How do gibbons solve social dilemmas?

Abstract

Social primates face conflicts of interest with other partners when their individual and collective interests collide. Despite living in small, primarily dyadic, groups compared to other social primates, gibbons are not exempt from these conflicts in their everyday lives. In the current task, we asked whether pairs of gibbons would solve a conflict of interest over food rewards. We presented pairs of gibbons with a situation in which one pair member, the actor, could release food rewards at a distance, giving the passive partner a chance to take an advantageous position to obtain the rewards. Gibbons participated in three conditions: A No Food control, an Altruistic situation in which the actor could not obtain a direct reward from the cooperative act and a Test condition in which the actor could secure a small fraction of the total rewards. We found that gibbons acted more often in the two conditions involving food rewards, and waited longer when no direct rewards were available for the actor, thus suggesting that they understood the mechanism and that they faced a social trade-off between making the rewards available and waiting for each other to act. However, we found that in a majority of pairs, acting individuals benefitted more than the passive partners in both altruistic and test conditions. Furthermore, in some occasions actors actively refused to approach the location where the food was released. These results suggest that while gibbons strategize to solve the social dilemmas, they often allowed their partners to obtain better rewards. Our results highlight the importance of social tolerance and motivation as drivers promoting cooperation in these pair-living species.

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How do gibbons solve social dilemmas?
Alejandro Sanchez-Amaro (  a5sanchezamaro@ucsd.edu )
University of California, San Diego
Robert Ball
Hunter College
Federico Rossano
University of California, San Diego
Research Article
Keywords: Gibbons, social dilemma, conict of interest, tolerance, cooperation
DOI: https://doi.org/10.21203/rs.3.rs-124133/v1
License:   This work is licensed under a Creative Commons Attribution 4.0 International License. 
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Abstract
Social primates face conicts of interest with other partners when their individual and collective interests
collide. Despite living in small, primarily dyadic, groups compared to other social primates, gibbons are
not exempt from these conicts in their everyday lives. In the current task, we asked whether pairs of
gibbons would solve a conict of interest over food rewards. We presented pairs of gibbons with a
situation in which one pair member, the actor, could release food rewards at a distance, giving the passive
partner a chance to take an advantageous position to obtain the rewards. Gibbons participated in three
conditions: A No Food control, an Altruistic situation in which the actor could not obtain a direct reward
from the cooperative act and a Test condition in which the actor could secure a small fraction of the total
rewards. We found that gibbons acted more often in the two conditions involving food rewards, and
waited longer when no direct rewards were available for the actor, thus suggesting that they understood
the mechanism and that they faced a social trade-off between making the rewards available and waiting
for each other to act. However, we found that in a majority of pairs, acting individuals benetted more
than the passive partners in both altruistic and test conditions. Furthermore, in some occasions actors
actively refused to approach the location where the food was released. These results suggest that while
gibbons strategize to solve the social dilemmas, they often allowed their partners to obtain better
rewards. Our results highlight the importance of social tolerance and motivation as drivers promoting
cooperation in these pair-living species.
Introduction
Social animals constantly face conicts of interest with other group members. In primates, decisions
about different travel routes, group defense, or access to limited resources may result in divergent
preferences for individuals and hence potential conicts between group members 1–6. To resolve these
conicts, primates may engage in different sets of strategies including dominance, mutual cooperation or
majority rules. For instance, Lar gibbon females(
Hylobates lar
) lead travel routes and access high value
rewards before males despite the lack of a clear dominance of one sex over the other 7. In olive baboons
(
Papio anubis
) instead, travel routes seem to be driven by majority rule despite their highly hierarchical
social system 5. However, while these observations allow us to understand different aspects of primate
behavior, it is often dicult to dissect the factors contributing to specic behavioral patterns in nature.
Accordingly, experimentally controlled studies with primates can shed crucial light on the decision-
making strategies underlying the observed behavior.
To that end, over the last years numerous studies have adapted game theory models to explore the
strategies that different primates use to overcome social dilemmas in which their interests come into
conict 8–11. That is, the free rider benets the most from the interaction. For instance, computerized
tasks have presented primate species including chimpanzees, capuchin monkeys and rhesus macaques
with different economic games borrowed from the game theory literature 12–14. These studies have found
that, in general, primates can converge to a Nash Equilibrium (i.e. the optimal outcome from an

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interaction given the strategy of your partner) during coordination and conict games between pairs.
Using a different approach, other researchers have presented great apes, mostly chimpanzees (
Pan
troglodytes
), with non-computerized social dilemmas in which apes had to decide between different
physical actions to obtain rewards from an apparatus. In general, these studies have found that
chimpanzees and bonobos develop strategies to outcompete their partners and obtain the most from the
social dilemma, either through monopolization of rewards 9, by waiting for their partner to act before
them 8,10,15 or by inuencing them to change their strategy 11 .
The use of game theory models to explore how different primate species coordinate actions for mutual
goals, as well as how they overcome conicts of interest, is a growing eld in comparative psychology 14.
For example, a very recent study presented squirrel monkeys, a primate species that rarely cooperates in
nature, with a set of computerized social dilemmas previously presented to more cooperative species
such as the capuchin monkeys (
Cebus apella
). Vale and colleagues 16 found that squirrel monkeys
(
Saimiri boliviensis
) behaved similar to capuchin monkeys in cooperative scenarios such as the stag hunt
game. In another recent study, Sánchez-Amaro and colleagues (under review) presented for the rst time
pairs of common marmosets (
Callithrix jacchus
) with a social dilemma modeled after the snowdrift
game 10,17. In this study, marmosets could access an unequal reward distribution in the form of a rotating
tray. In the social dilemma condition, the preferred reward could only be obtained by waiting for the
partner to act, with the risk that if none of the two accessed the tray they would both lose the rewards.
The authors explored whether cooperative breeding marmosets would engage in more cooperative
strategies due to their natural tendency to act proactively toward others in different contexts including
food sharing. They found similarities between marmosets and great apes’ strategies to maximize
benets (e.g. waiting for the partner to act before them). They also found sex differences between
females and males’ strategies, where the former was more willing to forego a cooperative act and
maximize rewards. The results t the natural history of this species in which males usually donate food
to females in food sharing tasks. However, we still know very little about the socio-cognitive strategies
that other primate species may develop to deal with similar conicts of interest.
Perhaps surprisingly, one of the primate families we know less about in terms of their socio-cognitive
abilities are the gibbons (family
Hylobatidae
). These small apes are key species in the sense that they are
closely related to both old world monkeys and great apes 18. Furthermore, unlike any other ape species,
gibbons primarily live in small groups mainly composed of a bonded breeding couple and their kin 19.
Thus, the study of their socio-cognitive abilities is crucial to understand whether some cognitive traits are
shared by common descent in all apes or have instead arise through convergent evolution in distinct
primate species. Up to date, the study of gibbons has mainly focused on elucidating aspects of their
biology, ecology and phylogeny 20,21, with little work assessing their socio-cognitive abilities in
experimentally controlled settings. For instance, in a recent literature review on primate cognitive studies
published by the Manyprimates initiative 22, it was found that gibbons only appeared in 2 of the 574
studies surveyed between 2014 and 2019.

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Gibbons may have been excluded from cognitive studies due to diculties securing a sucient sample
size to conduct experimental studies or due to their limited motivation to participate in cognitive tasks 23–
25. While limited sample size is a problem that a majority of comparative psychologists and
primatologists face when studying primate behavior, a lack of motivation is often the product of
experimental designs and methods not suited to the biology of the species. In the case of gibbons, early
work by Beck 26 already showed, for example, that adaptations to the way gibbons could interact with an
apparatus (e.g. lifting the access to the ropes instead of leaving them on the ground on a at surface)
improved their participation and performance when compared to previous studies 27,28.
Furthermore, as Liebal emphasizes 25, it has been sometimes assumed that gibbons’ are less interesting
than other primate species due to their relatively simpler social system based on pair-living, which
presupposes low socio-cognitive abilities in relation to other primates. However, mounting evidence over
the last 20years has challenged the assumption that gibbons are truly monogamous 19,20,29−31. Some
gibbon species have been found to engage in extra-pair copulations 32 and a number of studies have
reported different group structures in addition to pair-living 31,33−36. Furthermore, the fact that gibbons live
in reduced groups does not necessarily presuppose a lack of social complexity. According to Freeberg 37,
pair-bonded individuals would form more complex and intense relationships than those living in large
polygamous groups—possibly because they are more interdependent. In other words, social complexity is
not only a matter of group size but of relation quality. It is thus possible that gibbons would engage in
more prosocial strategies between closely bonded partners when conicts of interest take place.
Nevertheless, despite the lack of studies on gibbons’ socio-cognitive abilities in relation to other primate
species 22, researchers have made signicant advancements on this area. For instance, it has been
investigated whether gibbons recognize themselves in the mirror (see 25 for review) or whether they are
able to follow others gaze to discover an unseen object 38–42. In the case of gaze-following studies,
researchers have found that gibbons are able to shift their gaze in response to a previous experimenter
gaze shift but it remains unclear whether gibbons are taking the perspective of the experimenter into
account, including her mental states. However, a recent study found that when gibbons were presented
with a competitive scenario in which they could only retrieve uncontested rewards—when the
experimenter did not orient his body, head or eyes towards the rewards, gibbons avoided the contested
table by paying attention to the orientation of the body and the head of the experimenter but not to his
eyes 43. This later result suggests that, in line with previous socio-cognitive studies in other primate
species 44–46, gibbons may perform better in competitive settings compared to neutral ones. However, in
previous gaze-following studies the interaction only occurred between the human experimenter and the
ape and not between conspecics. Considering the competitive task presented by Sánchez-Amaro and
colleagues 43 as an example, the experimenter and the gibbon faced a conict of interest every time the
gibbon approached the contested table since the experimenter and the gibbon competed for the same
food reward. Therefore, one open question is how pairs of gibbons would solve conicts of interest in a

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more naturalistic context. That is, when they need to deal with other conspecics over access to
resources such as food rewards.
To answer this question, we presented pairs of gibbons with a simplied version of a social dilemma
resembling a snowdrift game 17. This is the rst time a game theory model has been implemented to
shed light on the nature of gibbons’ socio-cognitive abilities. In this situation, one pair member should
volunteer to provide a common good that becomes accessible for both of them. The dilemma occurs
when the passive individual takes advantage of her position. In other words, the costs to volunteer may
hinder the actor’s chances to benet herself in relation to the passive partner. In our task, one gibbon
could pull from a handle attached to a rope. By pulling the handle, the rope would lift a release
mechanism and rewards would fall at a distance to the actor, giving the passive individual the chance to
position herself in front of the released rewards and hence benet from the actors’ action (although the
actor could potentially benet from those rewards as well).
Pairs of gibbons were presented with three conditions varying in the number of available rewards: a Test
condition in which the actor could obtain one reward attached to the handle while releasing ve rewards
at a distance from herself; an Altruistic condition in which the actor did not obtain any reward from
pulling the handle but could still release the ve rewards at a distance and a No Food control condition
with no rewards involved.
We expected gibbons to act more often in the Test and the Altruistic conditions compared to the No Food
condition. This would show that gibbons understood the contingencies of the game. Furthermore, given
that gibbons were living with closely bonded partners one possibility is that they would cooperate to solve
the dilemma by sharing the volunteer costs. If that were the case, we would expect pairs of gibbons to
obtain similar amounts of food in both conditions. Furthermore, we would not expect pairs members to
show signicant differences in their rate of participation and in their latencies to release the rewards
between Altruistic and Test conditions.
If, in contrast, gibbons would react competitively as other great ape species did in previous social
dilemmas, we would expect them to try to maximize their food rewards by hesitating to act and by taking
advantage of their passive role—placing themselves in front of the release mechanism. In that sense, we
would expect passive partners to benet from their position and obtain more rewards than the actors.
Furthermore, we would also expect gibbons to be more likely to act in Test trials given that they could
obtain direct benets from their actions.
Results
Rate of participation and latencies
Gibbons decisions to participate (i.e. to manipulate the handle and release the food container) were
affected by the condition presented (GLMM:
χ
2 = 19.8, df = 2,
p
< 0.001, N = 532, Fig.1). Pair-wise
comparisons indicated that gibbons participated signicantly more often in the Test condition compared

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to the Altruistic (GLMM:
χ
2 = 19.8, df = 2,
p
< 0.001, N = 532, CI [4.05, 16.33]) and the No Food control
condition (GLMM:
χ
2 = 19.8, df = 2,
p
< 0.001, N = 532, CI [4.58, 16.76]). Gibbons pulled in all Test trials but
one while in Altruistic and No Food control trials gibbons pulled less frequently (62.5% and 53.1%
respectively). Interestingly though, gibbons pulled signicantly more often in the Altruistic condition
compared to the No Food control condition (GLMM:
χ
2 = 19.8, df = 2,
p
= 0.049, N = 532, CI [-1.02, 0.005]),
suggesting that the potential release of food rewards at a distance also motivated gibbons’ decisions and
that they could distinguish between the three conditions presented.
Next, we found that gibbons’ latencies to pull were also affected by the condition presented (see the
Electronic Supplementary Materials (ESM) Fig. S1). That is, in those trials in which individuals pulled
(71.8% of trials), they clearly acted faster in the Test condition (when one piece of food was directly
accessible from the pipe) compared to the Altruistic condition (coxme, HR = 4.97, p
<
0.001, N = 532, CI
[3.65, 6.11]) and to the No Food condition (coxme, HR = 6.38, p
<
0.001, N = 532, CI [4.6, 7.91]). Despite
gibbons being more likely to participate in Altruistic compared to No Food control trials, their latencies to
act did not statistically differ between those two conditions in those trials in which they accessed the
rope (coxme, HR = 0.78, p
=
0.075, N = 532, CI [0.59, 1.03]).
Strategies by actors and passive partners
We also analyzed whether actors received less rewards than passive partners. That is, whether there was
a conict of interest in place. To answer this question, we analyzed whether a previous volunteering
action would predict the amount of released food that individuals obtained across conditions.
Surprisingly, we found a signicant two-way interaction between condition and the presence/absence of
previous volunteer act suggesting that passive individuals obtained less food than actors and that this
difference was especially salient in Altruistic compared to Test trials (GLMM:
χ
2 = 7.57, df = 2,
p
=
0
.006, N
= 572, CI [-1.64, -0.19], see ESM Fig. S2). Therefore, actors were still able to obtain the majority of the food
released in both conditions, despite the fact that passive individuals could benet from their position in
Test trials compared to Altruistic trials—most likely due to the time that the actors required to obtain the
food from the pipe. In line with this nding, Fig.2 shows the percentage of trials in which individuals
release food against the number of rewards that each individual obtained in Altruistic and Test trials.
Interestingly, in only one pair (Betty and Khim Maung Win) the more active individual obtained less
benets than the passive partner in both conditions.
Next, we evaluated whether passive gibbons (those not releasing the food) acted strategically by placing
themselves in front of the ramp at the moment the actor released the food and whether they would act
more strategically in Test compared to Altruistic trials given that the actor gibbon would spend some time
retrieving the food baited inside the pipe.
There were 287 instances in which the food was released by the actor in the Altruistic and Test
conditions. In 235 (81.9%) of those trials the passive individual was not in front of the ramp by the time
the food was released. The passive individual only placed herself in front of the ramp by the time the
food was released in 52 trials (18.1%) and there were no differences between conditions (GLMM:
χ
2 =

Page 7/17
0.31, df = 1,
p
= 0.58, N = 287). We also explored whether gibbons would take an advantageous position
by the time the actor arrived to the location where the food had been released. That is, whether the
passive individual was there at that time. After releasing the food, the actor arrived to the food location in
237 of those 287 trials (82.6%). Passive individuals were not in front of the ramp in 161 trials (67.9%).
They were in front of the ramp by the moment the actor arrived in 76 occasions (32.1%). We found no
differences between conditions (GLMM:
χ
2 = 0.69, df = 1,
p
= 0.41, N = 237).
We found that actors did not approach the released food in 50 trials in which they released it. A majority
of those trials were Test trials (37 of 50, binomial test p < 0.001). This make sense since actors spent
some time obtaining the food reward placed inside the pipe. Nevertheless, in a great majority of these
trials (44 of 50 trials; 88%) the actors ended up obtaining their food reward from the handle before the
passive individual nished to obtain her last reward. Furthermore, we only found one case in which the
actor showed an intention to approach the partner by getting closer to him.
Social measurements: displacement and cofeeding events
We examined whether displacement and cofeeding events differed between Altruistic and Test
conditions. We only found 13 displacements out of 287 trials (4.5%). In a majority of displacement events
the passive was already in front of the food when the actor arrived (11 of 13) but only in 3 trials the
partner was already there from the moment the food was released. Furthermore, in 10 of 13 trials the
active individual displaced the passive one and 10 of these 13 displacements occurred in test trials.
Finally, we found cofeeding in 44 trials out of 287. A majority of cofeeding events occurred during Test
trials (27 of 44; 61%). However, we found that cofeeding did not signicantly varied between Altruistic
and Test conditions (GLMM:
χ
2 = 1.58, df = 1,
p
= 0.21, N = 287).
Discussion
The results of the study suggest that pairs of gibbons can solve a conict of interest where one pair
member has the opportunity to volunteer to activate a release mechanism containing potential food
rewards for both partners. In line with previous studies with great apes, gibbons avoided mutual defection
in a Test condition where they could obtain a direct reward from their actions 10,47. In contrast, in an
Altruistic condition where gibbons could not obtain direct benets, their likelihood to volunteer and
release potential food rewards dropped signicantly in comparison to the Test condition. In our opinion,
two primary reasons could explain this pattern of results.
First, it is possible that gibbons were only motivated to participate when they could directly benet from
their own actions. However, if that were the case, how can we explain that in the Altruistic condition
gibbons still participated signicantly more than in a No Food control condition? In that sense, gibbons
showed that they did not just act for the sake of pulling the rope. Most likely, their actions were motivated
by the prospect of obtaining rewards, especially when those were directly available.

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While this reason is plausible, it does not necessarily explain their refusal to participate in almost 40% of
Altruistic trials. The second possible reason is that the conict of interest was higher in the Altruistic
condition compared to the Test condition. In Altruistic trials, cooperative gibbons obtained no direct
benets from their participation. In these trials, gibbons ran the risk of losing any potential reward,
especially if their passive partner positioned herself in front of the release mechanism. Supporting this
view, gibbons waited for each other to act, often delaying their participation over the 90 seconds time
limit that they had to participate in a trial. In fact, gibbons did not differ in their latencies to pull from the
handle between Altruistic and No Food control trials.
Our results thus suggest that gibbons understood some aspects of the social dilemma, preferring to
participate in Test over Altruistic trials. As in previous studies with great apes 8,10, this propensity could be
explained through the combination of two opposing factors: the possibility to obtain direct benets and
the fear to lose all potential rewards.
So far, we have discussed gibbons’ decisions whether to participate or not. The next question is whether
gibbons strategized when they took a passive role. In other words, did they try to maximize their own
rewards when their partners volunteered? The main source of conict between participants lied on the
possibility that passive individuals could position themselves in front of the release mechanism during
Altruistic and Test trials. In Test trials, this could be particularly benecial for passive partners given that
actors could lose some valuable seconds trying to obtain the food reward attached to the PVC pipe.
However, we found that actors actually obtained most of the rewards in both conditions, with a special
advantage over passive partners during Altruistic trials. In fact, in only one pair of gibbons the passive
individual obtained more rewards than the actor in Altruistic and Test trials. Furthermore, passive
individuals rarely took advantage of the situation (they position themselves in front of the release
mechanism in 18.1% of trials) and they did not distinguish between conditions. That is, during Test trials
passive partners did not benet from the time that the actors spent trying to obtain the reward located
inside the PVC pipe. In that sense, gibbons did not solve the social dilemma cooperatively. For that to be
the case, pair members would have beneted more or less equally on both conditions and they would
have not hesitated to manipulate the PVC pipe in Altruistic trials. It is also very unlikely that gibbons’
decisions were driven by proactive prosocial motivations such as releasing food rewards to favor their
pair members given that actors beneted the most from their own actions.
One possible explanation to understand why actors obtained more rewards than passive partners is that
passive individuals tried to pull from the handle during Test trials. This possibility could partially explain
why passive gibbons rarely position themselves in front of the release mechanism during Test trials, but it
cannot explain why in Altruistic trials they did not take advantage of their passive role. An alternative
explanation is that only the most dominant individuals participated and obtained the majority of rewards.
However, there are numerous reasons to suggest that a dominance component cannot fully explain our
pattern of results as opposed to previous ndings in chimpanzees 9. First, only three of 12 individuals
released rewards in less than 20% of Altruistic and Test trials suggesting that in half of the pairs both

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individuals exchanged roles relatively often. Furthermore, our results are also in line with literature
suggesting that there is no clear dominance of one sex over the other 48. Importantly, conict avoidance
does not seem to support this alternative either. The number of co-feeding events tripled the number of
displacements events, suggesting that individuals were usually tolerant to each other. A third alternative
explanation is that once a gibbon decided to manipulate the handle, the other one totally disengaged
from the task. This could explain why passive partners did not take an advantageous position in front of
the release mechanism and why, as a consequence, actors did not face a social dilemma in many trials.
After all, actors benet more than passive partners despite the volunteer costs. Nevertheless, this is
unlikely because passive individuals still obtained a signicant fraction of the food rewards (38% of
rewards across conditions).
All these previous arguments cannot fully explain why passive partners rarely took advantage of their
position, especially given how successful this strategy was: passive partners obtained almost 75% of
rewards when they position themselves in front of the release mechanism by the time the actor
manipulated the pipe. We thus propose that the reason why passive gibbons did not always take
advantage of their partner actions could be explained through combined processes of motivation from
the side of the actor and a general high level of social tolerance towards inequities. That is, individuals
that were more motivated to obtain food and more attentive were also more likely to take the actors’ role
in our task. Through participation, they could become more aware of the situation as a whole and react
faster to obtain the rewards despite their volunteer costs –the distance they covered from the location of
the PVC pipe to the location where the rewards were released. This gave them an advantage over their
passive partners, who at the same time tolerated actors to obtain the majority of rewards (as if actors
would have called “dibs” on the rewards). Importantly, tolerance towards reward inequities also came
from the actors’ perspective. In support of this interpretation we also found that in a number of trials the
actors seemed to adopt a prosocial attitude towards their passive partners by letting them access all the
released rewards.
Future studies should improve different aspects of our setup to continue exploring gibbons’ decision-
making strategies when individuals’ interests collide. The main weakness of our design is that we were
not able to separate pairs of gibbons before the experimental sessions. In that sense we could not train
them with the different task contingencies as it is usually the case in this type of settings [10,50, 51 but
see 52–54]. It is thus possible that some individuals were more skillful than others, and that might have
affected our results. Nevertheless, all individuals approached the apparatus and obtained rewards and
only one never pulled from the PVC pipe during the course of the study. In addition, despite the fact that
gibbons distinguished between conditions with food (Test and Altruistic) and the No Food control, their
latencies to act did not differ between Altruistic and No Food control trials. We hypothesize that with
longer trials we would have found a signicant difference between gibbons’ latencies in Altruistic and No
Food conditions. In that sense, future changes in the trial time or the food rewards can better assess
whether gibbons strategize to solve conicts of interest. Given the quasi-experimental nature of our task,
we did not always capture the social dilemma scenario we envisioned. Future tasks should implement

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designs in which cooperative acts are clearly costly for those individuals willing to volunteer. In addition,
given our restricted sample size we could not test species differences or the presence of individual biases
(e.g. bias leading individuals towards becoming actors).
The present study advances our understanding of how primate species overcome conicts of interest in
experimental settings. Specically, we nd that gibbons are able to solve social dilemmas akin to those
previously presented to other great apes [10], suggesting a continuity in the way apes solve situations of
conict resembling a snowdrift dilemma. In addition, in our study gibbons exhibit high degrees of social
tolerance (in particular in the form of reactive, rather than proactive prosociality 54). Passive partners
tolerate that actors obtain higher benets in a majority of trials while actors often actively forego
opportunities to maximize rewards (e.g. the actor does not try to obtain any of the released rewards).
Relatedly, gibbons engaged in cofeeding events relatively often. One possibility is that such a high degree
of social tolerance towards conspecics results from gibbons’ unique pair-living social system compared
to other great apes, although future studies should inspect this relationship in more detail. Overall, the
inclusion of gibbons in studies exploring the nature of primates’ socio-cognitive abilities is crucial to
elucidate the nature of our prosocial motivations and their relationship to specic socio-ecological
pressures and ultimately how they have evolved since the last common ancestor with all living apes.
Materials And Methods
Subjects
Seven eastern hoolock gibbons (
Hoolock leuconedys
), two pileated gibbons (
Hylobates pileatus
), two
northern white cheeked gibbons
(Nomascus leucogenys
), and one Siamang (
Symphalangus syndactylus
)
participated in the current study (total N = 12, 6M:6F, age in years = 13.98 ± 6.99). See Table S1 in the ESM
for info about the subjects.
All subjects were housed at the Gibbon Conservation Center (GCC) in Santa Clarita (CA, USA). They lived
in pairs of the same species, with the exception of one eastern hoolock gibbon, who was housed with one
siamang. Subjects were tested together in their enclosure and participation was voluntary. Subjects were
fed multiple small meals per day outside of testing food. Testing took place between meals to ensure
motivation. Water was accessible
ad libitum
.
Ethical Statement
The current research was purely behavioral and non-invasive. The current research has been approved by
the IACUC committee of the Gibbon Conservation Center (GCC) and complied with the rules of the IACUC
oce at University of California, San Diego. The current research was carried out in compliance with the
ARRIVE guidelines.
Setup and materials

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One experimenter (hence E1) interacted with the apes during a test session while a second experimenter
recorded the session and scored the subjects behavior (hence E2). Each experimenter tested half of the
pairs. We used high quality rewards (blueberries) that would be easily visible to the subjects.
The apparatus (see Fig. 3) was composed of a plastic folding table with a square wooden plank clamped
to the top. At one end of the plank a transparent plastic bin was taped so that it could be lifted up or hang
down. The bin, at rest, would hang down and remain unmoved on the top of a wooden ramp. A hole big
enough to t blueberries was drilled on the back side of the bin so that when at rest on the ramp, the
experimenter could place ve blueberries into the bin. A thin purple rope was tied to the far end of the
plastic bin and was routed back to the opposite end of the wooden plank. This was set up so that pulling
on the purple rope would reliably lift the plastic bin, so blueberries could fall down the wooden ramp and
be easily accessible for subjects to obtain. The extreme end of the rope was attached to the mesh of the
enclosure. To allow reaching and pulling the rope, we attached a small, handheld, opaque white PVC pipe.
At the right tension, pulling on the PVC pipe would reliably lift the plastic bin. The PVC pipe could contain
a single blueberry inside depending on the condition presented. We used two PVC pipes of the same
dimensions and appearance to avoid contamination of blueberry leftovers after the trial.
The table with wooden plank would be set up at a distance so that it could not be grabbed by subjects
and the ramp was placed underneath so that blueberries would roll down and land in front of the
enclosure gate. E2 would then distract the two subjects to an opposite or adjacent side of the subjects’
enclosure with a handful of blueberries or cereal pieces while E1 tied the end of the purple rope with the
PVC pipe onto the mesh gate of the enclosure, roughly at the experimenter height, approximately 2 meters
to the right or left. The distance and location of the rope was kept constant for all trials of each pair;
however, because the enclosures differed in layout, the rope would go to the most convenient side. This
way, we ensured that the rope had proper tension to be pulled by gibbons and lift the plastic bin as well
as be distant enough from the ramp so that a subject could not easily pull on the rope and obtain food
from the ramp at the same time.
Procedure
Individual solo pre-testing of the mechanism of the apparatus was not possible because the separation
of the pairs was prohibited.
Three conditions were tested: Test, Altruistic, and No Food. In the Test condition, the following procedure
was performed. E1 would place ve blueberries in the plastic bin on the apparatus. To gain the attention
of the subjects, E1 would call the subjects names and show the food, if they were not already focused on
the food/experimenter. Once both subjects had observed the ve blueberries placed in the plastic bin, E1
would squeeze a single blueberry on top of the PVC pipe, so that the blueberry would be clearly visible.
The rope and pipe would be set up so that the pipe was just far enough from the enclosure (at
approximately 30-50 cm) in order for subjects to need to pull on the rope to obtain access to the pipe and
blueberry. Consequently, pulling the rope would also lift the plastic bin and drop ve blueberries down the
ramp, accessible to subjects. The experimenter would also call the names of the subjects when placing

Page 12/17
the single blueberry in the pipe. A choice was recorded when one of the subjects pulled the rope. If no
subject pulled the rope within 90 seconds, the trial ended and was recorded as no pull. If an experimenter
error was made, up to 3 repetitions of the trial would be completed. Environmental conditions such as
rain would also end test sessions to be continued the next day.
In the Altruistic condition, there was no single blueberry placed in the PVC pipe (but still followed the
same procedure of calling the subjects and mimicking putting a blueberry in the pipe). In the No Food
condition, no blueberries were used in the trial. In order to control for time and actions, we used the same
procedure of calling the subjects and touching both the box and the PVC pipe.
Design
The six pairs participated in 6 sessions of 15 trials each. Five trials of each condition were presented
within a session in a pseudorandomized order where no condition was done more than twice in a row. In
total, 540 trials were completed. There were no dropouts or removed subjects. One set of trials for one
pair had to be continued on the next day due to rain. 8 trials were lost due to errors in video recording. One
trial was excluded from model 3 due to a failure of the apparatus.
Coding and statistical analysis
Two cameras on tripods recorded footage concurrently. One was placed to the side of the experimenter in
order to capture a wide view of the trials, specically to show the positions of the subjects, their choices
and if they obtained blueberries. The other was placed close to the ramp to accurately count the quantity
of blueberries obtained by each subject. For all trials we coded the act of pulling or not pulling and the ID
of the puller (actor subject) and non-puller (passive subject in those cases in which one gibbon pulled).
We also coded the number of blueberries each subject ate and whether the actor subject ate the blueberry
from the pipe. Next, we coded whether a passive subject was present in front of the ramp at the moment
the plastic bin was lifted and at the moment the actor arrived at the release location. Additionally, we
coded instances of cofeeding and displacements. Cofeeding was coded when individuals feed within a
distance of 1 meter 55. Displacements occurred when an individual left her spot due to the partners
arrival. Additionally, we calculated the latency from the start of the trial (last frame experimenter touches
the handle) until the individual releases (opening of the plastic bin).
All analyses were conducted with R statistics (version 3.4.4). We used Generalized Linear Mixed Models
(GLMMs) to investigate gibbons’ binary choices (models 1, models 3-6). Variables were z-transformed
when required. Every full model was compared to a null model excluding the test variables. When the
comparison between the full and the null model was signicant, we further investigated the signicance
of the test variables and/or their interactions. We used the “drop1” function of the
lme4
package 56 to test
each variable signicance including interactions between test predictors. Non-signicant interactions
were removed and a new reduced model was produced when necessary. A likelihood ratio test with
signicance set at
p
< 0.05 was used to compare models and to test the signicance of the individual
xed effects. We ruled out collinearity by checking Variance Ination Factors (VIF). All VIF values were

Page 13/17
closer to 1 (maximum VIF value = 1.02). For every model we assessed its stability by comparing the
estimates derived by a model based on all data with those obtained from models with the levels of the
random effects excluded one at a time. All models were stable. We also tted a mixed-effects Cox
proportional hazards model (Model 2) to analyze gibbons’ latencies to act. For this purpose, we used the
“coxme” function from the
coxme
package 57. The results of Model 2 are reported as hazard ratios (HR).
An HR greater than one indicates an increased likelihood of acting (e.g. releasing the rewards) and an HR
smaller than 1 indicated a decreased hazard of acting. In addition, to obtain the p-values for the
individual xed effects we conducted likelihood-ratio tests.
The interobserver reliability was great based on the 19% of the data that was coded by a second rater.
Cohen’s Kappa values were calculated to assess the reliability of gibbons’ decisions and actions. Pearson
R2 values were calculated to assess the reliability of the latencies to manipulate the handle and release
the rewards and the quantity of rewards that each individual ate (participation and IDs of actor and
passive partner: Cohen’s Kappa = 1; latencies to manipulate the handles: Pearson R2 = 0.99; food
consumption and number of rewards obtained: Cohen’s Kappa = 1, Pearson R2 = 0.96; passive individual
in front of the ramp: Cohen’s Kappa = 1; passive individual in front of the ramp by the moment the subject
arrived to the rewards’ location: Cohen’s Kappa = 0.8; occurrence of cofeeding or displacements: Cohen’s
Kappa = 0.76).
Declarations
Ethical Statement
The current research was purely behavioral and non-invasive. The current research has been approved by
the IACUC committee of the Gibbon Conservation Center (GCC) and complied with the rules of the IACUC
oce at University of California, San Diego. The current research was carried out in compliance with the
ARRIVE guidelines.
Statement of informed consent
the person in Fig.1 is the rst author of the study and authorizes the consent to public the image.
Acknowledgments
We are grateful to Gabriella Skollar, Alma Rodriguez, Jodi Kleier and other staff at GCC for hosting the
current research and for their tremendous support. We thank Christina Ruiz-Mendoza for reliability
coding. ASA was supported by a DFG Fellowship Grant (SA 3716-1-1) for this project.
Additional information
The authors declare no competing interests.
Author contributions:

Page 14/17
ASA and FR designed the study. ASA and RB conducted the study. ASA and FR discussed the data
analysis. ASA and RB coded the data. ASA and FR analyzed the data. ASA drafted the manuscript. ASA,
RB and FR edited the manuscript.
Data availability: The data is located in the dryad repository:
https://datadryad.org/stash/share/vEjtboMhIKHTShoqSF49f_Po2XGZc2avM8PsDRMoLN8.
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