NOTICE: This is the author’s version of a work accepted for publication by Elsevier. Changes resulting from the publishing process,including peer review, editing, corrections, structural formatting and other quality control mechanisms, may not be reflected in thisdocument. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently
published in:
Nakamura M (2003) ‘Gatherings’ of social grooming among wild chimpanzees: implications for evolution of sociality. J Hum Evol
44: 59–71. DOI:10.1016/S0047-2484(02)00194-X‘Gatherings’ of social grooming among wild chimpanzees: implications
for evolution of sociality
Michio Nakamura
Graduate School of Science, Kyoto University, Japan
Abstract
Chimpanzees (Pan troglodytes) often groom in gatherings that cannot simply be divided into unilateral dyadic
grooming interactions. This feature of grooming is studied at two different levels: grooming cliques and grooming
clusters. Grooming cliques are defined as directly connected configurations of grooming interactions at any given
moment, and when any member of a clique successively grooms any member of another clique within 5 min andwithin a distance of 3 m, all the members of both cliques are defined as being in the same grooming cluster.
Twenty-seven types of cliques are observed, with the largest one consisting of seven individuals. Mutual and/or
polyadic cliques account for more than 25% of all cliques. The size of grooming clusters varies from two to 23 individuals, and almost 70% of the grooming time is spent in polyadic clusters. Although adult males groom the longest
in relatively smaller clusters (size = 2–4), adult females groomed the longest in clusters of five or more individuals. A
review of the literature implies that mutual and polyadic cliques occur less often in other primate species than inchimpanzees. The importance of overlapping interactions for these kinds of gatherings and its possible significance inthe evolution of sociality is discussed in this article.
Keywords: Chimpanzees; Social grooming; Mahale Mountains National Park; Overlaps of interactions; Gatherings; Evolution of sociality

1.Introduction
The evolution of sociality is one of the most important topics when we consider the overall evolution of
Homo sapiens and of primate taxa (e.g. Imanishi, 1960;
van Schaik, 1983; Itani, 1985; Wrangham, 1987). Sociality can be studied from various viewpoints. Somemay study macroscopic aspects such as ‘social structure’ or ‘social organization’, while others may study
microscopic aspects such as ‘social behaviors’ or ‘social tactics’ of an individual that are believed to ultimately increase its reproductive fitness. Both approaches are useful, but it is apparent that the social
structure cannot simply be constructed by summing up
individual social behaviors. Many more steps that connect these two aspects of sociality need to be studied inorder to understand how behaviors are integrated toform a society. One example of such attempts is thestudy of human social ‘gathering’ (Goffman, 1963).
Goffman defined gathering as “any set of two or moreindividuals whose members include all and only those
who are at the moment in one another’s immediate
presence”, and vividly described the aspects of everyday social life in modern human society. It is importantthat ‘gathering’ is neither a society itself nor an attribution of a particular individual, but lies somewhere in
between. Being a sociologist, Goffman did not conduct
his studies from the evolutionary perspective, but thesame kind of approach may be also useful for understanding primate sociality and its evolution.
Social grooming is one of the most common every-

day social interactions among chimpanzees (Pan troglodytes) as well as among various other primate species(Sparks, 1967; Goosen, 1987). Chimpanzees allocate alarge portion of their daytime hours grooming eachother. Some individuals spend more than 25% of theirwaking time grooming with others (Goodall, 1986;
Kawanaka, 1989). Being common behavior, grooming
is used frequently, and often too easily, as an index for
affiliation in primate studies (Cords, 1997). In suchcases, it is implicitly regarded that the groomer payssome cost and the recipient gains some benefit. With
this assumption, grooming is often regarded as being
exchanged for grooming itself, collaboration in fighting,copulation, or sometimes food. In this kind of analysis,
the grooming interaction is usually broken down into
dyadic and unilateral interactions such as ‘A/B’, andsuch segmented interactions are cumulated on a dyadic
basis.
However, chimpanzees often groom in gatherings,
sometimes called ‘grooming clusters’ (de Waal, 1982;
Goodall, 1986), in which 10 or more individuals groom
in the same session and the membership changes frequently (van Lawick-Goodall, 1968). In such gatherings, individuals can, and often do, participate in multiple interactions simultaneously; for example, an individual may groom another while being groomed by athird, or he/she may receive grooming from two or
more partners at the very same time. Such overlaps ofinteractions form polyadic chains of grooming (seepicture in Goodall, 1986, p. 393, for example). However, such gatherings of grooming and overlaps of

grooming interactions have not been examined in detail.
We do not know how often such gatherings are formed,
nor the characteristics of such gatherings compared tothe dyadic grooming interactions.
Humans often form social gatherings and interact
with multiple partners at the same time in everyday
interactions, such as conversations. Given the phylogenetic closeness of humans and chimpanzees, somesimilarity in sociality could be expected. Especially ingrooming, the most frequent everyday interactions in
chimpanzees, such polyadic characteristics of social
gathering are expected to be observed frequently. Accordingly, this article describes the frequencies ofvarious types and sizes of gatherings of chimpanzee
grooming and explains, by focusing on overlaps of theinteractions, as to how individuals perform grooming
behaviors in such scenes. Next, the age–sex differencesof participation in different size of gatherings arebriefly examined followed by a discussion on the implication of this phenomenon in chimpanzees on theevolution of sociality.

2.Methods
The grooming behavior of the M group chimpanzees in the Mahale Mountains National Park, Tanzania,
was studied between July 1996 and May 1997. For
detailed information on the research site, see Nishida(1990). Grooming is defined here as a series of behavioral elements, such as stroking the hair, picking thehair, removing things with hand(s) or lip(s), andscratching other individuals (Nakamura et al., 2000).
Self-grooming and leaf grooming (Zamma, 2002) were
not included in the analysis. In the course of my research, the M group consisted of 53 individuals: sevenadult males (over 15 years), 18 adult females (over 14
years), five adolescent males (9–14 years), five adolescent females (9–13 years), and 18 juveniles and infants(under 8 years).
To understand gatherings made for grooming, it is necessary to grasp the behaviors of multiple individuals at
any given time. However, ad libitum sampling of any
grooming is not suitable because large grooming clusters occurring in relatively open spaces are more likely
to be observed than smaller clusters occurring quietly inthe bush. Thus, in this study, the observational viewpoint is set on one individual for a day, and the grooming behaviors of the multiple individuals around him/
her were recorded even when they were not directly
grooming with the focal individual. In this methodology, all kinds of social situations that the target individual experienced in a day could be observed. Ten
males and 10 females of various age classes (juvenile toadult) were followed as focal target individuals (Table1). Total duration of follow-ups was 480 h, during
which 137 h of grooming (total accumulation of individual grooming including non-focal individuals) wererecorded. The entire non-focal grooming was included
in analysis unless otherwise stated.
2.1. Definition of grooming cliques
Grooming clique (Dunbar, 1993; Nakamura, 2000)

is defined as a configuration of directly connected individuals through grooming interactions at one moment.
For example, when individual A grooms individual B
and B grooms individual C at the same time (A/B/C),
this is a triadic clique in a serial chain. When any
change in composition or configuration occurs, thenewly formed groups are considered different cliques.
Polyadic cliques are cliques containing three or moremembers, and multi-interaction-cliques means polyadic
cliques plus dyadic mutual grooming cliques. This term
is used because in both polyadic and mutual cliques,
two or more grooming interactions (such as to groom
and to be groomed) simultaneously occur for at least
one individual.
2.2. Definition of grooming clusters
As a grooming clique corresponds to a momentarystate of direct grooming connections, its size and composition change quite frequently. In most cases, suchcliques do not occur independently, but are usually inparallel with other cliques, and they may often exchange their members with each other. Thus, all members of two or more cliques are defined to be in thesame grooming cluster when any member of a clique
successively grooms any member of another clique
within 5 min and within the distance of 3 m. Within
this distance, chimpanzees can change their grooming
partners easily with only slight movement. With thisdefinition of a cluster, an individual participating in acluster is connected to all the members of the cluster byeither direct or indirect (i.e. through other individuals)
grooming interactions. As long as someone continues togroom, a cluster lasts regardless of compositionalchange. In such a case, the cumulative number of themembers is used as the size of the cluster.
3.Results
3.1. Grooming cliques
Twenty-seven types of grooming cliques were observed (Fig. 1), and the largest consisted of sevenmembers. At a maximum, four interactions were observed for one individual at a time. For example incliques [12] and [24], an individual groomed one partner and was in turn groomed by three partners, including the one he was grooming; and in clique [22], an
individual was groomed by four individuals simultaneously.
Fig. 2 shows the proportion of total duration of eachtype of clique. Polyadic cliques ([3]–[27]) accounted
for 16.0% of all the cliques, and with the inclusion ofclique [2], multi-interaction-cliques accounted for
26.3%.
Among
varioustypes
ofmulti-interaction-cliques, chimpanzees groomed frequently in clique [2] (dyadic mutual grooming: 10.4%)
and clique [3] (triadic series chain grooming: 7.5%).
Total duration of clique [4] (dual grooming to the third:
4.0%) was less than that of clique [3], although both ofthem are triadic cliques with two grooming interactions.
The proportion of single and multiple grooming interactions in different age–sex classes are shown in Fig.
3 as mean of individuals in each class. The proportion

of multiple interactions varied among age–sex classes(Kruskal–Wallis, H = 28.2, p < 0.001). Significant differences were found between adult males (22.7% ± 7.8SD) and youngsters (infants and juveniles, 4.5% ± 6.8,
Bonferroni’s multiple comparison, p < 0.05).

df=6, F = 2.4, p<0.05). Grooming duration particularly
increased in adult females when they were in clusters offive or more individuals.

4.Discussion
3.2. Grooming clusters
The size of grooming clusters observed during thestudy varied from two to 23 individuals (almost half ofthe M group members). Day party size (the number ofindividuals observed in a day) surely limits the maximum size of clusters because the latter cannot exceed
the former. Therefore, I have examined the observed
maximum cluster size for each size of day party (2–50),
but there was no correlation between them (Spearman,
rs = 0.25, N = 38, p = 0.13). Relative frequencies ofclusters with different sizes are shown in Fig. 4 (left).
Overall, 926 grooming clusters were observed, of
which 66.6% (617/926) were clusters with two individuals. Clusters with five and more individuals occurred in only 7.8% (72/926) of all events. Small clusters seemed to be dominant if we only consider thenumber of events. However, when we look at the relative duration of clusters (Fig. 4, right), clusters with two
individuals accounted for only 32.4% (2133.2
min/6579.8 min), while clusters with five or more individuals accounted for 27.9% (1837.5 min/6579.8 min).
Therefore, the mean duration of clusters with two individuals was very short (3.5 min) compared to that ofclusters with five or more individuals (25.5 min). Although the previously mentioned durations include
even time when they were not actually engaged ingrooming, data on 20 focal individuals showed similartendencies: they virtually groomed an average of 42.0%(± 25.7), 16.9% (± 14.1), 19.9% (± 20.5), 21.5% (±20.7), and were groomed 37.5% (± 24.1), 20.4% (±16.3), 22.7% (± 20.9), 19.4% (± 21.8), in clusters of
two, three, four, and more than five individuals, respectively.
Fig. 5 shows the relative duration ofmulti-interaction-cliques of different sizes of clusters.
Polyadic cliques could not occur in clusters with two
individuals by definition. In every cluster larger than
three, polyadic cliques were observed at least more than
10% (11.6–48.4%) of the time. In the overall durationof such larger clusters, 22.7% of time was performed inpolyadic cliques. The larger cluster size did not affectthe occurrence of either the dyadic mutual cliques (rs =0.14, N = 12, p = 0.64) or the polyadic cliques (rs =0.47, N = 12, p = 0.11).
3.3. Grooming duration in clusters of different sizes
Fig. 6 shows mean total grooming duration of individuals of different age–sex classes in different sizes ofclusters. There are marked differences among theage–sex classes (ANOVA, df=2, F = 35.2, p<0.0001),
and adult males groomed the longest followed by adultfemales (Bonferroni test, adult males vs. adult females
p<0.01, adult males vs. others p<0.0001, adult females
vs. others p<0.0001). Although no significant differences were found among cluster sizes (ANOVA, df=3,
F = 2.5, p = 0.058), there was a significant interaction
between age–sex classes and cluster sizes (ANOVA,

4.1. Comparison with other primates
Although the aspects of polyadic grooming in
chimpanzees have been previously described, the results may not be easy to evaluate without comparisonswith other species. Unfortunately, almost no information is available on grooming clusters in other species.
However,
there
are
some
descriptions ofmulti-interaction-cliques from 13 Anthropoidea primates (Table 2). Most of the articles only note that suchcliques occur ‘sometimes’ or ‘rarely’ without any quantitative data (Table 2, (A)), and the scarceness of descriptions about such grooming in other primates may
indicate the actual rareness of such grooming. Whenthe four species with some quantitative data (namely,
Macaca assamensis, M. mulatta, H. sapiens and P.
troglodytes) are compared in Table 2 (B), chimpanzees
seem
to
be
more
often
engagedinmulti-interaction-cliques than other species (26.3% in
chimpanzees vs. 5.2–8.1% in others). Note that directcomparisons or statistical tests were not possible due tothe different measures among studies and the difficultyof identifying such measures from the previous articles(e.g. ‘episodes’ are not defined in Cooper and Bernstein,
2000). One famous example would also support thisview: Cheney and Seyfarth (1990) reported that when ahigh-ranking vervet monkey (Chlorocebus aethiops)
approached two lower-ranking conspecifics grooming
each other, the subordinate of the two almost always
moved away. This indicates that their grooming interactions are normally dyadic and rarely triadic, resulting
in this kind of competition over grooming partners.
Chimpanzees, on the other hand, can interact with anindividual who is already involved in grooming withanother, without manifesting competition, which may
obscure who is interacting with whom on a dyadic basis.
Judging from the fragmentary information, there
seems to be more frequent reporting on clique [4](dual grooming to the third) than clique[2] (dyadic mutual grooming) or clique [3] (triadic
serial chain grooming). Furthermore, in all of thosespecies with quantitative data, the proportion of clique[4] always exceeds that of [2] or [3], although thenumber of interactions are the same in these cliqueswith the only exception of chimpanzees, where [2]and [3] are much more frequent than [4]. In clique [4],such as A/B)C, although two interactions are overlapped on B, his/her role is still a groomee. On theother hand, in clique [3], such as X/Y/Z, Y plays thetwo roles of groomer and groomee. Playing two different roles is also the case in mutual grooming, where
both of the two participants play groomers andgroomees at the same time. Therefore, playing two
different roles simultaneously may be a key characteristic of chimpanzee grooming. It is also importantthat the former type of overlap (concentration on one

individual) cannot easily increase the size of a clique
because of the limitation of physical space. Five or
more individuals cannot groom one individual simultaneously, but the same number of individuals canmake a chain.
4.2. Sex differences in ‘gatherings’ of grooming
Female chimpanzees have been said to be relatively
inactive in social interactions with each other(e.g. Nishida, 1979; Wrangham and Smuts, 1980), andthe result that they groomed less often than longer thanmales in large grooming clusters. This adult males insmall clusters is consistent with this may be understoodby the difference in the social current knowledge.
However, they groomed even relationships between
males and between females. Adult male chimpanzees
are known to be very political (de Waal, 1982; Nishida,1983), and many of them usually compete for higher
status. One of the effective ways of doing this is tomake coalition partners. However, perhaps because of
the ascendancy of a young male or the betrayal of theallies, this coalitional relationship always faces thepossibility of change. Thus, these males always have torenew or confirm their relationships with each other.
This leads to their frequent and long grooming in relatively small clusters. On the other hand, adult females
do not have to maintain such relationships with particular individuals. They do not seem to compete forhigher status by forming particularly intimate allies.
Rather, it seems better for them to get along with many
individuals of the community. Therefore, females may
not have to focus their interactions on particular individuals in small clusters. Instead, they may prefer wider
interactions with various individuals at any given moment, making it practical to groom in larger grooming
clusters.
4.3. Implications for evolution of sociality
If chimpanzees groom more often in gathering thanother primates, what implications does it have on theirsocial life and the evolution of sociality? Three
non-exclusive hypotheses are subsequently proposed.
First, the formation of gathering in chimpanzees may
have something to do with the unique characteristics oftheir fission–fusion society (Nishida, 1968). As achimpanzee is not always together with a given member of the same community, there exists a period ofabsence, and this means that their opportunity for social
interactions are, to some extent, reduced. Members ofsuch a species would be more eager to make and maintain a gathering when there is an opportunity. It is suggested that grooming provides one such opportunity inchimpanzee society. If this is the case, individuals
would groom those others in a large cluster who do not
usually range with them in small parties.
Second, the relatively egalitarian sociality of chimpanzees may be one explanation. It is known that
dominance among chimpanzees (Bygott, 1974; Hayaki
et al., 1989), especially among females (Nishida, 1989),
is not as strict as that among macaques. Also, food is
occasionally shared among non-related adults (Nishida
et al., 1992; Nakamura and Itoh, 2001). Such a general
tolerance may permit many individuals to gather to-gether in one place and to participate in a single social
scene. If so, the more egalitarian bonobos would also
perform similar gatherings in some social scenes;
however, they do not seem to do this at least in grooming.
The other possibility is related to their cognitive
ability. Social behavior would require a more complex
cognitive process than the behavior with inanimate
objects, because the actor of the behavior has to coordinate its behavior with the response from the partner.
In this sense, it is more difficult for an individual tointeract with multiple individuals at the same time thanwith only one individual, and it is more difficult to play
two roles at the same time than one. Thus, it is likelythat at the preliminary stage of the evolution of sociality, individuals would have interacted with only one
partner at a time, even in groups of multiple members.
At the next stage, they would have started to interact
with two individuals duplicating the single interaction.
Then, they would have begun to interact with moreindividuals, sometimes performing two different roles
simultaneously. Investigating multiple social interactions in other great apes would test this hypothesis.
The previously mentioned three characteristics arealso common to humans, and we also form gatheringsof social interactions (especially in conversation),where we interact with multiple individuals at the sametime. In humans, mass (and often anonymous) individuals can be the targets of a behavior. The results ofchimpanzees do not directly lead to understanding of
the human gatherings, but it is possible that multiplying
or overlapping of social interactions may serve toenlarge society and make it more complex in both species.
Grooming is not the only social interaction inchimpanzees, and the current result is only a small step
toward connecting social behaviors to a society. However, the importance of overlaps of interactions forunderstanding sociality should be emphasized, because
an overlap of interactions cannot be attributed to a single individual, but rather exists among individuals: one
individual can groom another or perhaps can even demand to be groomed by another, but he cannot make‘gatherings’ only through his own effort. A ‘society’cannot simply be reduced to a behavior of a single individual or a relationship between individuals. Such an
emergent characteristic of sociality might have evolved
by expanding overlaps of interactions.

5.Summary
1. In chimpanzee grooming, 27 types of grooming
cliques were observed and the largest one consisted ofseven individuals.
2. The proportion of multi-interaction-grooming cliques
was more than 25% in chimpanzees, whereas it wasless than 10% in other primate species.
3. Overlap of grooming interactions (e.g. grooming andbeing groomed simultaneously or being groomed bymultiple individuals) accounted for about 20% of individual grooming. The maximum number of interactions
for one individual at one time was four.

4. Grooming clusters of two to 23 individuals wereobserved. Although the clusters of two individuals weremost frequent as events, clusters of three or moremembers accounted for about 70% of total duration.
5. Adult males groom longer than any other age–sex
classes in relatively small (size = 2–4) clusters, whereas
the duration of female grooming was as long as that ofmales in clusters with more than five individuals.
6. The overlap of interactions is important for understanding the evolution of sociality.

Acknowledgements
Thanks are due to Tanzania Commission for Scienceand Technology, Tanzania Wildlife Research Institute,Tanzania National Parks, Mahale Mountains Wildlife
Research Centre, and Mahale Mountains National Park,for granting permission to conduct the research. I am
also grateful to K. Kawanaka, S. Uehara, W.C.
McGrew, L.F. Marchant, H. Sasaki, R. Kitopeni, M.B.
Kasagula, M. Matumula, and K. Athumani for extending cooperation in the field and to F. Fukuda, T.
Nemoto, and H.Y. Kayumbo for the help rendered invarious ways in Tanzania. Thanks are also owed to T.
Nishida, T. Kano, S. Uehara, J. Yamagiwa, S. Suzuki,
and three anonymous referees for critical comments
and useful suggestions on an earlier version of themanuscript. The fieldwork and data analysis were supported by research grants from the Japanese Ministry of
Education, Science, Sports, and Culture (# 07041138 toT. Nishida and # 12740482 to M.N., respectively).

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Table 1 Hours of observation and grooming of focal individuals
Names

Sex

KalundeNsaba
FananaHanby
Dogurac
Bonobo
Alofu
CarterSinsi
Darwin

m
m
m
m
m
m
m
m
m
m

Gwekulo
FatumaIkochaNkonboPinky
Christina
Abi
Serena

f
f
f
f
f
f
f
f

Year of
birth
1963b
1973
1978b
1980
1981b
1981
1982
1985
1985b
1988
1962b
1963b
1965b
1970b
1972b
1975b
1982
1987

Maggy
Ai

f
f

1987
1988

Rank/status
2
1
3
5
4
8
5
Adolescent
Adolescent
Juvenile
Cycling
Lactating
Lactating
Cycling
Cycling
Lactating
Cycling
Cycling (adolescent)
Adolescent
Juvenile

Kinmo, ys, ys
mo, ybysso, daso, dasosomo, ysob
mo, ob, ys

Hours of total
observation
29.6
15.7
30.9
20.7
31.1
13.7
25.5
24.6
13.8
33.2

Hours ofgroominga
9.5
3.6
4.8
3.1
5.6
0.3
0.4
0.7
1.0
2.8

Groom/
observation (%)
32.0
23.0
15.6
14.7
17.9
2.5
1.5
2.9
7.3
8.3

25.8
22.8
26.2
28.0
26.8
23.2
23.8
21.3

5.5
4.3
4.2
3.3
2.9
3.3
4.2
1.9

21.2
18.9
15.9
11.8
10.9
14.0
17.8
8.8

22.4
21.0

0.6
1.8

2.7
8.5

mo, mother; ob, older brother; yb, younger brother; ys, younger sister; so, son; da, daughter.
a

Only grooming during their focal follow is shown here.
b

Estimated.
c

Although Dogura was still 15 years old, he was fourth-ranking (third-ranking in 1997), overtaking some older
males, and was the most important coalition partner for the beta male. Therefore, I treated the age as adult.

Table 2 Frequencies of mutual and polyadic grooming among various species of primatesSpecies

Clique [2](A) Primate species with no quantitave data
Ateles geoffroyi‘Never’Cebus apella–
Cercopithecus mitis ‘Never’
Macaca fuscata–
Macaca mulatta+–
Macaca nemestrina ‘Occasional’
Macaca radiata‘Occasional’–
Semnopithecus‘Rare’
entellus–
Trachypithecus–
cristatus
Pan paniscus–‘Rare’(B) Primate species with quantitave data
Macaca assamensis 0.8%(45/5397)
Macaca mulatta–
Homo sapiens (/Gui–bushman)
Pan troglodytes
10.4%(10.7/103.4)

Clique [3]Clique [4]Clique [5]–[27]

–
–
–
+
–
‘Sometimes’–––‘Sometimes’‘Often’‘Very rare’
+
–
+
–
–
+
‘Common’a

–
–
–
–
–
+
–
–
+

–

+
‘At times’++

Observation
units

Ahumada, 1992
Parr et al., 1997
Rowell et al., 1991
Furuya, 1957
Boccia, 1983
Sade, 1965
Kaufman and Rosenblum, 1966
Kaufman and Rosenblum, 1966
Simonds, 1965
Borries et al., 1994–

McKenna, 1978
Bernstein, 1968+a+‘Sometimes’
+

–
+

–
1.1%(17/1488)
0.5%(1/184)

7.1%(385/5397)
3.9%(58/1488)
7.1%(13/184)

0.1%(5/5397)b
0.1%(2/1488)d7.5%(7.7/103.4)

4.0%(4.1/103.4)

4.5%(4.7/103.4)–

SourceKuroda, 1980
Kano, 1998
Episodesc
Episodesc
Interactions

Cooper and Bernstein, 2000
Matheson and Bernstein, 2000
Sugawara, 1984

Hours

This study

For clique types, see Fig. 1. Single quotation marks indicate authors’ description of frequencies without data; +, description without any information on frequency; –, no description
in the article.
aTriadic grooming but no information on types.
bClique [10].
cAlthough these ‘episodes’ are not defined in the original articles, if the average duration of multi-interactional episodes was shorter than that of single-interactional episodes (which
is very likely), then the proportion with episodes shown here would be overestimated. Thus, if we could use duration as a common measure, this tendency (moremulti-interactional-cliques in chimpanzees than in other species) would likely be emphasized.
dCliques [9] and [10].

Fig. 1. Types of grooming cliques observed in the study. Circles in the figure indicate individuals, and arrows
indicate the flows of grooming interactions.

Fig. 2. Proportion of relative duration of each grooming clique type observed during study (for grooming
clique types, see Fig. 1).

Fig. 3. Age–sex differences in proportion of single/multiple interactions. Bars indicate mean relative durationof individuals in each age–sex class, where partner(s) of the interaction(s) can be individual(s) of any age–sex
class. Number of individuals in each class is shown in parentheses. Error bars represent standard deviations.

Fig. 4. Proportion of each size of grooming cluster with the number of events (left) and relative duration(right).

Fig. 5. Proportion of multi-interaction-cliques in different sizes of grooming clusters. Total does not reach
100% because single-interaction-cliques (namely clique [1] in Fig. 1) are not shown in this figure.
*Corresponds to cliques [3]–[27] in Fig. 1.
‡Corresponds to clique [2] in Fig. 1.

Fig. 6. Age–sex differences in grooming in different sizes of grooming clusters. Although data-points in the
figure represent mean total grooming duration of each age–sex class, individual data were used for ANOVA
test (Number of individuals in each class is indicated in the figure). Adolescents and youngsters in Fig. 3 werecombined into ‘others’ as they showed similar tendencies. Only data on giving grooming are shown here (dataon receiving grooming showed similar tendencies).