Proc. R. Soc. B (2007) 274, 367–372
doi:10.1098/rspb.2006.3733
Published online 31 October 2006

Spread of arbitrary conventions among
chimpanzees: a controlled experiment
Kristin E. Bonnie1,2,*, Victoria Horner1,3, Andrew Whiten3
and Frans B. M. de Waal1,21Living Links, Yerkes National Primate Research Center, Atlanta, GA 30329, USA
2
Department of Psychology, Emory University, Atlanta, GA 30322, USA
3
Centre for Social Learning and Cognitive Evolution, School of Psychology,
University of St Andrews, Fife KY16 9JP, UK

Wild chimpanzees (Pan troglodytes) have a rich cultural repertoire—traditions common in somecommunities are not present in others. The majority of reports describe functional, material traditions,such as tool use. Arbitrary conventions have received far less attention. In the same way that observationsof material culture in wild apes led to experiments to confirm social transmission and identify underlying
learning mechanisms, experiments investigating how arbitrary habits or conventions arise and spreadwithin a group are also required. The few relevant experimental studies reported thus far have relied oncross-species (i.e. human–ape) interaction offering limited ecological validity, and no study has successfully
generated a tradition not involving tool use in an established group. We seeded one of two rewarded
alternative endpoints to a complex sequence of behaviour in each of two chimpanzee groups. Eachsequence spread in the group in which it was seeded, with many individuals unambiguously adopting thesequence demonstrated by a group member. In one group, the alternative sequence was discovered by a
low ranking female, but was not learned by others. Since the action-sequences lacked meaning before theexperiment and had no logical connection with reward, chimpanzees must have extracted both the formand benefits of these sequences through observation of others.
Keywords: social learning; tradition; culture; primates

1. INTRODUCTION
The social transmission of interaction patterns not
involving a tool or foraging technique among chimpanzees(Pan troglodytes) was first documented by McGrew & Tutin(1978), who observed a customary hand-clasp grooming
posture in one wild community that was absent in another160 km away. When the same grooming posture originatedspontaneously in a captive chimpanzee group, it was foundto spread gradually along affiliative lines, consistent withtransmission through social learning (de Waal & Seres 1997;
Bonnie & de Waal 2006).
Social learning may also explain why communication
signals appear to carry different meanings in different
communities. We now know that variation in handclasp
grooming goes beyond the mere presence or absence of thisposture: neighbouring communities sometimes vary in thedetails of the posture (Nakamura 2002). Chimpanzees arealso known to orally tear apart dry leaves, a noise-making
action that serves as a solicitation for play in one community
yet as sexual courtship in another (Nishida 1980; Boesch
1996). The actions themselves are arbitrary; the significanceof the behaviour is defined only by the individuals within thegroup to create a convention. In each community, members
know the signal’s local meaning, reacting to it in a unique but
predictable manner. Tool use traditions are quite different,

because they are characterized by a functional, goal-oriented
task with foreseeable outcomes; for example, nuts must be
cracked to be eaten.
Compared to studies of tool use and foraging traditions,arbitrary conventions such as leaf clipping have received far
less attention (de Waal 2001) despite growing evidence fornon-subsistence behavioural variants in both monkeys andapes (Huffman 1984; de Waal 1989; Hirata et al. 1998;
Nakamura et al. 2000; Whiten et al. 2001; Perry et al.
2003a,b). Sapolsky & Share (2004) even report an entire
pacific social culture in wild baboons (Papio anubis). In thesame way that observations of material culture in wild apes
(e.g. Whiten et al. 1999; van Schaik et al. 2003; Lonsdorf
et al. 2004) led to experimental approaches in captivity toconfirm the putative social transmission and identify
underlying learning mechanisms (e.g. Whiten et al. 1996;
Hirata & Morimura 2000; Celli et al. 2001; Horner &Whiten 2005; Whiten et al. 2005; Horner et al. 2006),
experiments are also required to determine how habits,
arbitrary conventions or signals develop and are learned
within a group. Thus far, such experiments have included
the transmission of conflict resolution (de Waal &Johanowicz 1993) as well as human-induced mimickingof arbitrary gestures, body movements and actions
on objects (Tomasello et al. 1993; Custance et al. 1995;
Myowa-Yamakoshi & Matsuzawa 1999; Call 2001;
Bjorklund et al. 2002). However, all of these studies haverelied on cross-species interaction, making for limited
relevance to conventions within primate communities.

* Author for correspondence (kebonni@emory.edu).
Electronic supplementary material is available at http://dx.doi.org/10.
1098/rspb.2006.3733 or via http://www.journals.royalsoc.ac.uk.
Received 11 July 2006
Accepted 8 September 2006

367

This journal is q 2006 The Royal Society368 K. E. Bonnie et al. Spread of conventions among chimpanzees
In the only previous study of its kind, when one
chimpanzee was trained to employ arbitrary gestures togain food from a human, the behaviour failed to spread toother group members (Tomasello et al. 1997). This was a
relatively short, single attempt, but the authors took their
negative finding to mean that non-human species
probably lack the capacity to observationally learn thesignificance of arbitrary actions, a capacity considered
fundamental to human culture.
Here, we describe a new approach that demonstrates
this capacity experimentally in chimpanzees. We achieved
this by seeding in two ape groups two different
conventions concerning a sequence of actions using
exactly the same objects, making any spread of suchactions clear and unambiguous. The alternative sequences
involved collecting, transporting and depositing plastic
tokens into either a bucket receptacle or a pipe receptaclein order to gain a food reward from a separate, unrelated
location. Since the objects involved in the action sequence(tokens and receptacles) had no logical, causal connection
to a food reward, this experiment addresses the question ofwhether chimpanzees can socially learn arbitrary, non-toolconventions. Furthermore, including two alternative
endpoints (bucket versus pipe) to the same procedure
within a group diffusion paradigm (Whiten et al. 2005)
allowed us to control for individual learning, because bothgroups had the opportunity to discover both action
sequences and would be duly rewarded for either. Anybehavioural differences could thus be ascribed only to
ape-to-ape transmission.

2. MATERIAL AND METHODS(a) Subjects
Subjects were two groups of chimpanzees (FS1 and FS2)
housed at the field station of the Yerkes National Primate
Research Center, Atlanta, Georgia. The FS1 group comprised 15 individuals (two adult males), aged from 8 to 41
years. The FS2 group comprised 14 individuals (three adultmales), aged from 7 to 38 years. Additional information about
the subjects in each group can be found in electronic
supplementary material. Each group has access to indoor
areas and large outdoor compounds (700 and 520 m2,
respectively). Subjects have ad libitum access to chow andwater, and are provided with fresh fruits and vegetables
twice daily.
A pilot investigation conducted 1 year prior to the present
study showed that FS1 chimpanzees could learn to throw aplastic toy in the air, once the significance of this arbitrary
behaviour was demonstrated by a juvenile. The present study
employed a more systematic methodology and used highranking adult females as models. These models were chosen
based on their willingness to cooperate with experimentersand participation as models in a previous study (Whiten
et al. 2005).
(b) Materials
We provided each group with ‘tokens’ constructed from PVC
tube (lengthZ10 cm; diameterZ4 cm) painted orange.
Approximately 25 tokens were available to the chimpanzees
for each session.
Two receptacles, a ‘bucket’ and ‘pipe’, were available toboth groups throughout testing. The bucket (20!34!
61 cm) was a plastic bin modified for the testing purposes.
Proc. R. Soc. B (2007)

A diamond shaped hole (7.5!7.5 cm) was cut into the side of
the bucket at a height of 45 cm. The bucket’s lid and an
opaque barrier inside the bucket (added during the baseline
phase) prevented subjects from seeing any objects within. The
bucket sat on the ground and was secured to the mesh wall of
the subjects’ outdoor compound. The pipe was a larger PVC
pipe (lengthZ85 cm; diameterZ7.5 cm) angled downward
from 90 cm high into a covered opaque container. The pipe
was secured to the mesh, approximately 60 cm from thebucket. Both receptacles were available to subjects only
during the testing (see figure 1 for an overview of theexperimental set-up).
(c) Procedure
Testing involved four phases—habituation, baseline, modeltraining and transmission. With the exception of the training
phase in which the model was temporarily isolated from hergroup, all subjects could freely access both the indoor andoutdoor areas of their enclosures. To avoid biasing theattention of subjects towards the apparatus, the experimenter
observed the group from an observation tower at one cornerof the compound (figure 1).
(i) Habituation
Chimpanzees often show neophobic responses to novel
objects, so subjects were exposed to the apparatus prior tostarting the experiment. PVC tubes are commonly given as
enrichment, so only the bucket and the pipe were included inthis phase. For 72 consecutive hours, the receptacles weresecured to the mesh and the chimpanzees allowed to interact
with them freely. Chimpanzees exhibited no adverse
responses to the apparatus during informal observations
conducted in this phase.
(ii) Baseline phase
Baseline sessions were conducted to check if chimpanzees
would spontaneously deposit tokens in the receptacles prior
to observing a model demonstrate the task, and to gather
baseline data on the chimpanzees’ interactions with thetokens and apparatus. Six sessions were conducted in eachsocial group.
Just prior to each session, the bucket and pipe weresecured to the mesh of the compound. Each session lasted for
30 min and began when tokens were thrown from theobservation tower into the compound. At the end of thesession, the receptacles were removed. We made no attempt
to collect tokens not deposited in a receptacle, but many werecollected by care-staff during regular husbandry procedures,and put aside until the next session took place. Collected
tokens were thrown back into the compound at the start ofeach session, ensuring that 25 tokens were always available.
(iii) Model training
A high-ranking adult female from each group was selected asa model. Models were rewarded for depositing tokens into
one of the receptacles (FS1: GG—bucket, FS2: ER—pipe).
Training took place in the indoor area, where the model could
be temporarily isolated from her group. Training lasted
several days and continued until models would walkapproximately 10 m to find tokens and deposit them in only
one receptacle when both were available to them.
To check that the model would continue to perform thetrained behaviour while in the compound, we released her
while the other chimpanzees in her group were kept indoors.

Spread of conventions among chimpanzees K. E. Bonnie et al. 369

Figure 1. Illustration of the experimental set-up in the chimpanzee compounds at the field station of the Yerkes National Primate
Research Center. Chimpanzees are rewarded from the observation tower for depositing tokens into either of two receptacles.
This proved to be an uneasy situation for both the models, so
we allowed one additional individual (GG’s daughter LZ in
FS1 and ER’s daughter JA in FS2) outdoors. LZ and JA were
distracted by a second experimenter and did not interact withthe apparatus during this phase. Within two training sessionsof this type, both GG and ER deposited multiple tokens as
trained.
(iv) Transmission phase
In the transmission phase (20 sessions), subjects were able toobserve the model being rewarded for depositing tokens intoa receptacle. All subjects, including the model, were rewardedfor depositing tokens in either receptacle, such that bothalternatives were equally profitable. As with the baseline
phase, each session lasted 30 min (except for the first session,
which was 60 min) and began when tokens were thrown by anexperimenter into the outdoor compound.
To obtain a reward, chimpanzees needed to complete thefollowing behavioural sequence: (i) look for a token similar tothat handled by the model (i.e. requiring generalization fromthe original token to all tokens), (ii) collect the token, (iii)transport the token towards the containers (usually 10 m ormore), (iv) deposit the token into a container, and (v) turnand look up at the tower to catch the reward, which theexperimenter would throw into the actor’s hands or close by.
Whenever a token was deposited, one-sixth apple or onequarter banana was thrown to the appropriate chimpanzee. If
the chimpanzee was not able to keep the reward (as a result ofa poor throw or theft by another chimpanzee), this was noted,
Proc. R. Soc. B (2007)

and a second reward was thrown. At the end of each session,
the receptacles were removed.
After 12 sessions per group, it became apparent that two
individuals in each group, the original models plus two others(PE and ST), were able to monopolize the majority of thetokens or the receptacles. It seemed that these two individuals
might be preventing others from collecting and depositing
tokens. For the next three sessions (13–15), these individuals(and PE’s dependant daughter AZ) were kept indoors andcould not participate. This manipulation did not leave eithergroup without a model, as by this point at least three otherindividuals had learned the task. During sessions 16–18, allsubjects were able to participate. For sessions 19 and 20,
these same individuals plus two others in each group(LZ, SK, KE and AM) were again kept indoors.
(d) Data collection and analyses
Scan sampling was used to record the location of each subject(indoors or outdoors) and token handling at 3 min intervals
throughout sessions. In addition, all token deposits wererecorded. Data were collected via an audiocassette recorderby an experimenter located in the observation tower at eachcompound. A video camera located on the ground recorded
activity around the receptacles, and was used to confirm theoccurrence of token deposits and the identity of thechimpanzee involved. The video thus ensured reliability indata collection.
We used non-parametric statistics to compare the twogroups in terms of the total of bucket and pipe users in each,using a two-tailed Fisher’s exact test.

370 K. E. Bonnie et al. Spread of conventions among chimpanzees
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individual(a) MS
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Figure 2. Differential spread of the bucket and pipe conventions in two groups of chimpanzees. (a) FS1 group and (b) FS2 group.
Grey circles indicateR1 deposit to bucket and black circles indicateR1 deposit to pipe. Numbers within the circle indicate tokens
deposited. Horizontal bars indicate that no deposit was made during that session; X indicates that the individual was kept indoors.
Individuals are listed, from bottom to top, in the order of first deposit. Individuals GG and ER were the trained models.

3. RESULTS(a) Baseline phase
Prior to training a model, we conducted a baseline phase inwhich both the groups were provided with approximately
25 tokens and both receptacles, and their activities were thenmonitored over six sessions for a total of 3 h. No rewards
were given during this phase. Every 3 min, the locationof each individual was recorded, along with any occurrencesof token interactions (pick-up, deposit, etc.). Thirteen out of
15 individuals in the FS1 group and 10 out of 14 individualin the FS2 group were observed interacting with a token atsome point, but the overall level of token interactions was
low and declined across sessions.
During the first baseline session in the FS2 group, KE, ayoung adult female, deposited a token into the bucket. Later,
during the same session, VR and DA also deposited tokens
into the bucket after first looking inside, hence seeing theoriginal token deposited by KE. An opaque barrier was thenadded to the receptacles so that tokens disappeared fromview as soon as they were deposited. In subsequent baseline
sessions, we observed no further interest in the receptacles,
and no further depositions of tokens.
Data from the baseline phase provided evidence that,with the exception of KE, VR and DA, the tokens andreceptacles were not combined in a meaningful way. Thus,
we proceeded to determine if, after observing a group
Proc. R. Soc. B (2007)

member obtain a reward by depositing a token into one of
the receptacles, others would learn to do so. Because KE,
VR and DA deposited tokens in the bucket, the model in
their group was trained to use the alternative sequence;
deposit tokens in the pipe.
(b) Transmission phase
In 20 sessions (a total of 10.5 h), nine chimpanzees in the
FS1 group and six in the FS2 group deposited at least one
token in a receptacle and were rewarded for doing so(figure 2). Both models deposited tokens only in the
trained receptacle. In the FS1 group, tokens were
deposited exclusively in the bucket, the receptacle usedby the model in this group, yet never observed duringbaseline. In the FS2 group, all but one chimpanzee (DA)deposited tokens in the pipe (figure 2; Fisher’s exact testcomparing both groups, two-tailed, pZ0.002). Of thoseindividuals not observed to deposit a token in either the
bucket or the pipe (5 in FS1 and 7 in FS2), all but two had
been seen holding a token at least once during thetransmission phase. Moreover, failure to learn the task
was not owing to lack of opportunity for observation, as
there were no significant differences in either group in the
proportion of total scan samples in which individuals wereobserved outdoors between token users and non-learners(two-tailed Mann–Whitney U-test, NS).

Spread of conventions among chimpanzees K. E. Bonnie et al. 371
4. DISCUSSION
This study demonstrates that chimpanzees are capable ofduplicating a modelled sequence of initially arbitrary
actions with familiar objects with sufficient fidelity tosustain different conventions. The two-option design ofthe experiment allowed us to control for individual
learning, because both groups had the opportunity tolearn both sequences and were rewarded for performing
either. This design makes it unlikely that all individuals ina single group would discover one receptacle, but not theother. Moreover, data collected during the baseline phaseshow that chimpanzees were unlikely to develop the
differences in behaviour observed between the two groups
through individual experience. Any behaviouraldifferences can thus be ascribed to ape-to-ape transmission, probably based on an evaluation of the significance of the sequence of actions by successful
performers, generalization from tokens used by them tosimilar tokens, and stimulus enhancement, such as being
drawn to the receptacle favoured by others.
Our results lend support to the idea that thesignificance of certain object-directed behaviours in wild
chimpanzees is socially learned (Boesch 1996, 2003). The
result contrasts with an earlier failure of chimpanzees tolearn arbitrary gestures from each other (Tomasello et al.
1997). Differences in experimental design between thepresent study and that by Tomasello et al., including the
behaviours rewarded and the location of the experimenter
during data collection, may have contributed to thecontrasting findings.
Our study further shows unprecedented fidelity inexperimentally seeded alternative traditions in a nonhuman primate. Although one female in FS2 (DA)consistently deposited tokens in the ‘wrong’ receptacle,
the bucket, and was rewarded for doing so, no othermember of her group adopted this particular sequence.
This provides further evidence consistent with theconformity bias documented among the same chimpanzees in a previous study on foraging techniques (Whiten
et al. 2005). However, it is also consistent with theories
that high-status individuals are preferred as a source of
information (Henrich & Gil-White 2001; Laland 2004)
because DA is among the lowest ranking females in FS2,
whereas the group’s model, ER, is its alpha female.
Although the majority of individuals in each groupinteracted with tokens during the baseline phase, and werein a position to observe one or more group members being
rewarded for depositing a token in a receptacle during thetransmission phase, a number of individuals (nZ5 and 7
in FS1 and FS2, respectively) did not learn either
tradition. It is possible that social factors, such as
dominance, may explain these differences. Future research
could be directed at the social factors, which support orinhibit acquisition of traditions within a group.
This project was conducted at the Yerkes National Primate
Research Center’s field station, supported by the National
Institutes of Health (RR-00165) and the Living Links Centerof Emory University. Additional support was provided by the
National Science Foundation (K.B.), BBSRC (A.W., V.H.)
and the Leverhulme Trust (A.W.). We are grateful to Devyn
Carter, Marietta Dindo and Deborah Moore for logisticalsupport, and to Anna Garr and Jennifer Rybak forcontributions to the preliminary study. Two anonymous
reviewers provided helpful comments on a previous draft.
Proc. R. Soc. B (2007)

Figure 1 was drawn by Devyn Carter. Yerkes is fully
accredited by the American Association for Accreditation
for Laboratory Animal Care.

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