well as two further indices of hunting
ability (upper body strength and aim), along with visual acuity, to address this question.
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Materials and Methods
Study Population
The Hadza are an ethnolinguistic group living in the Lake Eyasi region of northern
Tanzania. Around 250 Hadza hunt and gather for the great majority of their diet: >95%
by some estimates (Wood and Marlowe 2013), although it appears probable, from
personal observation, that this proportion has decreased in recent years, following an
increased reliance on grain and other cultigens. Like many other forager groups (Bliege
Bird and Bird 2008; Marlowe 2007), the Hadza have a strong sexual division of labor:
hunting and honey-collecting among the Hadza are predominantly male activities,
while women gather the majority of tubers, berries and baobab fruit. This division of
labor appears at an early age (Crittenden et al. 2013; Froehle et al. 2018; Lew-Levy
et al. 2019). The great majority of Hadza mensforagingtrips(89%) are solitary
(Berbesque et al. 2016). Men hunt using bows and poisoned arrows and generally
ambush and shoot, then follow prey until they succumb to poison-induced cardiac
arrest (Marlowe 2010). The Hadza, like many foragers, never practice endurance
hunting. They have no cause to, partly because of to the efficacy of weapon poisons
and because the Eyasi region is shrubland (Marlowe 2010) with low visibility and
much cover. The Hadza exhibit a high degree of food-sharing and in some datasets
(Hawkes et al. 1991), though not others (Wood and Marlowe 2013; cf. Hawkes et al.
2014), a hunter and his family receive no more of the food he brings back than does any
other person living in the same camp. As with other hunter-gatherer populations, foods
collected by men are both more temporally variable (i.e., unreliably attained) and more
widely shared (Hawkes et al. 1991) than foods collected by women. Hadza men are
more willing than Hadza women to take on risk in experimental settings also (Apicella
et al. 2017). Hadza camps, usually composed of 2030 people (range = 6139), are
ephemeral, and most individuals move camp on average 6.5 times per year (Marlowe
2010). The Hadza show no assortative mating preferences for body height, weight, or
grip strength (Sear and Marlowe 2009), although both men and women cite the
importance of character, physical appearance, and foraging ability in a potential spouse
(Marlowe 2004b). Better hunters are also preferred as campmates (Smith and Apicella
2019; Wood 2006). As among many other forager populations (Dyble et al. 2015),
Hadza marital residence is multilocal (Marlowe 2004a); in-settlement relatedness is low
relative to most unilocal populations, and a mean of 27% of camp members share at
least one great-grandparent, only 16% higher than chance (Blurton Jones 2016:101).
Both Hadza women and men are afforded almost complete autonomy over who they
choose to marry or whether to divorce (Marlowe 2010).
In this study, I collected hunting reputation measures for 71 hunters, all male, aged
between 17 and 75 (mean = 39). Of these, 70 provided digit ratio data, 68 provided bow
pull strength data, and 64 participated in an archery contest. Most Hadza are nonliterate.
Accordingly, I explained at the beginning of each camp visit what would be involved in
each measurement. I explained, in lay terms, the purpose of each measurement. Hunters
were drawn from 17 camps during three separate field trips (17 August17 September
2013; 7 December 20136 January 2014; 19 October27 November 2014). On the last
of these trips, with the help of two research assistants, I conducted hunting reputation
interviews. The 67 interviewees whose answers I included in the final sample were of
Human Nature
both sexes (m = 36, f = 31). Participants were remunerated with gifts: shoes, blankets,
soap, petroleum jelly, plates, hammers, cold chisels, and other useful items. Some
interviewees in the Mangola region, where local laws required it, were remunerated
with money, equivalent to GBP (or ) Springer: choose one or the other 6 per camp
visit and shared between all camp members (including nonparticipants). I assured
people that they were free not to participate in any measures and to drop out of the
study at any time. I also made it clear that those who decided not to participate in some
measurements would still receive gifts when I left camp.
Hunting Reputation Measure
Reputation data interviews were conducted in the seclusion of a field vehicle, where
participantsanswers could not be overheard. I showed each participant a high-
resolution face-on photograph of each one of the 71 hunters in my sample. I asked
them to provide the hunters first name, his fathers name, and the length of time that
they last lived in the same camp with the interviewee. A hunters renown was defined
as the number of interviewees who knew both that huntersnameandhisfathersname.
In order to ensure that interviewees were familiar with the hunters they were ranking,
photographs were removed if the interviewee did not answer the first two questions
correctly, or had not lived with the photographed hunter within the previous two years.
Next, I set out the remaining photographs in a random order and asked interviewees to
remove the photograph of the best hunter. The photograph was removed, and the
process repeated, until each interviewee had provided a ranked list of every hunter
they knew in the sample, ordered from best to worst.
These ranked lists were then collated using the methods set out by Stibbard-Hawkes
et al. (2018). I took the proportional rank of each hunter in each list (i.e., the fraction of
the way up each list that each hunter appearedfor example, a hunter halfway up a list
of 20 would score 0.5). I then took the mean of these scores for each of the 71 hunters,
for each list in which that hunter appeared, giving me an aggregated index of hunting
reputationfor each of the 71 hunters in my sample. The measure is an aggregation of
ordinal rank data, although it behaves and is treated as a continuous variable. A
mathematical formalization of the procedure is provided in ESM §1.
This reputation measure showed a high degree of internal consistency, and it
significantly predicted skill on three hunting tasks as set out further by Stibbard-
Hawkes et al. (2018) and in ESM §2. For this reason, aggregated reputation data
appear to act as a serviceable proximate measure of true hunting skill (i.e., true skill at
finding and killing wild animals). Furthermore, and unlike previously used reputation-
based measures (e.g., Apicella 2014; Blurton Jones and Marlowe 2002; Marlowe
2000), the current method allowed fine-grained distinctions to be made between
hunters at all levels of ability across camps.
Digit Ratio
I measured the length of the index finger (second digit) and ring finger (fourth digit) on
first the right hand and then the left hand from the center of the basal crease to the distal
tip of the finger using battery-operated digital callipers. Digit ratios may be more
accurately collected using a scanner (Kemper and Schwerdtfeger 2009), but callipers
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have been elsewhere shown to be sufficiently reliable (Voracek et al. 2007), have been
successfully implemented in previous Hadza studies (Apicella et al. 2016), and were
deemed the most practicable method under field conditions. It should be noted,
however, that direct measures often yield higher ratios than indirect measures
(Ribeiro et al. 2016). Due to time constraints, digit lengths were measured only once
for each participant by a single observer. This constitutes a limitation.
A participants digit ratio was calculated by dividing the length of the second finger
by the length of the fourth finger for both hands. Where participants had sustained
injury to either hand, measures from that hand were excluded from the final analysis.
Hunting Ability Measures
Bow Pull Strength Bow pull strength was measured using an Easton Digital bow pull
scale. Hunters hooked the scale to the string of their own bow and drew the bow at peak
strength for fifteen seconds with their dominant arm. To allow direct comparison with
previous studies (e.g., Blurton Jones and Marlowe 2002), weight at peak pull was
recorded and reported in pounds (lbs).
Bow Aim I measured aim in an archery contest. Following the methods of Blurton
Jones and Marlowe (2002), I constructed a 61 by 61 cm cardboard target with an
opaque outer circle of 33 cm in diameter, a transparent inner circle of 20 cm in
diameter, and an opaque bullseyeof 4 cm in diameter. The target was attached to a
tree or other solid structure at shoulder height. Care was taken to ensure that terrain was
reasonably flat, and that view of the target was unobstructed by grass or foliage.
Although it was impossible, in field conditions, to completely standardize wind speed,
archery contests were conducted only at a Beaufort wind force of two or less, using the
Beaufort wind scale. Participants fired at the target using their own bows and arrows
from distances of 10, 20, and 30 m. They took three shots from each distance, nine
shots in total. Hits were scored at 100 for a bullseye, 50 on the inner ring and 25 on the
outer ring. It would have been ideal to conduct these tests without observers, although
given the excitement each archery contest generated, this proved impracticable.
Visual Acuity
I measured visual acuity for each eye using a 3 m Landolt C Optotype. This does not
require literacy or knowledge of the Latin alphabet. Since tests were conducted outside
in sunlight, I could not completely standardize lighting conditions between different
camps. However, I always ensured that the chart was clearly illuminated, free from
glare, and that the sun was behind the participant. Results were recorded as LogMAR
scores, and visual acuity in the best eye was used in the final analysis.
Research Clearance, Data Security and Availability
Research was approved by Cambridge Biological Research Ethics Committee and
conducted with permission from the Tanzanian Commission for Science and Technol-
ogy (COSTECH Permits: 2013271-ER-2000-80 and 2014317-ER200080).
Human Nature
Clearance to conduct research was also attained at the local level and from all
participants. All data were stored on a password-protected hard-drive encrypted using
Apple Firevault and anonymized using ID numbers known only to a small group of
Hadza researchers. Certain participants are known by their hunting reputations. For this
reason, to protect participants anonymity, I have not made hunting reputation, skill, or
age data freely available online. However, finger lengths and associated digit ratios,
which hold no value as individual identifiers, are available online at
Mean Hadza digit ratio in the current sample was 1.00 for right-hand measures (n=69,
SD = 0.04, range = 0.931.09) and also 1.00 for left-hand measures (n=69,SD=0.05,
range = 0.881.12). Neither departed significantly from the expectation of a normal
distribution in a Shapiro-Wilk test of normality (right hand, W=0.98, p= 0.22; left
hand, W=0.98,p= 0.45; Fig. 1). Descriptive statistics are provided for each variable in
the study in Table 1. As expected, in both hands, second- and fourth-finger lengths
showed a significant, strong positive linear relationship. Right second-finger length
increased 0.85 cm for each 1 cm increase in right fourth-finger length (p<0.00; R2=
0.70) while left second-finger length increased by 0.78 cm for each 1 cm increase in left
fourth-finger length (p<0.00; R2= 0.61). The regression lines for both hands had non-
zero intercepts (Fig. 2), and intercepts were significantly higher than zero for both right
(b=9.65, p= 0.03) and left hands (b=14.45, p= 0.01), implying that 2D:4D ratio
decreases as digit length increases in large samples. However, there was no significant
relationship between average digit length and digit ratio in the current dataset for either
the right-hand (F1,67 =0.05, R
2=0.00, p= 0.82) or left-hand (F1,67 =0.00, R
p= 0.98) measure.
0.90 0.95 1.00 1.05 1.10
Right Hand Digit Ratio (2D:4D)
0.90 0.95 1.00 1.05 1.10
Left Hand Digit Ratio (2D:4D)
Fig. 1 Histograms of right (left) and left (right) hand 2D:4D ratio for 67 Hadza men
Human Nature
Hunting reputation showed no significant relationship with digit ratio for either
hand. Although the slope of the regression line was in the expected direction for both
the right and the left hand, the value of R2was near zero in both cases (R = 0.01, 95%
CI = 0.11, 0.02; L = 0.03, 95% CI = 0.15, 0.01). Consistent with findings from many
other populations (e.g., Koster et al. 2019; von Rueden et al. 2008), hunting reputation
showed a quadratic relationship with age. It peaked at roughly 45 and gradually
declined thereafter. For this reason, and following the recommendations of Blurton
Jones (2016), I included both age and age2as controls. The effect of 2D:4D ratio did
Table 1 Descriptive statistics for each variable
Va r i ab l e NMean SD Median Min Max Range
Right Ring Finger (D4) Length 69 64.93 4.75 64.95 53.85 80.32 26.47
Right Index Finger (D2) Length 69 65.04 4.85 64.83 55.28 79.71 24.43
Right 2D:4D 69 1 0.04 1 0.93 1.09 0.16
Left Ring Finger (D4) Length 69 65.51 4.82 65.21 55.66 79.36 23.7
Left Index Finger (D2) Length 69 65.31 4.80 64.9 51.14 80.7 29.56
Left 2D:4D 69 1 0.05 1 0.88 1.12 0.25
Age 71 39.18 14.81 38 17 75 58
Aim 65 143.46 74.91 150 0 350 350
Bow Pull Strength (KG) 67 61.61 16.24 60.8 18.88 92.8 73.92
Best Eye Visual Acuity (LogMar) 70 0.03 0.23 0.1 0.3 0.8 62
Hunting Score 71 0.47 0.17 0.45 0.18 0.88 0.7
Renown (n= 89) 71 53.63 16.69 53 22 84 62
55 60 65 70 75 80
Right Hand 4th Digit Length (mm)
Right Hand 2nd Digit Length (mm)
55 60 65 70 75 80
Left Hand 4th Digit Length (mm)
Left Hand 2nd Digit Length (mm)
Fig. 2 Scatterplot of ring finger length by index finger length for all hunters in both right (left) and left (right)
hand measures, overlaid with a simple linear regression line and 95% confidence band
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not approach significance whether or not age and age2were included as controls
(Table 2), nor when including huntersrenownas a control (see ESM §3).
I also tested for a relationship between digit ratio and three other variables: draw
strength, best-eye visual acuity, and aim with a bow and arrow. In a simple linear
regression, there was no significant relationship between either right- (F1,63 =0.00,
R2=0.00, p= 0.60) or left-hand (F1,63 =0.00, R
2=0.00,p= 0.99) digit ratio and draw
strength. I calculated 95% confidence intervals for R2,whichwere0.09 and 0.03 for
the right-hand measure and 0.06 and 0.06 for the left-hand measure. Similarly, I
found no significant relationship between right- (F1,61 =0.55, R
2=0.01, p=0.46) or
left-hand (F1,61 =2.60, R
2=0.04, p= 0.11) digit ratio and aim with a bow and arrow.
The R295% confidence intervals were 0.03 and 0.11 for the right-hand measure and
0.18 and 0.00 for the left-hand measure. Furthermore, I found no significant rela-
tionship between either right- (F1,66 =1.7,R
2=0.01,p= 0.20) or left-hand (F1,66 =0.04,
R2=0.001, p= 0.84) digit ratio and visual acuity. The R295% confidence intervals
Table 2 Regression Models of Mean Hunting Score and 2D:4D Ratio with and without Age and Age2,first
for right hand, then for left hand measures. Regression coefficients are reported in their natural units and as
standardized βcoefficients
Model B SE B βR2Adjusted R2p
1.1 0.011 0.004 0.385
Intercept 0.895 0.489 0.072
Right Hand 2D:4D 0.426 0.488 0.106 0.385
1.2 0.064 0.036 0.111
Intercept 0.829 0.480 0.089
Right Hand 2D:4D 0.463 0.478 0.115 0.335
Age 0.003 0.001 0.230 0.057
1.3 0.103 0.062 0.068
Intercept 0.452 0.526 0.393
Right Hand 2D:4D 0.321 0.480 0.080 0.506
Age 0.016 0.008 1.344 0.051
Age20.0002 0.0001 1.133 0.010
2.1 0.028 0.013 0.171
Intercept 1.046 0.419 0.015
Left Hand 2D:4D 0.580 0.419 0.167 0.171
2.2 0.073 0.045 0.082
Intercept 0.949 0.416 0.026
Left Hand 2D:4D 0.577 0.413 0.166 0.166
Age 0.002 0.001 0.213 0.213
2.3 0.120 0.079 0.040
Intercept 0.637 0.442 0.154
Left Hand 2D:4D 0.517 0.407 0.148 0.208
Age 0.016 0.008 1.411 0.036
Age20.0002 0.0001 1.218 0.067
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were 0.01 and 0.15 for the right-hand measure and 0.07 and 0.05 for the left-hand
measure. Scatterplots are provided in Fig. 3.
Analogous analyses, accompanied by a widely applicable information criterion
(WAIC) model selection, were also conducted using a Bayesian framework. These
analyses yielded similar results to those reported here and did not provide any more
certainty about the relationships between the variables under investigation. Since
frequentist analyses are still both widely used and more widely understood, I have
opted to report frequentist results in the body of the article. Results of the Bayesian
reanalysis are reported in ESM §4.
Hadza Digit Ratio Compared with Previous Hadza Studies and Other Populations
Mean male Hadza digit ratios in the current study, 1.00 for both right- and left-hand
measures, are higher but almost within the range (0.970.99) of mean male digit ratios
in most other Hadza datasets (Apicella et al. 2016;Butovskayaetal.2012,2015)and
notably higher than those reported by Butovskaya et al. (2010) (R and L means = 0.95).
This is not because every hunter had an even digit ratio, but because participants had
digit ratios both above and below 1.0 (e.g., right-hand range = 0.931.09). These results
are dissimilar to those often found in Western populations (e.g., means: men = 0.95,
women = 0.97 among undergraduates from the University of Pittsburgh, reported by
Puts et al. 2004) and higher than the upper ranges of male digit ratio reported in a recent
review by Ribeiro et al. (2016) (i.e., 0.98 for right-hand measures in both US and Saudi
populations). Since this study was concerned with hunting, a typically male activity
among the Hadza, I did not collect any data on female digit ratio for comparison. For
this reason, although current data are consistent with Apicella et al.s(2016)finding
that Hadza men do not have significantly lower digit ratios than women, I lack the data
to replicate these findings in the current study.
It is unclear why Hadza mens digit ratios are higher than those reported for men in
many other study populations (e.g., Puts et al. 2004). This is relevant to the current
question because, drawing on the method of Longman et al. (2015), digit ratios are
treated in this study as a proxy measure of developmental androgen exposure. It is
possible that, as Apicella et al. have suggested, Hadza men have different hormonal
profiles than those in other populations, and the suppression of testosterone might be
advantageous under conditions of energetic stress, reducing muscle composition and
the metabolic requirements of its maintenance(2016:6). A study by Muller et al.
(2009), to my knowledge, contains the only published measure of testosterone levels
among the Hadza. Their results indicate that Hadza adult male salivary testosterone
may indeed be lower than is found in Western populations (pmol l1151 among the
Hadza vs. pmol l1250 in US study populations). However, their Hadza sample size,
at 27 individuals, was small.
This question may be fertile grounds for further enquiry. Butovskaya et al. (2015)
compared digit ratios between Hadza foragers and Datoga pastoralists and reported
higher mean digit ratios among the Hadza than among the Datoga (Hadza R = 0.97,
L = 0.98 vs. Dotaga R = 0.96, L = 0.96). Although the Datoga live in the same
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0.95 1.00 1.05
Right Hand Digit Ratio
Hunting Reputation Score
0.90 0.95 1.00 1.05 1.10
Left Hand Digit Ratio
Hunting Reputation Score
0.95 1.00 1.05
Right Hand Digit Ratio
Bow Pull Strength (KG)
0.90 0.95 1.00 1.05 1.10
Left Hand Digit Ratio
Bow Pull Strength (KG)
0.95 1.00 1.05
Right Hand Digit Ratio
0.90 0.95 1.00 1.05 1.10
Left Hand Digit Ratio
0.95 1.00 1.05
Right Hand Digit Ratio
Best Eye Visual Acuity (LogMar)
0.90 0.95 1.00 1.05 1.10
Left Hand Digit Ratio
Best Eye Visual Acuity (LogMar)
Human Nature
geographic region as the Hadza, they have much higher rates of polgygny, less paternal
care, and very different subsistence practices (Butovskaya et al. 2015;Mulleretal.
2009). However, Muller et al. (2009) have previously shown that the difference
between both morning and evening salivary testosterone levels from Hadza and Datoga
men was nonsignificant, and both had greatly lower salivary testosterone levels than is
generally observed in adult men from Western (US) populations analyzed in the same
Digit ratios have been reported from at least one other population who have
traditionally practiced hunting and gathering. Pettigrew et al. (2017)reportedthatmean
digit ratios directly measured among the San showed strong sexual dimorphism (Male
R = 0.96, L = 0.95 vs. Female R = 1.02, L = 1.03). San men in this sample had digit
ratios comparable to or lower than those often reported in Western men (Pettigrew et al.
2017). In a separate study, Worthman and Konner (1987) showed that blood plasma
testosterone measures among San hunters were not significantly different to Western
men, a different pattern to that observed by Muller et al. (2009) among the Hadza.
However, since Pettigrew et al. (2017)andWorthmanandKonner(1987)reportedtheir
results 30 years apart, working with different individuals, the grounds for comparison
are limited. Overall it is difficult to draw conclusions from these data, and more cross-
cultural comparative research examining digit ratios in a range of subsistence environ-
ments is warranted.
Digit Ratio and Digit Length
Kratochvíl and Flegr (2009)andLollietal.(2017) have proposed that sexual
dimorphism in digit ratio may not be a direct consequence of differences in
developmental androgen levels between men and women. They have instead
argued that sexual dimorphism in digit ratio may be the consequence of a
nonlinear scaling relationship between digit ratio and absolute digit length. In a
sample of 297 Czech biology students, Kratochvíl and Flegr (2009)notedthat
the relationship between 2D and 4D length had a positive intercept, implying a
decrease in the ratio between finger lengths as absolute finger lengths increase.
This, they argued, might account for patterns of sexual dimorphism in digit
ratio. Lolli et al. (2017) took this idea further and, in a study of 154 men and
262 women, also concluded that an allometric, nonlinear size-scaling relation-
ship was the most plausible model for normalizing 2D to 4D lengths. More-
over, after normalizing for scaling relationships, they found that womens digit
ratios were actually lower than mens.
The relationship between the length of the second finger and the length of
the fourth finger in the current dataset shows a very similar pattern to that
reported by Kratochvíl and Flegr (2009). As in Kratochvíl and Flegrsstudy,
the intercept of the regression line of left- and right-hand second and fourth
digit lengths was positive, implying that digit ratio decreases as finger length
increases (Fig. 2). Hadza men in the current study have lower absolute mean
digit lengths (2D = 65 mm, 4D = 65 mm) than men in Western populations (e.g.,
RFig. 3 Scatterplots of Hunting Reputation Score, Bow Pull Strength, Aim, and Visual Acuity by right- and
left-hand digit ratio, overlaid with simple linear regression lines and 95% confidence bands
Human Nature
4D = 77.46 mm and 2D = 73.04 mm),
suggesting that the relatively high digit
ratio observed in the study population may be a function of allometric scaling.
This evidence may cast doubt on claims that digit ratio is a direct consequence of
developmental androgen exposure. This relationship may, instead, be confounded in
many populations by sex difference in absolute finger length. However, the validity of
this interpretation is in turn unclear for at least five reasons.
First, there is no significant relationship between mean digit length and digit ratio
within the range of finger lengths measured in the current sample and, as with those
data presented by Kratochvíl and Flegr (2009), the assumed allometric relationship
between finger lengths and finger ratio is an implication of the slope of the regression
line. It is not a verifiable pattern in the current dataset. This pattern may be the result of
normal levels of error, rather than genuine evidence of allometry (Forstmeier 2011).
Second, the current data are from adult men only. I collected no data from women,
nor from those under the age of 16, and there are no data on either Hadza fetal androgen
levels or fetal digit ratios. It is, therefore, impossible to say whether the same finger-
length scaling relationship exists at all ages, and in other populations (Galis et al. 2010;
Hönekopp and Watson 2010), this appears not to be the case. A study of the digit ratios
of Hadza children, and perhaps including the Hadza womens digit ratios gathered by
Apicella et al. (2016), would shed further light on this question.
Third, although they cautioned against using ratios where scaling relationships are
evident, in a reanalysis of Kratochvíl and Flegrs data, Forstmeier concluded that
human digits are probably very close to being isometricand are fairly independent
of absolute size(2011:1860, although see Forstmeier 2018).
Fourth, and perhaps most problematic, is the fact that much recent evidence from
Western populations shows an absence of allometry effects on digit ratio. Galis et al.
(2010), in a study of fetuses, showed that sex differentiation in 2D:4D appears as early
as 14 weeks. Manning and Fink (2018) found that, although digit length increased and
sexual dimorphism in digit length changed across development, digit ratio was not
wholly age dependent. Furthermore Lombardo et al. (2018) found that throwing
performance among college-aged women was negatively related to digit ratio indepen-
dent of size-scaling effects.
Fifth and finally, Hadza digit ratios were higher than those reported among San
foragers (Pettigrew et al. 2017), even though San men had mean absolute second digit
lengths comparable to those of Hadza men in the current sample (e.g., Hadza R 2D =
65.04 mm, 4D = 64.93 mm, San R 2D = 65.15 mm, 4D = 67.7 mm).
Therefore, although the digit ratios of Hadza men reported here and elsewhere
(Apicella et al. 2016) are higher than in many other populations, this fact may not be
explained by size-scaling relationships.
Digit Ratio, Hunting Reputation, and Hunting Skill
In the current study there was no significant relationship between digit ratio for either
hand and hunting reputation. Furthermore, there was no significant relationship be-
tween digit ratio for either hand and either aim with a bow or the bow pull measure of
Statistics from a study of 849 Californian men published by Lippa (2003:183), one of the few studies to
report absolute finger lengths.
Human Nature
strength. Eyesight also showed no relationship to digit ratio in either hand. However,
although eyesight has been proposed as important to hunting success (e.g., Blurton
Jones 2016), Stibbard-Hawkes et al. (2018) found that visual acuity was unrelated to
both archery skill and hunting reputation within the normal range of human vision. The
analogous reanalysis using Bayesian methods (ESM §4) yielded comparable results
relationships between digit ratio and the other variables under investigation were weak.
Only one of the models was substantially preferred to the mean of the outcome variable
(the null model) in a WAIC model selection: bow aim was negatively related to left-
hand digit ratio. However, right-hand digit ratio showed a positive relationship to bow
aim, muddying the results.
Such results are difficult to interpret, and current findings leave open several
possibilities. It is possible that, among the Hadza, hunting might not, as Longman
et al. have suggested, act as either a marker of testosterone exposure(2015:3) or a
signal of reproductive potential(2015:6). It is further possible that digit ratio is
unrelated, among Hadza men, to developmental androgen exposure or that develop-
mental androgen exposure is unrelated to hunting success. It is finally possible that the
current sample size was not large enough to detect genuine but weak associations
between digit ratio and hunting ability, an interpretation consistent with the small effect
sizes often observed in digit ratio research. These possibilities I discuss further.
As set out in the introduction, Longman et al. (2015) argue that hunting ability
should be negatively associated with digit ratio. Their argument is based on the
following assumptions: (a) Hunting acts as a signal of athletic ability. (b)Athletic
ability is positively related to testosterone. (c) Adult testosterone is positively related to
fertility or other measures of reproductive fitness.(d) Adult testosterone is, further-
more, positively related to developmental androgen exposure. (e) 2D:4D ratio is a
marker of developmental androgen exposure.
Longman et al. (2015) present a logical argument, supported by data showing a
significant negative relationship between endurance running performance and digit
ratio. However, the model is complex and relies on a long chain of relationships
between variables. As a consequence, if any one of these associations is not valid or
does not apply in a particular case, the entire model is called into question. If, for
example, Hadza hunting reputation does not reliably reflect athletic ability, a negative
relationship between hunting reputation and digit ratio should not be expected. If digit
ratio is not negatively related to adult testosterone levels, as suggested by Hönekopp
et al. (2007), a negative relationship between digit ratio and reproductive outcomes
might not be expected. Furthermore, if the negative relationship between developmen-
tal androgen exposure and digit ratio is not universal (e.g., Apicella et al. 2016), a
relationship between athletic ability and digit ratio also should not be expected.
Similarly, the relationships between testosterone levels and fertility or other heritable
measures of reproductive fitnessare not straightforward (e.g., Scott et al. 2012),
further complicating Longman et al.smodel.
It is difficult to tell which assumption, if any, is incorrect or inapplicable to the
Hadza case. Furthermore, even if all the assumptions of the model are valid, if one or
more of the associations is, in general, weak, then the chain of associations may be
prone to weak or nonsignificant results overall. This may further account for the
absence of a significant association between digit ratio and hunting reputation in the
current study.
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There are two further difficulties in interpreting current results. Hunting reputation
here is an indirect proxy measure of true hunting skill. The measure is an aggregation of
ordinal rank data, although it behaves and is treated as a continuous variable. Because
of the significant associations found by Stibbard-Hawkes et al. (2018) between hunting
reputation and strength, aim, and ecological knowledge, it is likely that hunting
reputation, to a certain extent, reflects athletic ability and is externally valid. However,
much of the variance in hunting reputation remains unaccounted for, and although
participants showed high levels of agreement concerning the hunting ability of those
they were rating, some error has probably been introduced in aggregating ranked lists
with unequal numbers of cases. For this reason, any associations that exist between true
hunting ability and digit ratio may not be captured in the current measure of hunting
reputation. I here addressed this issue by relating 2D:4D ratio to two other component
hunting skills (a measure of strength and a measure of aim with a bow and arrow) as
well as eyesight. None of these variables show a significant relationship with 2D:4D
ratio in either direction. In the Bayesian analysis (ESM §4), left-hand digit ratio did
negatively predict aim (the expected direction), although conversely right-hand digit
ratio showed a positive relationship to aim.
The lack of significant associations in the current study could also be a consequence
of sample size. Those associations that do exist between digit ratio and spatial ability,
strength, sporting achievement, voice pitch, age-specific reproductive success, and so
on, in other populations usually have small or modest effect sizes. The sample sizes in
the current study (n=6367) are lowa fraction of those used elsewhere (e.g.,
Manning and Fink 2008). It is possible that, if there were small or even moderate
associations between digit ratio and hunting success, they would not be apparent in a
sample of this size. This is, perhaps, grounds for conducting a further study with a
larger sample size. However, given that fewer than 150 Hadza men are still hunting for
the majority of their diet, the current sample includes a reasonable proportion of the
total population. Even a study that included the entire population of hunters would
probably yield similar results. This is especially likely given that, in the current study,
associations between 2D:4D ratio and other variables were close to zero and did not
approach significance, as well as the fact that 95% confidence intervals suggested even
the highest probable value of R2was still low (0.15)a finding mirrored in the
Bayesian reanalysis (ESM §4).
In summary, results from the current study fail to support the prediction that digit ratio
should be related to hunting ability (Longman et al. 2015). Longman et al.smodelis
based on a large number of assumptions, and if the results do reflect a genuine absence
of relationship between digit ratio and hunting skill, it is unclear which of Longman
et al.s assumptions may not apply in the Hadza case. Due to the uncertainty inherent in
the reported results, it is further unclear whether there is a genuine absence of
relationship between the variables under study, or whether this is a consequence of
the small sample sizes typical of forager research, the small effect sizes commonplace
in digit ratio research, or, alternatively, noise and error in the variables used. Given the
small size of the adult male Hadza population, and some unavoidable imprecision in
Human Nature
hunting success measures (e.g., Hill and Hurtado 2009; Stibbard-Hawkes et al. 2018,
further research is sadly unlikely to produce more conclusive findings.
This study confirms that Hadza male digit ratios are higher than those generally
observed in Western populations (see Apicella et al. 2016;Butovskayaetal.2015;but
see Butovskaya et al. 2010) and among the San (Pettigrew et al. 2017). Results
corroborate previous evidence that Hadza digit ratios show little (Butovskaya et al.
2015), no, or reversed (Apicella et al. 2016) sexual dimorphism. Hadza men also have
shorter absolute finger lengths than men from many Western populations. However, the
possibility that high Hadza male digit ratios are the result of a size-scaling relationship
between relative second- and fourth-digit lengths (Kratochvíl and Flegr 2009; Lolli
et al. 2017) is called into doubt by the fact that San men have comparable second-digit
lengths but lower digit ratios than Hadza men (Pettigrew et al. 2017).
Whether or not there is an influence of allometry on 2D:4D ratio, current results
provide reason for caution in generalizing findings from Western, educated, industrial-
ized populations to other groups with differing diets, levels of pathogen exposure,
activity budgets, and modes of subsistence. This is especially important given that
findings from Western populations do not uniformly match findings from non-Western
populations in many other types of investigation (see Henrich et al. 2010). The current
study finds great uncertainty and no clear support for the idea that 2D:4D ratio and
hunting abilities are associated in a hunter-gatherer population. Results suggest that, if
such a relationship does exist, it is unlikely to be a strong one.
Acknowledgments Thanks to all those who have provided support throughout this project. Special thanks
to Frank Marlowe, Robert Attenborough, Jeremy Kendall, Coren Apicella, Sally Street, Enrico Crema, Trevor
Hawkes, and Charles Endeko. Thanks also to all those who participated in the project. Further thanks to
Bernhard Fink, Martin Voracek, and one anonymous reviewer for the help they have provided in revising and
improving this manuscript.
This project was funded with generous support from the Leakey Foundation, Robinson College Cambridge,
the Smuts Memorial Fund, the Ruggles-Gates Fund of the Royal Anthropological Institute, the Cambridge
Department of Archaeology and Anthropology, the Cambridge Centre for African Studies, the Anthony
Wilkin Fund, and the Ridgeway-Venn Fund.
Apicella, C. L. (2014). Upper-body strength predicts hunting reputation and reproductive success in Hadza
hunter-gatherers. Evolution and Human Behavior, 35(6), 508518.
Apicella, C. L., Tobolsky, V. A., Marlowe, F. W., & Miller, K. W. (2016). Hadza hunter-gatherer men do not
have more masculine digit ratios (2D:4D). American Journal of Physical Anthropology, 159(2), 223232.
Apicella, C. L., Crittenden, A. N., & Tobolsky, V. A. (2017). Hunter-gatherer males are more risk-seeking than
females, even in late childhood. Evolution and Human Behavior, 38(5), 592603.
Bennett, M., Manning, J. T., Cook, C. J., & Kilduff, L. P. (2010). Digit ratio (2D:4D) and performance in elite
rugby players. Journal of Sports Sciences, 28(13), 14151421.
Berbesque, J. C., Wood, B. M., Crittenden, A. N., Mabulla, A., & Marlowe, F. W. (2016). Eat first, share later:
Hadza hunter-gatherer men consume more while foraging than in central places. Evolution and Human
Behavior, 37(4), 16.
Bliege Bird, R., & Bird, D. W. (2008). Why women hunt: Risk and contemporary foraging in a Western Desert
Aboriginal community. Current Anthropology, 49(4), 655693.
Blurton Jones, N. G. (2016). Demography and evolutionary ecology of Hadza hunter-gatherers. Cambridge:
Cambridge University Press.
Human Nature
Blurton Jones, N. G., & Marlowe, F. W. (2002). Selection for delayed maturity. Human Nature, 13(2), 199
Blurton Jones, N. G., Hawkes, K., & OConnell, J. F. (2002). Antiquity of postreproductive life: Are there
modern impacts on hunter-gatherer postreproductive life spans? American Journal of Human Biology,
14(2), 184205.
Brosnan, M. J. (2008). Digit ratio as an indicator of numeracy relative to literacy in 7-year-old British
schoolchildren. British Journal of Psychology, 99(1), 7585.
Brown, W. M., Finn, C. J., & Breedlove, S. M. (2002a). Sexual dimorphism in digit-length ratios of laboratory
mice. Anatomical Record, 267(3), 231234.
Brown, W. M., Hines, M., Fane, B. A., & Breedlove, S. M. (2002b). Masculinized finger length patterns in
human males and females with congenital adrenal hyperplasia. Hormones and Behavior, 42(4), 380386.
Brown, K. S., Marean, C. W., Jacobs, Z., Schoville, B. J., Oestmo, S., Fisher, E. C., Bernatchez, J., Karkanas,
P., & Matthews, T. (2012). An early and enduring advanced technology originating 71,000 years ago in
South Africa. Nature, 491(7425), 590593.
Burley, N. T., & Foster, V. S. (2004). Digit ratio varies with sex, egg order and strength of mate preference in
zebra finches. Proceedings of the Royal Society B: Biological Sciences, 271(1536), 239244.
Butovskaya, M. L., Burkova, V., & Mabulla, A. Z. (2010). Sex differences in 2D:4D ratio, aggression and
conflict resolution in African children and adolescents: A cross-cultural study. JournalofAggression,
Conflict and Peace Research, 2(1), 1731.
Butovskaya, M. L., Vasilyev, V. A., Lazebny, O. E., Burkova, V. N., Kulikov, A. M., Mabulla, A., Shibalev, D.
V., & Ryskov, A. P. (2012). Aggression, digit ratio, and variation in the androgen receptor, serotonin
transporter, and dopamine D4 receptor genes in African foragers: The Hadza. Behavior Genetics, 42(4),
Butovskaya, M., Burkova, V., Karelin, D., & Fink, B. (2015). Digit ratio (2D:4D), aggression, and dominance
in the Hadza and the Datoga of Tanzania. American Journal of Human Biology, 27(5), 620627.
Cain, K. E., Bergeon Burns, C. M., & Ketterson, E. D. (2013). Testosterone production, sexually dimorphic
morphology, and digit ratio in the dark-eyed junco. Behavioral Ecology, 24(2), 462469.
Crittenden, A. N., Conklin-Brittain, N. L., Zes, D. A., Schoeninger, M. J., & Marlowe, F. W. (2013). Juvenile
foraging among the Hadza: Implications for human life history. Evolution and Human Behavior, 34(4),
Direnzo, G. V., & Stynoski, J. L. (2012). Patterns of second-to-fourth digit length ratios (2D:4D) in two
species of frogs and two species of lizards at La Selva, Costa Rica. Anatomical Record, 295(4), 597603.
Dunbar, R. I. M., & Wlodarski, R. (2016). Reply to Falcon. Biology Letters, 12,34.
Dyble, M., Salali, G. D., Chaudhary, N., Page, A., Smith, D., Thompson, J., Vinicius, L., Mace, R., &
Migliano, A. B. (2015). Sex equality can explain the unique social structure of hunter-gatherer bands.
Science, 348(6236), 769798.
Falcon, R. G. (2016). Stay, stray or something in-between? A comment on Wlodarski et al. Biology Letters,
Forstmeier, W. (2011). Women have relatively larger brains than men: A comment on the misuse of general
linear models in the study of sexual dimorphism. The Anatomical Record, 294, 18561863.
Forstmeier, W. (2018). Avoiding misinterpretation of regression lines in allometry: Is sexual dimorphism in
digit ratio spurious? bioRxiv, 298786.
Froehle, A. W., Wells, G. K., Pollom, T. R., Mabulla, A. Z., Lew-Levy, S., & Crittenden, A. N. (2018).
Physical activity and time budgets of Hadza forager children: Implications for self-provisioning and the
ontogeny of the sexual division of labor. American Journal of Human Biology, 31(1), 113.
Galis, F., Ten Broek, C. M. A., Van Dongen, S., & Wijnaendts, L. C. D. (2010). Sexual dimorphism in the
prenatal digit ratio (2D:4D). Archives of Sexual Behavior, 39(1), 5762.
Hawkes, K., & Bird, R. B. (2002). Showing off, handicap signaling, and the evolution of menswork.
Evolutionary Anthropology, 11,5867.
Hawkes, K., OConnell, J. F., & Blurton Jones, N. G. (1991). Hunting income patterns among the Hadza: Big
game, common goods, foraging goals and the evolution of the human diet. Philosophical Transactions of
the Royal Society, B: Biological Sciences, 334(1270), 243251.
Hawkes, K., OConnell, J. F., & Blurton Jones, N. G. (2014). More lessons from the Hadza about menswork.
Human Nature, 25(4), 596619.
Hawkes, K., OConnell, J., & Blurton Jones, N. G. (2018). Hunter-gatherer studies and human evolution: A
very selective review. American Journal of Physical Anthropology, 165(4), 777800.
Henrich, J., Heine, S. J., & Norenzayan, A. (2010). The weirdest people in the world. Behavioral and B rain
Sciences, 33(23), 6183.
Human Nature
Hill, K., & Hurtado, A. M. (2009). Cooperative breeding in South American hunter-gatherers. Proceedings of
the Royal Society B: Biological Sciences, 276(1674), 38633870.
Hönekopp, J., & Watson, S. (2010). Meta-analysis of digit ratio 2D:4D shows greater sex difference in the
right hand. American Journal of Human Biology, 22(5), 619630.
Hönekopp, J., Voracek, M., & Manning, J. T. (2006). 2nd to 4th digit ratio (2D:4D) and number of sex
partners: Evidence for effects of prenatal testosterone in men. Psychoneuroendocrinology, 31(1), 3037.
Hönekopp, J., Bartholdt, L., Beier, L., & Liebert, A. (2007). Second to fourth digit length ratio (2D:4D) and
adult sex hormone levels: New data and a meta-analytic review. Psychoneuroendocrinology, 32(4), 313
Howlett, C., Setchell, J. M., Hill, R. A., & Barton, R. A. (2015). The 2D:4D digit ratio and social behaviour in
wild female chacma baboons in relation to dominance, aggression, interest in infants, affiliation and
heritability. Behavioural Ecology and Sociobiology, 69,6174.
Kemper, C. J., & Schwerdtfeger, A. (2009). Comparing indirect methods of digit ratio (2D:4D) measurement.
American Journal of Human Biology, 191,188191.
Koster, J., McElreath, R., Hill, K., Yu, D., Glenn, S. J., Van Vliet, N., Gurven, M., Kaplan, H., Trumble, B.,
Bliege Bird, R., et al. (2019). The life history of human foraging: Cross-cultural and individual variation.
Kratochvíl, L., & Flegr, J. (2009). Differences in the 2nd to 4th digit length ratio in humans reflect shifts along
the common allometric line. Biology Letters, 5(5), 643646.
Lee, R. B. (1979). The !Kung San: Men, women, and work in a foraging society. Cambridge: Cambridge
University Press.
Lew-Levy, S., Boyette, A. H., Crittenden, A. N., Hewlett, B. S., & Lamb, M. (2019). Gender-typed and
gender-segregated play among Tanzanian Had za and Congolese BaYaka hunter-gatherer children and
adolescents. Child Development.htt ps:// dev.13306.
Lieberman, D., Tooby, J., & Cosmides, L. (2007a). The architecture of human kin detection. Nature,
445(7129), 727731.
Lieberman, D. E., Bramble, D. M., Raichlen, D. A., & Shea, J. J. (2007b). The evolution of endurance running
and the tyranny of ethnography: A reply to Pickering and Bunn (2007). Journal of Human Evolution,
53(4), 439442.
Lippa, R. A. (2003). Are 2D:4D finger-length ratios related to sexual orientation? Yes for men, no for women.
Journal of Personality and Social Psychology, 85(1), 179188.
Lolli, L., Batterham, A. M., Kratochvíl, L., Flegr, J., Weston, K. L., & Atkinson, G. (2017). A comprehensive
allometric analysis of 2nd digit length to 4th digit length in humans. Proceedings of the Royal Society B:
Biological Sciences, 248,17.
Lombardo, M. P., Otieno, S., & Heiss, A. (2018). College-aged women in the United States that playoverhand
throwing sports have masculine digit ratios. PLoS One, 13(9), e0203685.
Longman, D., Stock, J. T., & Wells, J. C. K. (2011). Digit ratio (2D:4D) and rowing ergometer performance in
males and fe males. American Journal of Physical Anthropology, 144(3), 337341.
Longman, D., Wells, J. C. K., & Stock, J. T. (2015). Can persistence hunting signal male quality? A test
considering digit ratio in endurance athletes. PLoS One, 10(4), e0121560.
Manning, J. T. (2002). The ratio of 2nd to 4th digit length and performance in skiing. Journal of Sports
Medicine and Physical Fitness, 42(4), 446450.
Manning, J. T., & Fink, B. (2008). Digit ratio (2D:4D), dominance, reproductive success, asymmetry, and
sociosexuality in the BBC internet study. American Journal of Human Biology, 20(4), 451461.
Manning, J. T., & Fink, B. (2018). Sexual dimorphism in the ontogeny of second (2D) and fourth (4D) digit
lengths, and digit ratio (2D:4D). American Journal of Human Biology, 30,17.
Manning, J. T., & Hill, M. R. (2009). Digit ratio (2D:4D) and sprinting speed in boys. American Journal of
Human Biology, 21(2), 210213.
Manning, J. T., & Taylor, R. P. (2001). Second to fourth digit ratio and male ability in sport: Implications for
sexual selection in humans. Evolution and Human Behavior, 22(1), 6169.
Manning, J. T., Scutt, D., Wilson, J., & Lewis-Jones, D. I. (1998). The ratio of 2nd to 4th digit length: A
predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen.
Human Reproduction, 13(11), 30003004.
Manning, J. T., Barley, L., Walton, J., Lewis-Jones, D. I., & Trivers, R. L. (2000). The 2nd:4th digit ratio,
sexual dimorphism, population differences, and reproductive success: Evidence for sexually antagonistic
genes. Evolution and Human Behavior, 21,163183.
Human Nature
Manning, J. T., Morris, L., & Caswell, N. (2007). Endurance running and digit ratio (2D:4D): Implications for
fetal testosterone effects on running speed and vascular health. American Journal of Human Biology, 19,
Marlowe, F. W. (2000). The patriarch hypothesis. Human Nature, 11(1), 2742.
Marlowe, F. W. (2004a). Marital residence among foragers. Current Anthropology, 45(2), 277284.
Marlowe, F. W. (2004b). Mate preferences among Hadza hunter-gatherers. Human Nature, 15(4), 365376.
Marlowe, F. W. (2007). Hunting and gathering: The human sexual division of foraging labor. Cross-Cultural
Research, 41(2), 170195.
Marlowe, F. W. (2010). The Hadza: Hunter-gatherers of Tanzania. Los Angeles: University of California
Muller, M. N., Marlowe, F. W., Bugumba, R., & Ellison, P. T. (2009). Testosterone and paternal care in East
African foragers and pastoralists. Proceedings of the Royal Society B: Biological Sciences, 276(1655),
Nelson, E., Hoffman, C. L., Gerald, M. S., & Shultz, S. (2010). Digit ratio (2D:4D) and dominance rank in
female rhesus macaques. Behavioural Ecology and Sociobiology, 64(6), 10011009.
Oświecimska, J. M., Ksiazek, A., Sygulla, K., Pyś-Spychała, M., Roczniak, G. R., Roczniak, W., Stojewska,
M., & Ziora, K. (2012). Androgens concentrations and second to fourth-digit ratio (2D:4D) in girls with
congenital adrenal hyperplasia (21-hydroxylase deficiency). Neuroendocrinology Letters, 33(8), 787
Paul, S. N., Kato, B. S., Hunkin, J. L., Vivekanandan, S., & Spector, T. D. (2006). The big finger: The second
to fourth digit ratio is a predictor of sporting ability in women. British Journal of Sports Medicine, 40(12),
Pettigrew, J. D., Bhagwandin, A., Spocter, M. A., Davimes, J., & Manger, P. R. (2017). Hands of living San
resemble those in Palaeolithic stencils, not modern Europeans. Transactions of the Royal Society of South
Africa, 73(1), 17.
Pickering, T. R., & Bunn, H. T. (2007). The endurance running hypothesis and hunting and scavenging in
savanna-woodlands. Journal of Human Evolution, 53(4), 434438.
Puts, D. A., Gaulin, S. J. C., Sporter, R. J., & McBurney, D. H. (2004). Sex hormones and finger length: - what
does 2D:4D indicate? Evolution and Human Behavior, 25(3), 182199.
Rammsayer, T. H., & Troche, S. J. (2007). Sexual dimorphism in second-to-fourth digit ratio and its relation to
gender-role orientation in males and females. Personality and Individual Differences, 42(6), 911920.
Ribeiro, E., Neave, N., Morais, R. N., & Manning, J. T. (2016). Direct versus indirect measurement of digit
ratio (2D:4D): A critical review of the literature and new data. Evolutionary Psychology, 14(1), 18.
Scott, I. M., Clark, A. P., Boothroyd, L. G., & Penton-Voak, I. S. (2012). Do mens faces really signal heritable
immunocompetence? Behavioural Ecology, 24(3), 579589.
Sear, R., & Marlowe, F. W. (2009). How universal are human mate choices? Size does not matter when Hadza
foragers are choosing a mate. Biology Letters, 5(5), 606609.
Smith, K. M., & Apicella, C. L. (2019). Partner choice in human evolution: The role of character, hunting
ability, and reciprocity in Hadza campmate selection. PsyArXiv.
Stibbard-Hawkes, D. N. E. (2019). Costly signaling and the handicap principle in hunter-gatherer research: A
critical review. Evolutionary Anthropology, 28,144157.
Stibbard-Hawkes, D. N. E., Attenborough, R. D., & Marlowe, F. W. (2018). A noisy signal: To what extent are
Hadza hunting reputations predictive of actual hunting skills? Evolution and Human Behavior, 39(6),
Tobler, M., Healey, M., & Olsson, M. (2011). Digit ratio, color polymorphism and egg testosterone in the
Australian painted dragon. PLoS One, 6(1), e16225.
Trivers, R., Manning, J., & Jacobson, A. (2006). A longitudinal study of digit ratio (2D:4D) and other finger
ratios in Jamaican children. Hormones and Behavior, 49(2), 150156.
von Rueden, C., Gurven, M., & Kaplan, H. (2008). The multiple dimensions of male social status in an
Amazonian society. Evolution and Human Behavior, 29(6), 402415.
Voracek, M., Manning, J. T., & Dressler, S. G. (2007). Repeatability and interobserver error of digit ratio (2D:
4D) measurements made by experts. American Journal of Human Biology, 19(1), 142146.
Voracek, M., Reimer, B., & Dressler, S. G. (2010). Digit ratio (2D:4D) predicts sporting success among female
fencers independent from physical, experience, and personality factors. Scandinavian Journal of
Medicine and Science in Sports, 20(6), 853860.
Wadley,L.,Trower,G.,Backwell,L.,&DErrico, F. (2015). Traditional glue, adhesive and poison used for
composite weapons by Ju/hoan San in Nyae Nyae, Namibia. Implications for the evolution of hunting
equipment in prehistory. PLoS One, 10(10), e0140269.
Human Nature
Wilkie, D. S., & Curran, B. (1991). Why do Mbuti hunters use nets? Ungulate hunting efficiency of archers
and net-hunters in the Ituri rain forest. American Anthropologist, 93(3), 680689.
Wlodarski, R., Manning, J., & Dunbar, R. I. M. (2015). Stay or stray? Evidence for alternative mating strategy
phenotypes in both men and women. Biology Letters, 11(2), 20140977.
Wood, B. M. (2006). Prestige or provisioning? A test of foraging goals among the Hadza. Current
Anthropology, 47(2), 383387.
Wood, B. M., & Marlowe, F. W. (2013). Household and kin provisioning by Hadza men. Human Nature,
24(3), 280317.
Wood, B. M., & Marlowe, F. W. (2014). Toward a reality-based understanding of Hadza menswork.Human
Nature, 25(4), 620630.
Worthman, C. M., & Konner, M. J. (1987). Testosterone levels change with subsistence hunting effort in
!Kung San men. Psychoneuroendocrinology, 12(6), 449458.
Publishers Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Duncan Stibbard-Hawkes received his BA and PhD in biological anthropology from the University of
Cambridge. He conducted his PhD research with the Hadza in northern Tanzania, where he examined hunting
reputation and the costly signaling hypothesis. He currently holds an honourary fellowship in evolutionary
anthropology at Durham University.
Human Nature
... The digit ratio (2D:4D) is known as a key parameter in the selection of highly qualified athletes [3] and refers to the ratio between the index finger, the second digit length (2D) and the ring finger, the fourth digit length (4D) [4]. This concept was first introduced by Baker in 1888 [5,6], confirming that the length of a person's fingers is usually determined in the 13th week of foetal development [7] and remains almost constant throughout life [8], and since then it has been widely studied in various fields. ...
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Background: Previous studies have found a correlation between digit ratio (2D:4D) and performance in sporting events which require physical competence. Such correlation, however, has not been well studied in technique-oriented sports. This study explored the correlation between 2D:4D and sporting performance of Chinese elite archers. Also it provides some references for sports research in other fields. Methods: We conducted the study with 31 Chinese elite archers (15 males and 16 females) preparing for the 2020 Tokyo Olympic Games and collected their 2D:4D ratios by measuring their index and ring fingers. The study also asked them to take physical fitness tests including push-ups, decline sit-ups, half-squat and a 4000-meter run, and obtained their results in former ranking tournaments and Olympic elimination series. The study recorded every entry of data in a “mean ± standard deviation” (M ± SD) format and processed them in SPSS 24.0 to produce One-Way ANOVA and Pearson correlations. Results: Our study found no significant difference between varied sporting levels in the left 2D:4D, and between world-class and national-level archers in the right 2D:4D (p > 0.05), yet a significant one between left-right 2D:4D of world-class, national-level, and level-1 archers (p < 0.01). We also found no significant correlation between the left 2D:4D and specific physical fitness test results, and between those archers’ results. Similarly, there was a significant negative correlation between the right 2D:4D and specific test results (p < 0.05), and a strong negative correlation between that and results of ranking tournaments (p < 0.01) and Olympic elimination series (p < 0.01). Conclusions: Chinese elite archers with lower right 2D:4D are of better physical fitness and enhanced sport performance. Hence, the right 2D:4D may act as a critical indicator in selecting excellent archers.
... The GLM ANCOVA tests conducted in our study demonstrated the significant effects of sex and population origin (medium size), and a small effect size for age as predictors of 2D:4D. However, the average finger length was not a significant predictor for the right 2D:4D in accordance with recently reported data for adult Hadza males by other authors 77 . The effect of height as another potential marker of allometry on 2D:4D has not been detected in a limited sample of respondents until the age of 30; however, the data on height were tested for Europeans only. ...
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Recently, a number of authors have claimed that sexual dimorphism in the second-to-fourth digit ratio (2D:4D) is simply dependent on digit length and is an artifact of allometry. The goal of our study is to verify the validity of these assumptions. The study sample comprised 7,582 individuals (3,802 men and 3,780 women) from three large world populations: Europeans (n = 3043), East Africans (n = 2844), and Central Asians (n = 1695). The lengths of the second and fourth digits on both hands were measured. Digit ratios were computed according to standard procedures. Analyses were conducted separately for each hand for the whole sample and in succession for the three large populations. Additionally, we separately tested four age cohorts (≤ 13, 14–18, 19–30, and 31 ≥ years) to test the effect of developmental allometry. The second and fourth digits showed strong positive linear relationships on both hands, and demonstrated an increase with age; digit length in women from the youngest age cohort was longer or equal to that of men, and shorter than men in older age cohorts. However, the 2D:4D magnitude and its sexual dimorphism remained stable throughout the ontogeny. To test for an allometric effect on 2D:4D, the average digit lengths were calculated. Both sex and population origin were permanent reliable predictors of 2D:4D, whereas average digit length was not. Height was applied as another measure of allometric effect on the limited sample (≤ 30 years) from the European population, along with sex and age. No allometric effect was observed in this case. We conclude that sex differences in 2D:4D are not an artifact of allometry.
... Hunting reputation is often employed as a proxy for a hunter's ability (e.g., Blurton Jones, 2016;Marlowe, 2000b;Stibbard-Hawkes, 2019b Jones & Marlowe, 2002;Marlowe, 2000b), allows granular distinctions to be made between hunters at all levels of perceived ability. ...
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Objectives: The incentives underlying men's hunting acquisition patterns among foragers are much debated. Some argue that hunters preferentially channel foods to their households, others maintain that foods are widely redistributed. Debates have focused on the redistribution of foods brought to camp, though the proper interpretation of results is contested. Here we instead address this question using two nutritional variables, employed as proxies for longer-term food access. We also report on broader patterns in nutritional status. Materials and methods: We measured male hunting success, hemoglobin concentration and body fatness among bush-living Hadza. Hunting success was measured using an aggregated reputation score. Hemoglobin concentration, a proxy for dietary red meat, was measured from fingerprick capillary blood. Body fatness, a proxy for energy balance, was measured using BMI and bioelectrical impedance. Results: We find no statistically significant relationship between a hunter's success and any measure of his nutritional status or that of his spouse. We further find that: women are, as elsewhere, at greater risk of iron-deficiency anemia than men; men had slightly lower BMIs than women; men but not women had significantly lower hemoglobin levels than in the 1960s. Discussion: The absence of an association between hunting reputation and nutritional status is consistent with generalized food sharing. Null results are difficult to interpret and findings could potentially be a consequence of insufficient signal in the study measures or some confounding effect. In any event, our results add to a substantial corpus of existing research that identifies few nutritional advantages to being or marrying a well-reputed Hadza hunter.
Objectives Myopia rates are increasing globally. This epidemic is linked to increased school participation, decreased outdoor activity and the proliferation of near-work occupations. The Tanzanian Hadza have traditionally subsisted as hunter-gatherers. School participation has historically been low and near-work otherwise minimal. Previous studies have reported exceptionally low myopia rates among hunter-gatherers, though such studies are few. The present study aims to expand this dataset. We report Hadza myopia rates and compare them to those from other economic/subsistence niches. We look for temporal changes in eyesight, in line with changing Hadza subsistence. Further, we assess the impact two known myopia risk factors, gender and educational participation, on Hadza eyesight. Materials and Methods We measured visual acuity among 182 bush-living Hadza aged 10–75 using a non-Latin optotype. From these measures, we estimate age-specific myopia prevalences. Results We find age-specific myopia prevalences between 5% and 10% for individuals under 40, increasing thereafter. This is low compared to industrialized populations, although not atypical for rural and non-industrialized populations. Unlike previous studies of hunter-gatherers, myopia was not exceptionally rare. We find that Hadza men have better distance vision than Hadza women. Though the Hadza have experienced subsistence change, we find no statistical evidence of associated decreases in visual acuity between 2006 and 2013/14 after controlling for gender imbalances. Finally, we find no support for our prediction that schooling participation reduces visual acuity, though so few attended school (13 of 58) that this analysis lacked statistical power and probably represents a false negative.
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The use of sport as a conceptual framework offers unprecedented opportunities to improve our understanding of what the body does, shedding new light on our evolutionary trajectory, our capacity for adaptation, and the underlying biological mechanisms. This approach has gained traction over recent years. To date, sport has facilitated exploration not only of the evolutionary history of our species as a whole, but also of human variation and adaptation at the interindividual and intraindividual levels. At the species level, analysis of lower and upper limb biomechanics and energetics with respect to walking, running and throwing have led to significant advances in the understanding of human adaptations relative to other hominins. From an interindividual perspective, investigation of physical activity patterns and endurance running performance is affording greater understanding of evolved constraints of energy expenditure, thermoregulatory energetics, signaling theory, and morphological variation. Furthermore, ultra‐endurance challenges provoke functional trade‐offs, allowing new ground to be broken in the study of life history trade‐offs and human adaptability. Human athletic paleobiology—the recruitment of athletes as study participants and the use of contemporary sports as a model for studying evolutionary theory—has great potential. Here, we draw from examples in the literature to provide a review of how the use of athletes as a model system is enhancing understanding of human evolutionary adaptation.
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The statistical analysis of allometry (size-dependence of traits) is fraught with difficulty that is often underestimated. In light of some recent controversies about statistical methods and the resulting biological conclusions, I here discuss the interpretation of regression lines and show how to avoid spurious effects. General linear models based on ordinary least square (OLS) regression are often used to quantify sexual dimorphism in a trait of interest that is modelled as a function of sex while controlling for size as a covariate. However, an analysis of artificially generated data where males and females differ in size only, but are otherwise built according to the same principles, shows that the OLS method induces a spurious dimorphism where there is none. Hence, OLS-based general linear models should not be regarded as a fail-proof tool that automatically provides the correct answer to whatever question one has in mind. Here I show how to avoid misinterpretation and how to best proceed with answering the recent debate about sexual dimorphism in digit ratio, a trait that is thought to reflect sex-hormone levels during development. The limited data, currently available to me, suggests that the widely accepted sexual dimorphism in digit ratio might well be only a by-product of an allometric shift in shape, urgently calling for a re-examination in larger data sets on humans and other vertebrates.
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Human adaptation depends upon the integration of slow life history, complex production skills, and extensive sociality. Refining and testing models of the evolution of human life history and cultural learning will benefit from increasingly accurate measurement of knowledge, skills, and rates of production with age. We pursue this goal by inferring individual hunters' of hunting skill gain and loss from approximately 23,000 hunting records generated by more than 1,800 individuals at 40 locations. The model provides an improved picture of ages of peak productivity as well as variation within and among ages. The data reveal an average age of peak productivity between 30 and 35 years of age, though high skill is maintained throughout much of adulthood. In addition, there is substantial variation both among individuals and sites. Within study sites, variation among individuals depends more upon heterogeneity in rates of decline than in rates of increase. This analysis sharpens questions about the co-evolution of human life history and cultural adaptation. It also demonstrates new statistical algorithms and models that expand the potential inferences drawn from detailed quantitative data collected in the field.
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It has been argued that men's hunting in many forager groups is not, primarily, a means of family provisioning but is a costly way of signaling otherwise cryptic qualities related to hunting ability. Much literature concerning the signaling value of hunting draws links to Zahavi's handicap principle and the costly signaling literature in zoology. However, although nominally grounded in the same theoretical paradigm, these literatures have evolved separately. Here I review honest signaling theory in both hunter‐gatherer studies and zoology and highlight three issues with the costly signaling literature in hunger‐gather studies: (a) an overemphasis on the demonstration of realized costs, which are neither necessary nor sufficient to diagnose costly signaling; (b) a lack of clear predictions about what specific qualities hunting actually signals; and (c) an insufficient focus on the broadcast effectiveness of hunting and its value as a heuristics for signal recipients. Rather than signaling hunting prowess, hunting might instead facilitate reputation‐building.
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The ability to choose the partners we interact with is thought to have been an important driver in the evolution of human social behavior, and in particular, our propensity to cooperate. Studies showing that humans prefer to interact with cooperative others is often cited as support for partner choice driving the evolution of cooperation. However, these studies are largely drawn from Western samples, where conditions for partner choice to operate may be especially favorable. Here, we investigate qualities associated with being a preferred partner (i.e., campmate) in Hadza hunter-gatherers of Tanzania in 2016 and 2019. A total of 156 Hadza participants from 17 camps ranked their campmates on cooperative character traits (i.e., hard work, generosity, and honesty), foraging ability, and their preference for them as future campmates. In 2016, we found a weak, but positive relationship between character traits and being a preferred campmate, with evidence suggesting a stronger preference for better hunters as campmates; however, the relationship between character traits and being a preferred campmate was greater in 2019 than in 2016, suggesting that campmate preferences are changing. Further, in 2019, Hadza with greater exposure to outside cultural institutions (e.g., schooling, having a job, or living in a village) had a stronger preference for generous campmates than those with less exposure. The results are consistent with theories that culture and norms play a role in shaping partner choice psychology and undermine the relevance of partner choice for understanding the evolution of human cooperation.
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Objectives To determine the effects of age and sex on physical activity and time budgets of Hadza children and juveniles, 5‐14 years old, including both in‐camp and out‐of‐camp activities. Methods Behavioral data were derived from ~15 000 hourly in‐camp scan observations of 76 individuals and 13 out‐of‐camp focal follows on nine individuals. The data were used to estimate energy expended and percentage of time engaged in a variety of routine activities, including food collection, childcare, making and repairing tools, and household maintenance. Results Our results suggest that (1) older children spend more time in economic activities; (2) females spend more time engaged in work‐related and economic activities in camp, whereas males spend more time engaged in economic activities out of camp; and (3) foraging by both sexes tends to net caloric gains despite being energetically costly. Conclusions These results show that, among the Hadza, a sexual division of labor begins to emerge in middle childhood and is well in place by adolescence. Furthermore, foraging tends to provide net caloric gains, suggesting that children are capable of reducing at least some of the energetic burden they place upon their parents or alloparents. The findings are relevant to our understanding of the ways in which young foragers allocate their time, the development of sex‐specific behavior patterns, and the capacity of children's work efforts to offset the cost of their own care in a cooperative breeding environment.
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Athletic prowess in both males and females is negatively correlated with the ratio between the lengths of the second and fourth fingers (2D:4D), a correlate of prenatal testosterone exposure. Because multiple lines of evidence suggest that prenatal testosterone exposure is associated with sports interest, motivation, and athletic performance we measured the digit ratios of 77 non-athletes, 103 varsity athletes, and 78 club sport athletes to test 8 hypotheses about the relationship between digit ratio and the athletic behavior of college-age women in the USA. Using independent samples t-tests, we found no significant differences between the digit ratios of women that (1) were athletes and non-athletes, (2) were varsity or club sport athletes, (3) had played or were currently playing individual or team sports, (4) played contact and non-contact sports, (5) played sports involving a ball and those that do not, (6) played sports where the outcome was determined by a score or the outcome of direct physical competition or subjectively by judges, or (7) were starters or reserves on their teams. However, women that played overhand throwing sports softball and water polo had significantly smaller digit ratios than did women that played other sports. These differences were not due to scaling effects. The independent samples t-test results were supported by subsequent Monte Carlo bootstrap, Bayesian, Random Forest, and multiple linear regression analyses. We suggest that the organizational consequences of prenatal testosterone exposure may influence the anatomy and physiology of women that leads to success playing overhand throwing sports.
Few data exist on gender‐typed and gender‐segregated play in hunter‐gatherer societies, despite their unique demographic and cultural features which may influence children’s gendered play. Using naturalistic observations of Hadza (N = 46, 41% female) and BaYaka (N = 65, 48% female) hunter‐gatherer 3‐ to 18‐year‐olds from Tanzania and the Republic of Congo, we showed that access to playmates was negatively associated with playing in mixed‐gender groups. Young boys did not engage in more rough‐and‐tumble play than girls, but adolescent boys participated in this type of play more than adolescent girls. Children were also more likely to participate in work‐themed play which conformed to gender norms within their society. Findings are discussed within the context of gendered division of labor, child autonomy, and demography.
Objective Sex differences are often reported in digit lengths and digit ratio (2D:4D). However, the ontogeny of these sex differences and their interrelationships are less well known. Methods We considered sex differences in the lengths of the 2nd (2D) and 4th (4D) digit and 2D:4D in children aged 2 to 18 years (Sample I, n = 680) and adults aged 18 to 30 years (Sample II, n = 89,246). Digit length was determined by direct experimenter‐measurement (Sample I) and direct self‐measurement (Sample II). The data were tested with two‐factor ANOVA's (sex and year‐group). Results In both samples, there were significant main effects of sex and year‐group, and a significant interaction effect on digit length. Digit length was positively related to age in both samples. Boys had longer digits than girls but only after 13 years. Men had longer digits than women and the dimorphism increased from 18 to 30 years. There were significant sex differences in 2D:4D (males < females), but no significant effect of age and no interaction effect of age and sex on 2D:4D in children or adults. Conclusions Between 2 and 30 years, the lengths and the sexual dimorphisms of 2D and 4D are dependent on age. In contrast, 2D:4D is not age‐dependent. We discuss our findings in the context of the ontogeny of digits and in the light of recent claims on the presence of static allometry in 2D and 4D lengths.
The measurement of hunting ability has been central to several debates about the goals of men's hunting among the Hadza and other hunter-gatherer populations. Hunting ability has previously been measured indirectly, by weighing the amount of food individuals bring back to camp over an extended period, their central place hunting return rate, and by conducting hunting ability interviews. Despite the centrality of the hunting ability concept, some authors (Hill & Kintigh, 2009) have expressed scepticism that such measures accurately capture individual differences in actual hunting ability. In the current study, we introduce a novel measure of hunting reputation which, unlike previous ones, allows fine-grained distinction between hunters of all reputations. To assess the suitability of this measure as a viable proxy for hunting ability, we address two further questions. First, to what extent do interviewees agree about the hunting ability of their present and former campmates? Second, to what extent does this measure of hunting reputation reflect success in four tasks expected to capture important components of hunting ability? We demonstrate that these measures of hunting reputation appear to reflect variation in these skills. We argue, however, that hunting reputation appears too noisy an index of these skills and, we infer, hunting ability in general for hunting to act, as some have suggested (e.g. Hawkes & Bird, 2002), as an honest signal of cryptic qualities related to hunting ability.