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RESEARCH ARTICLE
Women Favour Dyadic Relationships, but
Men Prefer Clubs: Cross-Cultural Evidence
from Social Networking
Tamas David-Barrett
1
*, Anna Rotkirch
2
, James Carney
1
, Isabel Behncke Izquierdo
1
,
Jaimie A. Krems
3
, Dylan Townley
1
, Elinor McDaniell
1
, Anna Byrne-Smith
1
, Robin I.
M. Dunbar
1
1Department of Experimental Psychology, University of Oxford, South Parks Rd, Oxford, OX1 3UD, United
Kingdom, 2Population Research Institute, Väestöliitto, Kalevankatu 16, 00101, Helsinki, Finland, 3
Department of Social Psychology, Arizona State University, Tempe, AZ, 85287, United States of America
*tamas.david-barrett@psy.ox.ac.uk
Abstract
The ability to create lasting, trust-based friendships makes it possible for humans to form
large and coherent groups. The recent literature on the evolution of sociality and on the net-
work dynamics of human societies suggests that large human groups have a layered struc-
ture generated by emotionally supported social relationships. There are also gender
differences in adult social style which may involve different trade-offs between the quantity
and quality of friendships. Although many have suggested that females tend to focus on inti-
mate relations with a few other females, while males build larger, more hierarchical coali-
tions, the existence of such gender differences is disputed and data from adults is scarce.
Here, we present cross-cultural evidence for gender differences in the preference for close
friendships. We use a sample of *112,000 profile pictures from nine world regions posted
on a popular social networking site to show that, in self-selected displays of social relation-
ships, women favour dyadic relations, whereas men favour larger, all-male cliques. These
apparently different solutions to quality-quantity trade-offs suggest a universal and funda-
mental difference in the function of close friendships for the two sexes.
Introduction
The recent literature on both the evolution of sociality and the network dynamics of human
and animal societies [1–8] suggests that large social groups cannot be fully connected: they
have a layered structure that is generated by emotionally supported social relationships [9–15].
Although there are structural aspects to social organisation, individual behaviour is crucially
shaped by dyadic relationships. It may be that preferred patterns of relationships vary by gen-
der in a way that typically reflects sex differences in reproductive strategies [16–19].
For instance, friendships or close and prolonged affiliation with non-kin are characterised
by homophily, so that people typically choose friends of the same age and gender (for recent
PLOS ONE | DOI:10.1371/journal.pone.0118329 March 16, 2015 1/15
OPEN ACCESS
Citation: David-Barrett T, Rotkirch A, Carney J,
Behncke Izquierdo I, Krems JA, Townley D, et al.
(2015) Women Favour Dyadic Relationships, but
Men Prefer Clubs: Cross-Cultural Evidence from
Social Networking. PLoS ONE 10(3): e0118329.
doi:10.1371/journal.pone.0118329
Academic Editor: Luo-Luo Jiang, Wenzhou
University, CHINA
Received: April 12, 2014
Accepted: January 3, 2015
Published: March 16, 2015
Copyright: © 2015 David-Barrett 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: This study was supported by EU FP7 EINS
grant agreement No 288021 to TDB, European
Research Council Advanced grant to RD, EU Marie
Curie Fellowship, grant agreement No 297854 to JC.
The funders had no role in study design, data
collection and analysis, decision to publish, or
preparation of the manuscript.
Competing Interests: The authors have declared
that no competing interests exist.
reviews see [20–22]). Sex differences in reproductive strategies also shape adult social behav-
iour [20,23–26] and may reflect different trade-offs between the quantity and quality of friend-
ships. Thus, it has been suggested that females invest more heavily in a few, high-quality and
time-consuming friendships, while males prefer groups with less investment per member, and
higher group cohesion [27–30].
There is reason to believe these gender differences have evolutionary roots. First, sex differ-
ences in friendship emerge early [31], are quite apparent already in small children and appear
to increase with age [21,32,33]. Second, similar gender patterns exist in some non-human pri-
mates [13,34]. One obvious explanation is that male peer sociality evolved to enable hunting,
coalitionary support for within-group dominance, and/or defence in larger groups, while wom-
en’s peer sociality is to a greater extent shaped by their higher investment in reproduction and
child rearing, as well as their historically more frequent experience of out-migration and the
need to integrate into a patrilocal society with few, if any, kin [18,20,25]. Patrilocality is also the
norm for chimpanzees and bonobos, lending further support to female transfer being the norm
throughout hominin evolution [35,36].
However, the scope of human gender differences is disputed. Several studies found few sex
differences in the number of close non-related friends that an individual turns to for help and
assistance [4,37–42] while others detect crucial differences in the quantity and intensity of
male and female peer ties [16,23,24,30,32,43–45]. Sex differences also depend on which compo-
nent of friendship is being studied, as well as the age and culture of the subjects. For instance,
the review by Rose and Rudolph [21] found that girls have a greater preference for extended
dyadic interactions and prosocial behaviour, while boys interact more in peer groups with a
high network density and clear dominance hierarchy. But gender differences are negligible con-
cerning the expectations males and females have of friends and in the symmetrical reciprocity
they expect from them [22]. In any case, documented sex differences tend to be small or medi-
um-size rather than large [22,46].
All reviews stress the need for more friendship research on adults and on people from other
than ‘WEIRD’[47] societies [21,22,46]. Most research on friendships has involved children or
teenagers, and there is to date only limited and mixed evidence for gender differences in adult
human friendships [20,38,48–50]. Here, we use data from a social media site to explore gender
differences in close peer relations. We hypothesised that social relations among same-aged
adults would exhibit gender homophily and would vary by gender, such that men would exhib-
it higher numbers of friends compared to women.
Methods and Data
To investigate close friendships in the two sexes, we used Facebook Profile Pictures following
the example of recent literature that deals with social networking data [51–54]. Facebook is the
most popular global social networking site, the primary function of which is to fulfil psychoso-
cial needs for belonging and self-presentation [55]. Upon signing up, each user may choose a
Profile Picture, which represents the user to the rest of the Facebook community, is public, and
appears at the top of the user’s profile and as the icon next to the user’s name wherever he or
she posts on the site. Each user can have only one Profile Picture at a time. Usually the picture
features the user only. When the picture displays peers they tend to be the user with friends or
acquaintances [51]. The choice of profile pictures is related to the user’s desired impression for-
mation [55]. This impression is not, however, too detached from the real world: Facebook user
profiles have been found to reflect actual rather than idealised identities [56]. We thus assume
that Profile photographs of peers are likely to align with behavioural inclinations, and thus pro-
vide a reliable proxy for relationship preferences.
Women Favour Dyadic Relationships, but Men Prefer Clubs
PLOS ONE | DOI:10.1371/journal.pone.0118329 March 16, 2015 2/15
Data collection
We used random search terms to select 309 users (seeds) of Facebook who shared their batch
of friends publicly. (The 309 users had on average 362 friends.) In the first wave of data collec-
tion, we located the Profile Pictures of all the friends of each of these users (111,863 Profile Pic-
tures in total); each photograph was categorised with respect to the type of the picture, and the
number and gender of the persons displayed (see Table 1). Data collection took place between
July 2011 and January 2012.
Additional information was collected in two further waves. In the first of these (wave 2),
geographical data based on geographical location was collected to control for cultural differ-
ences. The coders were provided with a list of geographic regions: Europe (56 seeds), Central
and South Asia (16), Latin America (9), Middle East and North Africa (14), North America
(14), South East Asia (43), Sub-Saharan Africa (7), Australia (9), East Asia (19), or ‘can’t tell re-
gion’(21). In the second (wave 3), we used a finer classification of the number of people shown
in Profile Pictures: the first two waves classified all pictures with four or more individuals as a
single category, but in wave 3 this was extended to specify individual numbers up to 20.
The coding was done by 8 coders, all but two of whom were research assistants at the Uni-
versity of Oxford. All coders bar one (the lead author) were blind during the coding phase,
knowing only that the project was to study gender differences on social networking sites. Cod-
ers were instructed to avoid any discussion of the project amongst themselves during the cod-
ing phase. All results presented in this paper are robust to the elimination of each of the coders,
each of the regions, and the type of Profile Pictures displayed by the seeds.
We used only publicly available pictures. No pictures or other information associated with
this research was either separately downloaded or stored. The research project was approved
Table 1. Coding categories.
Code Description Tally %
NH Not human picture: e.g., object, landscape, monster, car, or any picture with an
animal
13,861 12.9
NA Not publicly available (or Facebook default profile) 1,626 1.5
NP Multiple people but not peer: e.g., mother-child, a family 2,651 2.5
CB Child or baby 3,470 3.2
MP Multiple pictures, collage 1,961 1.8
CTG Can’t tell gender 2,447 2.3
1F 1 female 32,208 30
2F 2 females 3,508 3.3
3F 3 females 945 0.9
4F 4 females 384 0.4
1M 1 male 30,279 28.2
2M 2 males 2,326 2.2
3M 3 males 935 0.9
4M 4 males 388 0.4
1F1M 1 female + 1 male (e.g. a couple) 6,769 6.3
1F2M 1 female + 2 males 329 0.3
2F1M 2 females + 1 male 359 0.3
1F3M 1 female + 3 males 106 0.1
2F2M 2 females + 2 males (e.g. two couples) 185 0.2
3F1M 3 females + 1 male 110 0.1
5+ Five or more people on the picture 2,433 2.3
doi:10.1371/journal.pone.0118329.t001
Women Favour Dyadic Relationships, but Men Prefer Clubs
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by the Central University Research Ethics Committee of University of Oxford, and each coder
received a full research ethics briefing before joining the coding team.
Data exclusion
As adult interpersonal processes were our focus here, we excluded all Profile Pictures that con-
tained a non-human figure, a baby or child, people of markedly different ages (as judged by the
coders), pictorial collages (i.e. pictures composed of multiple, distinct or coloured photos), a
person or people whose gender(s) were unidentifiable or that did not contain a person. 81,246
Profile Pictures remained, of which 19,984 (26%) contained more than one person. For an
overview of the data see Fig. 1.
Results
Gender homophily
First, we studied gender homophily in profile pictures. As expected, if the Profile Pictures dis-
played three or more persons (7.6% of Profile Pictures displaying adults), they tended to be the
same gender, confirming our gender homophily hypothesis (Fig. 1).
Fig 1. The ratio of men in Profile Pictures (with same age adults only) as a function the number of persons in the picture. The value corresponding to
each nadds up to 1. The size of the disks denotes the share in the ratio at that particular bin. Crossing points on the grid are the only possible points given the
discrete nature of the data.
doi:10.1371/journal.pone.0118329.g001
Women Favour Dyadic Relationships, but Men Prefer Clubs
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The gender ratio on Profile Pictures persons exhibited a tri-modal distribution (see Fig. 2).
In Profile Pictures with 5–12 persons, the frequencies for women-only, men-only, and equal
gender ratios are 15.5%, 40.6%, 15.7%. (Note that for n<5 the trimodality cannot exist, while
for very large nthe male groups dominate to such an extent that only one modality is left.)
These are above the frequencies for all other combinations: pictures with both genders but un-
equally represented (such as one woman and two men) were significantly less common. Given
that the population mean is approximately balanced, this finding suggests a strong
gender homophily.
This preference for same-sex friends was further supported by the subsample that contains
Profile Pictures displaying fewer than 5 people. While pictures with two persons showed a
higher mixed than same gender frequency (sample average (2F+2M)/1F1M-1 = -0.123 (boot-
strapping mean -0.122 and s.d. 0.044), likely indicative of pictures of romantic couples, both
the 3-person pictures (sample average 2(3F+3M)/(1F2M+2F1M)-1 = 4.34 (bootstrapping
mean 4.36 and s.d. 0.39) and the 4-person pictures (sample average 3(4F+4M)/(1F3M+2F2M+
3F1M)-1 = 4.78 (bootstrapping mean 4.75 and s.d. 0.51) had a much higher same-gender fre-
quency compared to mixed-gender frequency (Fig. 3A).
Male propensity towards displaying more people
Studying the gender composition of peer groups indicated that, when Profile Pictures display a
large group of people, they tend to be all male, which was in line with our expectations. In
Fig 2. Relative frequencies of different gender ratios for groups where 5<=n<= 12. The distribution shows a trimodal pattern with women-only,
gender-equal and men-only gender ratios significantly above zero.
doi:10.1371/journal.pone.0118329.g002
Women Favour Dyadic Relationships, but Men Prefer Clubs
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Fig 3. Bootstrap histograms. Panel (a): the ratio of same gender Profile Pictures compared to mixed
gender profile pictures (probability corrected); green, red, and blue lines correspond to n = 2, 3, and 4. Panel
Women Favour Dyadic Relationships, but Men Prefer Clubs
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groups of three or four persons, men and women had the same propensity to appear in same-
gender Profile Pictures (Table 1). However, larger groups were predominantly male, and in-
creasingly so as the group size grows (Fig. 4).
The male propensity to be part of larger groups was further supported by the fact that men
appear together with more people than do women (Fig. 3B). For groups with 2–4 peers of the
same gender, men appeared in pictures with 2.47 people on (bootstrapping mean 2.47, s.d.
0.01), as opposed to 2.35 of women (bootstrapping mean 2.35, s.d. 0.01), which are significantly
different from each other (p<0.0001). For groups with 2–20 peers of any gender, men appeared
with 2.90 (bootstrapping mean 2.90, s.d. 0.07), as opposed to 2.54 of women (bootstrapping
mean 2.54, s.d. 0.05) which are also significantly different from each other (p<<0.0001). This
suggests that independently of gender combinations and group size, men tend to appear with a
larger number of people displayed in Profile Pictures.
(b): the number of people on a Profile Picture men (red) or women (blue) appear on; straight lines: same
gender with nbetween 2 and 4, dashed lines: mixed gender with nbetween 2 and 4, dotted lines: mixed
gender with nbetween 2 and 20. Panel (c): the ratio of the frequency of same-gender pictures between the
genders; green: pictures with 1 person, red: 2 persons, brown: 3 persons, and blue: 4 persons. (100,000
bootstrapping repeats.)
doi:10.1371/journal.pone.0118329.g003
Fig 4. The ratio of same-gender women-only to men-only frequencies (nF/nM as function of n). Gray lines denote a one-standard-deviation band from
bootstrapping. Above n>4, men-only groups dominate, while women-only groups become extremely rare. The linear OLS coefficient of nF/nM as a function
of n is negative, with R
2
= 0.86, and p<0.01
doi:10.1371/journal.pone.0118329.g004
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Women focus on dyadic relationships
Finally, we detected an unexpectedly strong female focus on same gender dyads. Women not
only tended to appear in Profile Pictures displaying a smaller number of people, as expected in
the second research hypothesis, but there was also a strong preference towards pictures con-
taining two women (Fig. 3C). There were 50.8% more pictures with two female peers than pic-
tures with two male peers (bootstrapping mean 51.5, s.d. 14.8). This is especially remarkable
given that same-gender pictures with 1, 3, or 4 or more people had an almost perfect gender
balance: there were only 6.4% more pictures with one woman only in our dataset than with one
man only (bootstrapping mean 7.2, with s.d. 14.1), only 1.1% more pictures with three women
than with three men (bootstrapping mean 1.7, s.d. 10.8), and 1.0% fewer pictures with four
women compared to four men (bootstrapping mean 0.5, s.d. 12.5).
Cultural variation
Although there was substantial variation across the different world regions, the main patterns
of our findings were present in each of them(Fig. 5A-C), with only the magnitude of the effect
varying. While a selection bias may theoretically have affected the relatively small local varia-
tion of our results, we find this unlikely: the seeds were randomly selected and at the time of
the data collection 17% of the global adult population was using Facebook. Furthermore, mea-
sures for other than our three main findings reported above were less uniform among the
world regions (Fig. 5D).
Follow-Up Studies
It is possible that the relative prevalence of female-female Profile Pictures in comparison to
male-male ones may not reflect friendship behaviour but either (a) a female preference for put-
ting up pictures of two people of any gender, or (b) a reluctance among men to display pictures
with two male friends, especially in regions where homophobia is common. We tested both of
these alternative explanations.
First, we tested whether there is a gender difference in the preference for Profile Pictures
containing two people only. We randomly selected 960 new profiles on Facebook with two
same-aged individuals on them. Out of the 960, we were unable to determine the gender of the
account user in 11 cases. Out of the remaining 949 cases, 493 pictures belonged to a man and
456 belonged to a woman. Given the fact that the overall Facebook participation gender ratio is
almost balanced, this result suggests that the probability of women having a strong preference
to put up pictures of two people of any gender is negligible.
Second, we tested if homophobia in the country where the seed owner of the Facebook ac-
count lived would affect the ratio between female and male same-gender two-person pictures.
(To correct for the fact that different countries have different gender ratios among Facebook
users, we used the (2F/1F)/(2M/1M)-1 measure.) As a measure of homophobia, we used the
country level geographical codes of our data, and the corresponding homophobia index for
these countries as calculated by Pew Global [57]. As the homophobia index of Pew Global cov-
ers only the largest 52 countries, we could not test this hypothesis on our entire database. For
the remaining seeds, countries were coded to be low on homophobia if they scored between 0
and 40 for the question “Homosexuality is a way of life that should be accepted by society”and
to be high homophobic countries if they scored between 60 and 100. This gave us 66 seeds liv-
ing in countries with low homophobia and 47 seeds in countries with high homophobia. The
(2F/1F)/(2M/1M)-1 mean for the two groups were 0.63 and 0.52, respectively (Fig. 6). As the
difference is not statistically significant (p = 0.12), and is in the opposite direction to that
Women Favour Dyadic Relationships, but Men Prefer Clubs
PLOS ONE | DOI:10.1371/journal.pone.0118329 March 16, 2015 8/15
predicted by the hypothesis, we can conclude that male homophobia is not associated with the
difference between 2F vs 2M frequencies in our dataset.
Discussion
Human societies are complex, large-scale communities of multi-generational social networks.
At base, however, these networks are built of the small-scale personal networks of individuals.
Our data shed light on the dynamics of these personal networks by providing strong cross-
cultural evidence for the universality of a male propensity to prefer a higher number friend-
ships compared to women. While large women-only groups were almost non-existent in self-
selected Profile Pictures, males were more likely to present themselves as part of large all-male
groups—arguably an essential element of male-male coalitional competition. Our results are
Fig 5. Cultural variation in the main finding. Panel (a): the ratio between same-gender and mixed gender pictures of different group size (corrected by the
probability of appearance, see text). Panel (b): number of close friends (same gender, with groups size 2 to 4). Panel (c): the ratio between same-gender
Profile Pictures as a function of group size (1F/1M normalised to 0). Panel (d): cultural variation in the proportion of single person pictures within all Profile
Pictures in a given global region. (Region codes of Panels a-c: green: Central and South Asia, blue: Europe, dashed blue: Latin America, dashed green:
Middle East and North Africa, dotted blue: North America, dotted green: South-East Asia, red: Sub-Saharan Africa, black: Australia, dashed black: East
Asia.)
doi:10.1371/journal.pone.0118329.g005
Women Favour Dyadic Relationships, but Men Prefer Clubs
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broadly in line with many previous studies of human friendship [27,30,58,59] which also
found strong gender homophily and male preference for coalitions. This difference in the pre-
ferred number of friends may signal different solutions to the quantity-quality trade-off in so-
cial ties. The emotional quality of a relationship is a positive function of the time invested in it
[39,60], and the closer and more time-demanding a relationship one has, the less time can be
devoted to others [11,24]. At the same time, the amount of social capital available that individ-
uals have to distribute among the members of their personal social networks is limited [61,62].
It thus appears as if women build a ‘dense’network, while men make alliances based on ‘loose’
networks. We also found gender similarity in the preferences for three and four friends,
which may explain part of the inconclusive results in previous studies on sex differences in
friendship numbers.
The male propensity to form coalitions could have emerged from the sexual division of la-
bour in ancestral environments. It was a male responsibility to defend the group against attack
from outsiders, and to do so successfully it was necessary that men band together [20,24]. In
males but not females, then, out-group defence called for coalitional cooperation
and behaviour.
Our finding that women prefer to picture themselves with fewer friends, and thus appear
more often to focus their social capital on only one person at a time, suggests a strong female
preference for dyadic relations. The social benefits of such a female dyadic social style are
harder to pin down, but three alternative hypotheses might be suggested. First, females may
have developed a propensity to form dyadic same-sex friendships as a response to the chal-
lenges of their social environments. Given the likelihood of ancestral patrilocality
Fig 6. Homophobia is not related to the frequency of 2F Profile Pictures, bootstrapping distributions. Blue line: highly homophobic countries; red line:
countries with low homophobia (see text for definitions). Dashed black line: the (2F/1F)/(2M/1M)-1 mean of the entire database. (100,000 bootstrapping
repeats.)
doi:10.1371/journal.pone.0118329.g006
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[35,36,63,64], adolescent females would often have entered communities where they had few
or no close kin. For females especially, the presence of kin is fitness-enhancing, as has been re-
peatedly shown for both anthropoid primates [65,66] and humans [67,68]. Thus the formation
of emotionally intense, exclusive and “sisterly”dyadic bonds may have been a means to essen-
tially replace kin [25] and to defend against male and inter-female aggression in the new com-
munity where she did not have female kin [25,69,70]. This mirrors the case of patrilocal
bonobos (Pan paniscus), where females enter foreign communities in adolescence and integrate
into their new group through intense bond formation with another (typically older) female
[71]. A second explanation posits that, since females are the driving agents in human pair-
bond formation, it may be a female-specific sexual strategy to form exclusive dyadic relation-
ships. In this framework, the high frequency of female-female dyads in women’s lives might be
a by-product of a preference for pairbonding [12]. A third explanation focuses on females’
unique capacity for intense empathic relationships, derived from the mother-infant bond. In
this model, heightened female empathy creates an emphasis on individual relationships as a
consequence of the psychological toolbox of mothering [72,73]. In comparison, males generally
neither have nor require this capacity, and hence they form less emotionally close bonds, those
of friendship included.
These three explanations for gender differences in social style—patrilocality, pair-bonding
and maternal empathy—are difficult to tease apart. Not only is the evolutionary origin of all
primate bonding likely to have arisen out of the mother-infant relationship [74], whatever
forces shaped female friendship thereafter, different ultimate causes (e.g., defence against ag-
gression in a patrilocal society, or assistance among maternal kin) may have used similar proxi-
mate mechanisms (e.g., high reliance on intimate disclosure) making it hard to dissociate
them. Furthermore, recent studies of close friendship as a function of age suggests that women
switch their primary focus from female-female friendships to pairbonding and then to mother-
daughter bonding at different stages of the life cycle [75].
The importance for a female of maintaining close relationships once she has left her natal
group sheds light on the strategies that women use during intrasexual aggression (notably ex-
clusion and relationship-ending gossip [29,69]). If a female’s bonds to friends and her spouse
are crucial for accessing resources—from food to information—then breaking these bonds
and/or excluding the female all together can radically affect that individual’s fitness, to the ben-
efit of her competitors.
There are, inevitably, some potential limitations to our data. We cannot be sure that co-ap-
pearance on Profile Pictures always reflects real-life social ties. Future research is needed in
order to assess gender differences in offline sociality. However, no existing research suggests
that profile pictures would include imagined or random social relations to any significant ex-
tent (not least because the other person is likely to object) and our results are in line with other
recent findings from online social communities [50]. Displaying Profile Pictures with two or
more people compared to only one person may also reflect some unknown personal psycholog-
ical characteristics or specific life-events of account users; however, such possible characteris-
tics should not affect our main results.
In summary, our results point to striking gender differences in intimate friendship strate-
gies: women prefer close dyadic bonds (with evolutionary origins in either pairbonding or so-
cial insurance purposes, or both), whereas men use their bonding capacity to build multimale
groups (in effect, clubs). This concurs with work on chimpanzees, where females form tight,
kin-based networks and males make loose, easy-to-break alliances [76]. Since similar gendered
bonds are found in our closest primate relatives, they may long predate the evolution of our
species. Among cercopithecine primates, females are disproportionately more likely to invest
in core female ‘friendships’with matrilineal relatives as group size gets larger, apparently in
Women Favour Dyadic Relationships, but Men Prefer Clubs
PLOS ONE | DOI:10.1371/journal.pone.0118329 March 16, 2015 11 / 15
order to maximise the effectiveness with which these relationships function as social buffers
[1,12,65]. By contrast, chimpanzees [34] and humans may show a tendency to form close
friendships with unrelated females in addition to those they might form with close
female relatives.
Supporting Information
S1 Data. Anonymised profile picture frequency database.
(CSV)
Acknowledgments
We thank Elena Denaro, Joshua de Gastyne, Erin Simmons, and Cathal Power for assistance in
data collection.
Author Contributions
Conceived and designed the experiments: TDB. Performed the experiments: TDB AR DT EM
ABS. Analyzed the data: TDB. Wrote the paper: TDB AR JC IBI JK RD.
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