Personality homophily drives female friendships in a feral ungulate

Abstract

Similarity or homophily in personality drives preferential strong social bonds or friendships in humans and some non-human primate species. However, little is known about the general behavioral “decision rules” underlying animal friendships in other taxa. We investigated a feral and free-ranging population of water buffalo (Bubalus bubalis) to determine whether homophily in personality drives female friendships (n=30) in this social ungulate. Close spatial proximity served as an indicator of friendship, validated by affiliative body contact. A “bottom-up” method revealed three personality traits – social tension, vigilance, and general dominance. We found that individuals with lower personality differences (i.e., more similar) in social tension and general dominance traits exhibited higher spatial associations, suggesting that friendships in buffalo can form based on personality homophily. Our findings offer crucial insights into the role of personalities driving complex social patterns in species beyond primates.

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Article
iScience
Personality homophily drives female friendships in a
feral ungulate
Graphical abstract
Highlights
dFemale water buffalo exhibit preferential strong social
association or friendships
dSimilarities in personality predict friendships in buffalo
dPersonality homophily principle of friendship applies to
species beyond primates
Authors
Debottam Bhattacharjee, Kate J. Flay,
Alan G. McElligott
Correspondence
bhattacharjee.debottam@gmail.com
(D.B.),
alan.mcelligott@cityu.edu.hk (A.G.M.)
In brief
Wildlife behavior; Zoology; Evolutionary
biology
Bhattacharjee et al., 2024, iScience --, 111419
--, 2024 ª2024 The Author(s). Published by Elsevier Inc.
https://doi.org/10.1016/j.isci.2024.111419 ll

iScience
Article
Personality homophily drives female friendships in a
feral ungulate
Debottam Bhattacharjee,
1,2,4,
*Kate J. Flay,
3
and Alan G. McElligott
1,2,
*
1
Department of Infectious Diseases and Public Health, Jockey Club College of Veterinary Medicine and Life Sciences, City University of Hong
Kong, Hong Kong, Hong Kong SAR
2
Centre for Animal Health and Welfare, Jockey Club College of Veterinary Medicine and Life Sciences, City University of Hong Kong, Hong
Kong, Hong Kong SAR
3
Department of Veterinary Clinical Sciences, Jockey Club College of Veterinary Medicine and Life Sciences, City University of Hong Kong,
Hong Kong, Hong Kong SAR
4
Lead contact
*Correspondence: bhattacharjee.debottam@gmail.com (D.B.), alan.mcelligott@cityu.edu.hk (A.G.M.)
https://doi.org/10.1016/j.isci.2024.111419
SUMMARY
Similarity or homophily in personality drives preferential strong social bonds or friendships in humans and
some non-human primate species. However, little is known about the general behavioral ‘‘decision rules’’ un-
derlying animal friendships in other taxa. We investigated a feral and free-ranging population of water buffalo
(Bubalus bubalis) to determine whether homophily in personality drives female friendships (n=30) in this so-
cial ungulate. Close spatial proximity served as an indicator of friendship, validated by affiliative body con-
tact. A ‘‘bottom-up’’ method revealed three personality traits – social tension,vigilance, and general domi-
nance. We found that individuals with lower personality differences (i.e., more similar) in social tension and
general dominance traits exhibited higher spatial associations, suggesting that friendships in buffalo can
form based on personality homophily. Our findings offer crucial insights into the role of personalities driving
complex social patterns in species beyond primates.
INTRODUCTION
In social species, including humans, the strength of affiliative
‘‘ties’’ in a social network is not homogeneous, indicating varying
degrees of relationships among individuals. Preferential strong
social associations or friendships are of particular interest as
they positively correlate with health, well-being, and survival
benefits.
1,2
Although conceptualized relatively recently in non-
human animals (hereafter, animals),
3
growing convergent evi-
dence suggests that human-like friendships can form in a variety
of species.
4,5
Similar to humans, animal friendships can be sta-
ble, long-lasting,
6,7
and form beyond the extent of kin relation-
ships.
8
The primary evolutionary explanation of friendship lies
in their low levels of uncertainty, where, in comparison to a
non-friend, a friend provides assured fitness benefits, and these
benefits surpass the costs of maintaining friendships.
9
For
instance, friendships foster cooperation, which minimizes the
probability of ‘‘cheating’’
10–13
; the strong emotional underpin-
ning of friendship can act as a ‘‘social buffer’’ during aversive sit-
uations, improving the physiological states of the friends and
enhancing their survival
14–16
; and friendships, even same-sex,
can positively influence reproductive outputs.
17,18
Although the
evolutionary implications are well established, the proximate
mechanisms of animal friendships are relatively understudied.
Particularly, how animals choose their partners in a ‘‘biological
market’’ is poorly understood.
19
In other words, whether and
how animals choose and compete to be chosen as friends
with different social (such as sociability and cooperative ten-
dencies) and non-social traits (such as problem-solving abilities)
are not clear. Consequently, empirical evidence on the behav-
ioral decision rules that apply in animal societies to choose
preferred partners or friends is lacking.
Several behavioral decision rules have been proposed as
proximate mechanisms of animal friendships,
20
of which the
‘‘homophily principle’’ received much attention. The homophily
principle, proposed originally to characterize human social net-
works, suggests that individuals similar in terms of their age,
gender, ethnicity, and interests are more likely to become
friends.
21
To some extent, these propositions fundamentally
overlapped with that of another behavioral decision rule, the
symmetry-based reciprocity.
22,23
Yet, friendships in animals
can form irrespective of age, sex, and kin relationships, and
thus, these variables could not solely explain the emergence of
friendships.
24
Personality – the consistent inter-individual differ-
ences
25
– due to their immense adaptive values,
26
have been
proposed as an alternate mechanism influencing friendships.
Similarities or differences in traits are calculated at the dyadic
levels to evaluate how personalities may influence friendships.
Interestingly, when extended to personalities, researchers have
found strong evidence of homophily explaining the emergence
and sustenance of friendships in both humans
27,28
and anim-
als (chimpanzee,
29
bonobo,
30
Assamese macaque,
31
baboon
32
).
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Given that these findings are restricted to (non-human-)pri-
mates, testing the hypothesis in phylogenetically diverse taxa
is required to ascertain if personality homophily is a general
behavioral decision rule of friendship.
Ungulates represent a wide range of species with varying so-
cial organizations,
33
providing a suitable system to expand the
investigation of the emergence of friendships. Several ungulate
species exhibit complex sociality patterns, including marked
preferences for specific group members over others (e.g., cat-
tle,
34
goat,
35
Przewalski’s horse,
36
bison
37
). However, existing
research has predominantly used demographic characteristics,
e.g., age and sex, often coupled with kinship and dominance
rank relationships, to explain sociality patterns in ‘‘non-random’’
or heterogeneous networks. In recent years, studies have as-
sessed the personality traits of various ungulate species and
identified their influence on cognitive performance and deci-
sion-making,
38,39
autonomic nervous system reactivity,
40
domi-
nance hierarchy,
41
and welfare,
42
among others. To our knowl-
edge, no systematic attempt has been made to investigate the
effects of personality homophily on the emergence of ungulate
heterogeneous social networks, including friendships. More-
over, most research on ungulate social behavior and personality
is performed in captivity, often as livestock, thus partly limiting
the broader ecological value of the findings.
43
Water buffalo (synonymously, buffalo) are domesticated ungu-
lates with an estimated global population of 208 million.
44
Almost
all research on buffalo is restricted to their production capacities
(for meat and milk) as livestock. In feral and free-living condi-
tions, female buffalo can form clans of up to 30 individuals,
comprised of kin members (grandmothers, mothers, daughters,
and sisters), whereas multiple non-kin or distantly related clans
can form herds of up to 500 individuals; Males typically disperse
and form bachelor groups of up to 10 individuals.
45
However, our
understanding of their social interaction patterns is very limited.
Despite this considerable gap in knowledge, some research,
predominantly conducted in captivity, suggests that buffalo are
highly social animals, exhibiting complex patterns of sociality,
46
sociability and affiliative behaviors,
46,47
and dominance-rank
relationships.
48
Thus, to fully understand their social behavior,
it is necessary to carry out research on populations that are
largely free of human interventions, living in wild or feral states.
Such (semi-)natural populations can also prove to be valuable
in investigating the effects of personality homophily on social
associations.
Here, we studied a feral and free-ranging population of buffalo.
To assess friendships in female buffalo, we utilized two behav-
ioral indicators: proximity and affiliative body contact.
20
Unlike
non-human primates and many ungulate species, allogrooming,
a strong indicator of social bonds, is predominantly absent in
buffalo.
49
In species with limited direct affiliative interactions
(e.g., allogrooming), spatial associations (e.g., proximity and
body contact) can sufficiently capture the varying social bonds
among individuals.
50
We conducted extensive behavioral obser-
vations and assessed the consistent inter-individual differences
or personalities in female buffalo. Notably, we used a ‘‘bottom-
up’’ approach to assess personalities; hence, traits were not
predetermined.
51–54
We hypothesized that if friendships among
female buffalo are formed based on personality homophily,
then homophily in personality is a general behavioral decision
rule for partner preference. In particular, we predicted that lower
score differences in social personality traits would be positively
associated with higher dyadic friendship values.
RESULTS
Friendships in buffalo
We validated the dyadic proximity index, i.e., our measure of
buffalo social associations, by comparing it with the dyadic
body contact index (see STAR Methods). We found a moderately
strong positive correlation (Spearman rank correlation: n= 274,
r= 0.42, p< 0.001), suggesting that the dyadic proximity index
sufficiently captured affiliative social relationships (Figure 1). On
average, the proximity index had a value of 1.0 (range = 0.15 to
2.19), with higher values indicating stronger social associations.
For instance, a dyadic value of 2.19 indicates 2.19 times stro-
nger associations between two individuals than the population
average. To investigate whether the patterns of dyadic proximity
were temporally consistent, we calculated dyadic proximity index
values by equallydividing the scan data into two phases (i.e.,a first
set of 150 scans and a second set of 150scans). We compared the
dyadic proximity index values between the two phases and found
no difference (WilcoxonSigned Rank Test: Z = 0.66, p= 0.50). This
suggests that these relationships were temporally stable.
Personality traits in buffalo
We extracted three principal components (or personality traits)
from a principal component analysis (PCA) that included six
repeatable behavioral variables (Figure 2, also see STAR
Methods). The three PCs cumulatively explained 80.15% of the
variance (Table S1). The PCs were labeled based on the behav-
ioral variables that were loaded on them (Table S2). PC1 had
three positively loaded variables: approach,self-groom, and
avoid. We labeled it as social tension. PC2 included only one var-
iable, vigilance; hence, it was given the same label, i.e., vigilance.
PC3 had two loaded variables, sit and displace, loaded positively
and negatively, respectively. We labeled PC3 general domi-
nance. We did not find any evidence of behavioral syndrome
based on phi matrix values.
We found that sit and displace behaviors were (linearly) non-
correlating, yet they loaded on a single PC. While non-correlating
variables can load on a single PC if they capture variance of the
data similarly, we further investigated for the presence of any po-
tential non-linear relationship between sit and displace behav-
iors. We conducted a generalized additive model but found no
evidence of any non-linear relationship between the two vari-
ables (Adjusted R
2
=0.03, p= 0.70). However, due to high con-
fidence interval values, we created a residual model plot and re-
investigated their relationship visually. We found the presence of
an ‘‘arch effect’’ between sit and displace, suggesting a complex
non-linear rather than a simple linear relationship (Figure S1).
From each PC or personality trait, individual factor scores
(synonymously personality scores) were extracted, and score
differences were calculated for all combinations of dyads. As
personality traits are often considered age-independent behav-
ioral constructs, we tested for correlations between individual
age and personality scores for the three traits separately.
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Personality traits and age had no significant associations
(Spearman rank correlation tests, social tension: r=0.01,
p= 0.93, vigilance: r=0.01, p= 0.92, general dominance:
r= 0.006, p= 0.97).
Effects of personality traits, age differences, kinship,
and rank differences on buffalo friendships
We found significant relationships between personality trait dif-
ferences and the dyadic proximity index (Figure 3,Table S3).
Lower dyadic score differences in social tension were associ-
ated with higher dyadic proximity index values (GLMM: z =
5.147, Cohen’s d = 0.62, 95% CI = [0.39, 0.86], p< 0.001, Fig-
ure 3A). Differences in vigilance did not affect the dyadic prox-
imity index (GLMM: z = 0.768, p= 0.44, Figure 3B). Like social
tension, dyads with lower score differences in general domi-
nance had higher proximity index values (GLMM: z = 4.427,
Cohen’s d = 0.53, 95% CI = [0.30, 0.77], p< 0.001, Figure 3C).
We did not find any effect of age differences on the dyadic prox-
imity index (GLMM: z = 1.468, p= 0.14). Similarly, kinship had no
effect on the dyadic proximity index (GLMM: z = 1.110, p= 0.26,
Figure 3D).
These results suggest that dyads with more similar social ten-
sion and general dominance traits were more likely to be in close
Figure 1. The spatial positions of females in
a feral and free-ranging population of buf-
falo
(A) The black rectangle shows two female buffalo
sitting in affiliative body contact. The red rectangle
shows two female buffalo sitting in proximity (one
body length distance).
(B) Proximity network of female buffalo in the Lo
Wai Tsuen herd.
(C) Proximity network of female buffalo in the Lo Uk
Tsuen herd. In (b) and (c), each dark green circle or
node indicates an individual, and the lines or edges
between them indicate the dyadic proximity index
values, with thicker edges indicating higher index
values or strong social associations [Photo credit
(a): Debottam Bhattacharjee, Location: Pui O,
Hong Kong], See also Table S5 and Figure S2.
proximity than dyads with greater trait dif-
ferences. This full model differed from the
null model (Likelihood ratio test: c
2
=
57.918, df = 5, p< 0.001). We found no
collinearity among the fixed effects (VIF
range: 1.01–1.12), and the model resid-
uals followed normality assumptions (Uni-
formity or normality – Kolmogorov-Smir-
nov test: p= 0.98; Dispersion – Fitted vs.
simulated residuals: p= 0.91; Outliers –
Binomial test: p= 0.73).
Some behavioral variables (i.e., avoid
and displace), which constitute social ten-
sion and general dominance personality
traits, are often used in combination with
other behaviors, such as agonistic inter-
actions, to construct dominance hierar-
chies.
55
However, we found that these direct interactions were
very rare during scans, leading to >94% ‘‘unknown’’ dominance
rank relationships among the individuals. Thus, dominance hier-
archies could not be constructed based on the data collected
during scans. Yet, to investigate whether dominance hierarchies
were associated with friendships, we assessed hierarchies
based on our overall continuous focal data. We extracted avoid,
displace, physical aggression, and flee behaviors (see Table S2)
from focal observations and calculated within-herd dominance
rank relationships. The buffalo herds had low to moderately
steep dominance hierarchies (Steepness: Lo Wai Tsuen =
0.61 ±0.07, Lo Uk Tsuen = 0.53 ±0.04). However, we did not
find any effect of dominance rank differences on the dyadic
proximity index (GLMM: z = 0.166, p= 0.87, Table S4).
DISCUSSION
The emergence of friendships in animals has long been consid-
ered a phenomenon governed by similarities in demographic
characteristics, such as age and sex.
4
However, the sustenance
of age- and sex-independent friendships has impelled resea-
rchers to propose alternate explanatory mechanisms, such as
homophily in personality.
29
Yet, empirical research explicitly
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testing homophily as a behavioral decision rule of friendship is
mainly restricted to primates. We investigated adult same-sex
friendships in a free-ranging and feral population of a social un-
gulate species, the water buffalo.
56
Our results show that non-
random female-female social associations form in feral buffalo,
where stronger associations may indicate friendships,
20
and ho-
mophily in certain personality traits can predict them. Similarities
in age and dominance rank relationships did not explain the
dyadic social associations. Also, we found that kin and non-kin
dyads show comparable levels of dyadic associations. Based
on these results, we show that homophily in personality is a prox-
imate behavioral decision rule of friendship that applies to spe-
cies beyond primates.
Friendships in buffalo
The temporally stable dyadic proximity index, validated by affili-
ative body contact, captured the differential strengths of buffalo
social associations, including friendships. Friendships in buffalo
can have considerable socio-ecological and adaptive implica-
tions, which can range from foraging and collective movement
to resting. The ‘‘‘conspecific attraction hypothesis’’ suggests
that the presence of conspecifics can be used as social cues
during foraging site selection, especially when resources are
patchily distributed
57
(also see
58
). However, social aggregations
during foraging are often considered a by-product of sharing
space (i.e., random associations) and not outcomes of preferen-
tial associations. Although the empirical testing of such assump-
tions is limited, conscious behavioral synchronization
59
or exhi-
Figure 2. Correlation matrix plot of behav-
ioral variables loaded under the three buf-
falo personality traits
Behavioral variables and their factor loading
scores under each personality trait are provided,
see also Table S1,Table S2,Figure S1,Table S6,
and Figure S3.
bition of the same behavior by strongly
bonded individuals in different species is
evident. Synchronization of activities,
such as foraging and collective move-
ment, in a multi-level society of feral hors-
es is influenced by dyadic social associa-
tions, with stronger associations leading
to better synchronization.
60
Stronger affi-
nitive bonds result in more successful
spatiotemporal movements in horses,
too.
61
Desert baboons follow their friends
during the departure phase of collective
movements.
62
In addition, microhabitat-
level space use patterns, in terms of
resting site selection, can be governed
by preferential strong social associations.
Cape buffalo and forest buffalo choose
specific resting sites (e.g., forest clearing)
that enable them to sit in very close prox-
imity to each other (often including body
contact) and facilitate social interac-
tions.
63–65
Close spatial associations can foster allopreening in
parrots and corvids, too.
66
These results suggest that preferen-
tial social associations can form during foraging, collective
movements, and resting, and such associations may not neces-
sarily be random and simple by-products of sharing a common
space. Therefore, while the general herd-level social associa-
tions in space use patterns may strengthen group cohesion
and help avoid predators through ‘‘many eyes,’’
67
preferential
social associations or friendships can still be sustained.
As evident in a species such as water buffalo, where direct
affiliative interactions, such as allogrooming, are predominantly
absent,
49
spatial proximity patterns can be of vital importance
for maintaining friendships (cf.
50
). Hence, instead of fully
relying on the assumption of the ‘‘gambit of the group,’’
68
spe-
cies-specific ‘‘norms’’ of friendships can be present, i.e., without
necessitating direct affiliative interactions among friends
(cf.
20,29,49,50,69
).
Our assessment of buffalo friendships encompassed all as-
pects of grazing, movement, and resting in dry and wet seasons.
The strength of overall observed differential dyadic associations
and their temporal stability suggest that friendships were likely
sustained throughout contexts and climatic conditions. Friend-
ships were both age- and kin-independent, but note that the
number of kin-dyads was lower than non-kin dyads in our buffalo
herds. Kinship can govern close social associations potentially
mediated through ‘‘familiarity.’’ Nonetheless, friendships may
form both within genetically closely related and unrelated mem-
bers, suggesting that kinship is not necessarily a prerequisite for
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the emergence of friendships.
12,20,29
Additionally, dominance
rank differences did not predict friendships in buffalo. In highly
despotic societies with steep dominance hierarchies, rank-
based cooperation among group members or selectivity in
benefiting group members is evident.
12,70
The low to moderate
steepness values in dominance hierarchies in female buffalo
might explain why rank differences are not suitable drivers of
friendships.
Personality traits in buffalo
Using a bottom-up method, we reported three personality
traits in feral buffalo: social tension, vigilance, and general
Figure 3. Predicted effects of buffalo personality trait differences and kinship on dyadic proximity index
(A) Effect of social tension trait differences on proximity index (p< 0.001).
(B) Effect of vigilance trait differences on proximity index (p= 0.44).
(C) Effect of general dominance trait differences on proximity index (p< 0.001).
(D) Effect of kinship on proximity index (p= 0.26). Solid dark green circles indicate data points and pvalues indicate significance. In (a), (b), and (c), shaded areas
indicate 95% confidence intervals. In (d), solid black dots and vertical lines indicate predicted mean values and 95% confidence intervals, respectively, see also
Tables S3 and S4.
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dominance. While these personality traits are subjectively
labeled (cf.
30,41,51–54
), the underlying repeatable behaviors may
have socio-ecological implications for buffalo. The social tension
trait consisted of three behaviors: approach,self-groom, and
avoid. Approaching and tolerating conspecifics or even hetero-
specifics shows a tendency toward sociability,
71
which has sub-
stantial adaptive implications (see
43
for a detailed discussion).
Self-groom, or broadly, self-directed behavior in animals, is a
widely used indicator of stress-related responses, contributed
by social (e.g., hierarchical group structure) or non-social (e.g.,
presence of ectoparasites) factors.
52,72–74
Finally, avoiding (spe-
cific) conspecifics can be attributed to the hierarchical social
structure of buffalo (see
48
), which may help reduce conflicts,
competition, and disease transmission within the herd.
75
How-
ever, counterintuitively, approach behavior positively co-varied
with self-groom and avoidance (of conspecifics) in our personal-
ity construct, indicating aspects of both sociability and tension.
The availability of partners, space, and distribution of food re-
sources in social and ecological niches can potentially explain
why behaviors of two extremes may have positively co-var-
ied.
76,77
In other words, individuals may show consistent
behavioral ‘‘tactics’’ to fulfill their social and ecological needs.
Alternatively, the counterintuitive loadings can simply be ca-
use-and-effect relationships, where the more an individual ap-
proaches herd members, the more situations arise where they
are approached by others, leading to avoidance and self-groom
behaviors.
The second personality trait, vigilance, was labeled after the
only behavioral variable loaded in it. Vigilance, in general, has a
high adaptive value associated with predator avoidance; howev-
er, our study population of feral buffalo has no natural predators.
Thus, proximately, this behavior can be attributed to focusing on
within-herd events, such as ongoing physical fights, or paying
attention to nearest neighbors.
78
In addition to resident males
within the herds, other territorial males who were not part of
the observed herds were present in the study area. Subse-
quently, frequent territorial fights among resident and non-resi-
dent males, often followed by female guarding, were observed
(personal observations, D.B., 2023–2024). The approach of the
resident male for mate guarding can make nearby females vigi-
lant. However, individuals who are not within a guarding radius
can avoid being vigilant. Further, individuals with relatively low
dominance ranks can collect spatiotemporal information on
aversive events (such as fights and aggression) and nearest
neighbors (such as an approaching male or other higher-ranking
herd members) and benefit by escaping or avoiding conflicts.
Therefore, consistent variations in vigilance behavior in feral buf-
falo can provide substantial benefits even in the absence of nat-
ural predators.
The third and final personality trait, general dominance, has
two inversely co-varying behaviors: sit and displace. In the
context of water buffalo, the behavior sit can extend beyond
resting. The swamp and marshland habitats allowed our study
population to ‘‘rest’’ in the waterlogged fields. While the behavior
was coded independently of wallowing,
79
sitting in such terrain
may include attributes of thermoregulation, ectoparasite
removal, and so forth, thus potentially serving valuable physio-
logical functions. However, such semi-naturally occurring water-
logged areas are not uniformly distributed,
80
especially during
the dry season of the year, leading to competition (including
displace behaviors) among individuals for access. Therefore,
the inversely co-varying behaviors can have consistent inter-in-
dividual differences and be labeled general dominance. Besides,
this trait is often linked to boldness and exploration
81
(but see
82
),
which are highly beneficial when predator pressure is low or ab-
sent. In contrast, individuals with low general dominance scores
can benefit by avoiding potential conflicts and aggressive inter-
actions. General dominance has further been reported as a
manifestation of leadership behavior in animals,
83
which could
be helpful in decision-making, e.g., in collective movements.
To what extent variations in general dominance represented
the dominance hierarchy of the herds remains to be investigated,
which could not be assessed in this study to avoid data
dependence.
We found that personality traits in buffalo partially resemble
the generally accepted traits of exploration avoidance, bold-
ness-shyness, activity, sociability, and dominance in animals.
25
While some aspects of sociability and dominance have been
captured, we did not find any direct evidence of exploration
and boldness-like traits. This could be attributed to the solely
observation-based methodology of our study, through which
‘‘rare’’ behaviors such as exploration, boldness, and persistence
are challenging to capture (see
12,29,51,53
). It would be interesting
to conduct field experiments (such as novel objects and novel
puzzle tasks) to investigate whether buffalo exhibit those traits.
Personality homophily and friendships
As hypothesized, we found evidence that personality homophily
is a behavioral decision rule of friendship in female water buffalo.
Social tension and general dominance, but not vigilance,
predicted the preferential strong social associations. Unlike vig-
ilance, social tension and general dominance are social person-
ality traits (i.e., traits that include social interactions
84
). In general,
social personality traits, such as extraversion and agreeable-
ness, are known to foster friendships and cooperation in hu-
mans.
85
Certainly, the buffalo social personality traits do not fully
resemble the human social personality traits, yet justification can
be made by highlighting the underlying basis of personality ho-
mophily: its ability to help form trust and reduce uncertainty
among similar individuals.
4,21
Consistent with this idea, similarly
(socially) tensed buffalo can form emotionally mediated attitudes
toward herd members through social associations, leading to the
preferential selection of partners.
52,86
Likewise, differences in
general dominance have significant implications for adaptive
benefits. A low dominance rank difference or high dominance
rank similarity can be associated with better cooperation and co-
ordination.
12,87
Friendships based on similar general dominance
status can reduce the monopolization of resources such as food
by facilitating tolerance. In line with previous findings, the non-
social vigilance trait did not influence friendships,
31
which may
suggest that the emergence of friendships in buffalo may depend
on social but not on non-social personality traits (but see
29
).
Conclusion
The evolution of social relationships has several complex under-
pinnings beyond simple demographic characteristics. Using an
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underrepresented social ungulate species, we provide valuable
insights into the proximate mechanisms driving preferential
close social relationships or friendships. We identify and discuss
the emergence of friendships based on similarities in personal-
ities, a behavioral decision rule that has so far been tested pri-
marily on primates. However, the involved psychological and
emotional processes employed for the selection of friends in wa-
ter buffalo should carefully be assessed before generalizing
them with that of humans and other non-human primates.
Although we highlight the broader evolutionary implications of
friendships in buffalo by drawing parallels with other social spe-
cies, significant gaps in knowledge of feral water buffalo ecology
and behavior have rendered identifying the species-specific
norms of sociality and friendships challenging. These limitations
call for in-depth investigations to validate how the broad spec-
trum of ecological relevance pertaining to a host of other taxa
aligns specifically with water buffalo.
Limitations of the study
Our study relied on behavioral observations, and we could not
conduct experiments to assess traits that are otherwise chal-
lenging to observe (such as persistence and exploration). Thus,
the reported traits may not fully represent buffalo personalities.
Future studies should include field experiments while evaluating
the personalities of buffalo. Due to data density and model
convergence issues, we had to discard the body contact index
and only use the proximity index as an indicator of friendship.
Although we checked for the correlation between the two indices
(and indeed found a positive association), more opportunistic
observations using All occurrences sampling might be helpful
in collecting data on body contact and other direct affiliative
behavioral interactions. Such a dataset would greatly benefit
the construction of more robust friendship indices, such as the
dynamic dyadic sociality index (DDSI). Additionally, domi-
nance-rank relationships could not be constructed based on
the scan data. Therefore increasing the number of scans or em-
ploying the All occurrences sampling method might prove to be
more effective in future research. Nonetheless, we utilized the
focal data to calculate dominance rank relationships, which,
however, could not be modeled together with personality traits
to assess friendships. Finally, 29 of the 30 individuals in our study
are sterilized, which could potentially influence their social rela-
tionships. While it is not possible to compare social behaviors
between sterilized and non-sterilized individuals, we emphasize
that the daily interactions among individuals in our study herds
are not controlled by humans. In other words, individuals are
free to interact with other herd members, thus still promoting
ecological relevance.
RESOURCE AVAILABILITY
Lead contact
Further information and requests for resources should be directed to and will
be fulfilled by, the lead contact, Debottam Bhattacharjee (bhattacharjee.
debottam@gmail.com).
Materials availability
Besides data and R codes, this study did not generate any new reagents or
materials.
Data and code availability
dData: All data generated in the study have been deposited at Open Sci-
ence Framework and are publicly accessible at Open Science Frame-
work Data: https://doi.org/10.17605/OSF.IO/AU3VP which is also listed
in the key resources table.
dCode: All original codes used in the study have been deposited at Open
Science Framework and are publicly accessible at Open Science
Framework Data: https://doi.org/10.17605/OSF.IO/AU3VP which is
also listed in the key resources table.
dAny additional information required to reanalyze the data reported in this
paper is available from the lead contact upon request.
ACKNOWLEDGMENTS
We thank Wong Ching Ki and Tin Ka Yuen for their help with data collection and
coding. We sincerely thank Jean Leung for providing the demographic data on
the buffalo. We are grateful to Jorg Massen, Elham Nourani, George Hodgson,
and Tania Perroux for their valuable feedback on an earlier version of this
article. This study was supported by a Lantau Conservation Fund (Hong
Kong SAR Government-funded program) grant (RE-2021-01) awarded to
A.G.M. and K.F.J.
AUTHOR CONTRIBUTIONS
D.B., K.J.F., and A.G.M. conceived and designed the study; D.B. performed
research, collected, coded, and organized data; D.B. conducted the formal
analysis of the data; D.B. wrote the first draft of the article; D.B., K.J.F., and
A.G.M. edited the article. K.J.F. and A.G.M. acquired funding and supervised
the study.
DECLARATION OF INTERESTS
We declare that we have no competing interests.
STAR+METHODS
Detailed methods are provided in the online version of this paper and include
the following:
dKEY RESOURCES TABLE
dEXPERIMENTAL MODEL AND STUDY PARTICIPANT DETAILS
dMETHOD DETAILS
BData collection
dQUANTIFICATION AND STATISTICAL ANALYSIS
BCoding
BAssessment of friendships
BAssessment of personality
BEffects of personality trait differences on friendships
BAssessment of dominance hierarchies and their effects on friend-
ships
BStatistical packages
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.isci.
2024.111419.
Received: September 7, 2024
Revised: October 15, 2024
Accepted: November 13, 2024
Published: December 5, 2024
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STAR+METHODS
KEY RESOURCES TABLE
EXPERIMENTAL MODEL AND STUDY PARTICIPANT DETAILS
We conducted this study on a feral and free-ranging population of water buffalo present in Pui O in the Southern Lantau Island of Hong
Kong SAR, China. Pui O is a small village at the edge of Lantau South Country Park, and the marshlands of Lo Wai Tsuen (2214032.700 N
11358042.3" E) and Lo Uk Tsuen (2214029.300 N11358031.1" E) in Pui O are home to two different herds of feral buffalo. At the begin-
ning of the study, the Lo Wai Tsuen herd had a size of 19 (females = 7, males = 12), whereas the Lo Uk Tsuen herd comprised 29 in-
dividuals (females = 25, males = 4), of which two females were below two years of age (i.e., calves). We collected data on thirty adult
females (age range: 4 to 19; mean age in years ±standard deviation = 10.38 ±4.33) from the two different herds (Lo Wai Tsuen = 7, Lo
Uk Tsuen = 23, Table S5) between 17
th
July 2023 and 6
th
February 2024, including a wet and a dry season.
88
While the number of fe-
males remained the same throughout the study period, the number of males changed in both herds due to death and dispersal. Two
male buffalo from the Lo Wai Tsuen herd dispersed in October, and one individual died in late December (due to natural causes). One
male buffalo from the Lo Uk Tsuen herd dispersed in January. Although the two herds live adjacently, inter-herd interactions among
females were never observed (personal observations, D.B., 2023–2024, also see
89
). The buffalo fed on grass and other natural vege-
tation but occasionally (three to four times a week between December and February) received supplementary food (hay and sweet
potato leaves) from local citizen groups. Fresh water was available ad libitum from small waterbodies in and around the marshlands.
A cattle management team from the Agriculture, Fisheries and Conservation Department of Hong Kong (AFCD) routinely sterilized
the buffalo population.
90
All adult females were sterilized except one from the Lo Uk Tsuen herd, and their status did not change dur-
ing the study period. Although sterilized, we emphasize that the daily behavioral interactions among individuals are not controlled by
humans (personal observations, D.B.). That is, individuals are free to engage in interactions (such as affiliative and agonistic) with
other herd members. Moreover, like affiliative interactions, we witnessed guarding behavior and territoriality in our study population
(unpublished data). These observations suggest that management by AFCD in the form of sterilization has limited potential effect on
the natural behavioral interactions among buffalo on the Lantau Island. While over 50% of the adult females (16 of 30) had numbered
ear tags (administered by the AFCD), we relied on morphological features, such as horn shape and structure, relative horn length, and
scar marks, to identify the remaining individuals. A photo catalog was prepared to ensure the reliable identification of the individuals.
Kinship and individual age details were collected from a local non-government citizen group, documenting the buffalo population’s
demographic information over the past 20 years.
Ethical approval was obtained from the Animal Research Ethics Sub-Committee of City University of Hong Kong. The behavioral
observations of buffalo were conducted from a distance of at least 20 m without direct human intervention. We adhered to the ethical
guidelines of the ASAB/ABS for conducting the research.
91
METHOD DETAILS
Data collection
Continuous focal and scan sampling methods were used to collect behavioral data.
92
Each observation day was divided into three
time periods: morning (0900–1159 h), afternoon (1200–1459 h), and late afternoon (1500–1800 h), and each period was equally
REAGENT or RESOURCE SOURCE IDENTIFIER
Deposited data
Raw data and code This paper Open Science Framework: https://doi.
org/10.17605/OSF.IO/AU3VP
Experimental models: Organisms/strains
Water Buffalo (Bubalus bubalis) Lantau Island, Hong Kong SAR N/A
Software and algorithms
R (version 4.3.1) R Core Team https://www.Rproject.org/
Adobe Illustrator (version 25.3) ADOBE https://www.adobe.com/products/
illustrator.html
Other
Video Camera (Panasonic HC-V785) PANASONIC https://www.panasonic.com/in/consumer/
cameras-camcorders/camcorders/hc-
v785.html
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sampled. An extensive ethogram was developed with all behaviors exhibited by the buffalo (Table S2), which was used during data
collection and coding.
Focal sampling
Each continuous focal observation session was 20 min long. We collected 720.03 ±0.32 min (mean ±standard deviation) of
focal data per individual. Thus, each individual was observed a total of 36 times. We used a semi-randomized order to observe
the individuals in different time periods on an observation day, and the same individual was not observed more than once per
day. We conducted one to nine focal observations daily (5.17 ±1.96 focal observations/day), at least three times a week. Focal
data were recorded using a video camera (Panasonic HC-V785) mounted on a tripod or collected live using digital data sheets.
All state (durations) and event (occurrences) behaviors of the focal individuals, as well as their interactions with non-focal herd mem-
bers, were noted using the ethogram.
Herd scan sampling
We followed predetermined routes during scan sampling. The sampling distances of Lo Wai Tsuen and Lo Uk Tsuen were 2.1 km and
2.3 km, respectively. In complex sampling routes (like in areas of human settlement in our study), the same roads had to be walked
more than once to go to different parts of an area, but data were recorded only for the first time while walking on the same road (Fig-
ure S2). This step helped avoid re-sampling of the same individuals during a given scan. During each scan, we recorded the spatial
positions of the individuals relative to each other in a herd. We collected data on the occurrence of two levels of dyadic spatial po-
sitions: dyadic proximity (one body length distance to each other) and affiliative body contact (body parts of two individuals excluding
legs or horns touching while standing, sitting, or lying down) (Figure 1A, Table S2). Dyadic proximity and affiliative body contact were
mutually exclusive, suggesting that we did not consider proximity when two individuals were in affiliative body contact, We conduct-
ed 3–5 scans per day (3.84 ±0.74 scans/day) at least twice a week. The interscan interval was 30 min, and we conducted 300 scans of
each herd. Notably, all individuals from their respective herds were present during all the scans.
QUANTIFICATION AND STATISTICAL ANALYSIS
Coding
Videos were played using Pot Player (version 240315, 1.7.22129) and coded in a frame-by-frame manner. Following the ethogram,
we coded the duration (in seconds) and occurrence of all state and event behaviors, respectively (cf. Table S2). Since each individual
was observed for the exact duration (i.e., 720 min), no time correction was deemed necessary. D.B. coded all the videos and
compiled them with the focal data recorded in digital datasheets. Another person, unaware of the goals of the study, coded 10%
scan and 10% of the focal data. The reliability between D.B. and the blind coder was excellent (Intraclass correlation tests: focal
data - ICC (3,k) = 0.93, p< 0.001; scan data - ICC (3,k) = 0.95, p< 0.001).
Assessment of friendships
We created all possible combinations of dyads at the herd level (Lo Wai Tsuen herd = 21 dyads and Lo Uk Tsuen herd = 253 dyads).
Four of the 21 dyads from the Lo Wai Tsuen herd consisted of kin members, whereas only 14 were kin dyads out of the 253 dyads
from the Lo Uk Tsuen herd. Kinship specifically included mother-daughter, grandmother-granddaughter, and sibling (i.e., sisters) re-
lationships. From the herd scans, the occurrences of proximity and affiliative body contact were used to calculate a dyadic sociality
index (DSI),
93,94
a widely used indicator of strong social associations and friendship in animals. Dyadic proximity and body contact
occurrences were divided by the respective population mean values to calculate corresponding index (i.e., dyadic proximity index
and body contact index) values. These scaled index values were combined and averaged to obtain the DSI values. However, the data
on the occurrence of body contact was zero-inflated, with only close to 30% of the potential dyads exhibiting affiliative body contact.
To avoid any statistical model convergence issues, we chose dyadic proximity index as the only indicator of friendship (see
20,69
).
Nevertheless, we investigated the validity of dyadic proximity index by comparing it with the body contact index (Figure 1A). To
further investigate whether social associations were temporally consistent, we calculated and compared the dyadic proximity index
between two equally divided phases, each phase consisting of 150 scans.
Assessment of personality
A standardized ‘bottom-up’ approach was used to assess the personality traits,
51–53
where any potential bias of predetermined clus-
tering of variables could be avoided. By definition, personality traits should be consistent across time and contexts
25
; therefore, we
divided the focal data into two phases (with 360 min/focal individual for each phase) to investigate whether they were repeatable. In
other words, the first 360 min of focal data were compared with the rest of the 360 min. The repeatability of behavioral variables was
assessed using a two-way mixed model intraclass correlation (ICC (3,1)) test.
51–53
Forty-eight behavioral variables (cf. Table S2) were
considered for the bottom-up approach. However, variables with low occurrences were dropped before conducting the ICC analysis.
Twenty-seven variables were dropped as more than half of the individuals did not exhibit them, i.e., the variables had zero occur-
rences. Subsequently, twenty-one variables were retained for the ICC analysis (Table S6). We used a conservative ICC cut-off value
of 0.5 for a variable to be considered repeatable.
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Fourteen of the twenty-one variables were repeatable, with ICC values ranging
from 0.521 to 0.850 (Table S6).
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A principal component analysis (PCA) was conducted using the fourteen repeatable variables. The average values of the
repeatable variables between the two phases were calculated, standardized, and included in the PCA. However, due to low
Kaiser-Meyer-Olkin sampling adequacy (MSA) values (MSA cut-off = 0.6) and inadequate percentage communality values (commu-
nality cut-off = 70%),
96
the number of variables was further reduced to six by a step-by-step process (cf.
51,52
). The percentage of
communalities of the remaining six variables ranged between 73.02 and 91.33, with an overall MSA value of 0.69. A scree plot
was generated using an unrotated PCA, and the eigenvalue of each potential principal component (PC) was retrieved. The number
of PCs was subsequently decided based on the eigenvalues (with eigenvalue R1) and by visual inspection of the scree plot (Fig-
ure S3).
96
All other assumptions of PCA were met (Bartlett test: p< 0.001, and cumulative variance explained by PCs > 60%). As
personality traits can be correlated and form behavioral syndromes,
97
we used an oblique rotation technique (direct oblimin rotation).
We created a phi matrix and obtained the correlational values among the personality traits. The factor loadings R0.6 (positive and
negative) were considered salient.
98
Effects of personality trait differences on friendships
A generalized linear mixed-effect model (GLMM) analysis was conducted to investigate the effects of personality score- and age-dif-
ferences, and kinship on dyadic proximity index values. We used a Gaussian error distribution with an ‘identity’ link function. In the full
model, the dyadic score differences of each personality trait, absolute dyadic age differences, and kinship (yes/no) were included as
fixed effects, and the proximity index as a response variable. Individual identities of a dyad (i.e., individual 1 and individual 2) were
included as random effects. The null model included the response variable and the random effects but lacked fixed effects.
Assessment of dominance hierarchies and their effects on friendships
Using focal data, we created four occurrence-based matrices per herd where all individuals were placed as initiators and receivers of
avoid, displace, physical aggression, and flee behaviors (see Table S2 for definitions). We created the four matrices separately based
on the number of occurrences of each behavior. The numbers were later added to build a combined matrix, each for a herd, but after
adjusting for their directionality (i.e., avoid, being displaced, being physically aggressed, and flee). We used a Bayesian Elo-rating
method to assess herd-level hierarchies.
99
The method involved the calculation of winning probabilities from the combined matrices.
Upon construction, individuals were plotted ordinally along their respective hierarchies based on the estimated Elo values. The
ordinal rank differences were calculated for the dyads and standardized to account for the varying herd sizes. Since personality
and dominance data are not independent, to avoid any bias of data dependence, rank difference was not included as a fixed effect
in the GLMM that investigated the effects of age and personality differences on friendships. In a separate GLMM with a Gaussian
error distribution and ‘identity’ link function, dyadic rank difference was included as a fixed effect and the proximity index as a
response variable. Individual identities of a dyad were included as random effects. The null model included the response variable
and the random effects but lacked the fixed effect.
Statistical packages
All statistical analyses were performed in R (version 4.3.1).
100
ICC analyses were conducted using the psych package.
101
We used
Elosteepness package to construct the dominance hierarchies.
99
We used the package glmmTMB for the GLMM analyses.
102
The
null vs. full model comparisons were checked with the lmtest package.
103
Collinearity among fixed effects was investigated using the
performance package,
104
and a variation inflation factor (VIF) of >3 was considered a threshold for high correlation.
105
GLMM model
diagnostics (normality assumptions) were checked using the package DHARMa.
106
The significance value was set at 0.05 for all sta-
tistical tests.
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