# Mutual gaze and facial mimicry as drivers of shared engagement in macaque play fighting

Luca Pedruzzi, Giulia Facondini, Simone Aere, Martin Böye, Alban Lemasson,

Elisabetta Palagi

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Luca Pedruzzi, Giulia Facondini, Simone Aere, Martin Böye, Alban Lemasson, et al. Mutual gaze and facial mimicry as drivers of shared engagement in macaque play fighting. Animal Behaviour, 2025, 224, pp.123215. 10.1016/j.anbehav.2025.123215. hal-05079241

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# Mutual gaze and facial mimicry as drivers of shared engagement in macaque play fighting

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Luca Pedruzzi<sup>a, b, *, 1 ID</sup>, Giulia Facondini<sup>a, 1</sup>, Simone Aere<sup>a</sup>, Martin Böye<sup>c</sup>, Alban Lemasson<sup>b, d, 2</sup>, Elisabetta Palagi<sup>a, e, 2 ID</sup>

## ARTICLE INFO

**Article history:**

Received 3 September 2024

Initial acceptance 6 December 2024

Final acceptance 28 February 2025

Available online xxx

MS. number: 24-00547R

**Keywords:**

coordination

facial expression

joint action

Macaca mulatta

monkey

play signal

primate communication

Human and nonhuman animals engage in complex joint actions, characterized by interactions where individuals coordinate their behaviours in space and time. This synchronization points to the presence of an underlying shared social motivation driving joint actions. However, the use of signals to form, maintain and seal joint commitments during joint actions seems to be exclusive of hominin species. Here, we explored how rhesus macaques, *Macaca mulatta*, use mutual gaze (MG) to open and close naturally occurring play fighting, and its relation to facial mimicry of playful facial expressions during the main body of the interaction. In macaques, the exchange of MG opening joint play contributed to longer successful playful interactions, possibly signalling a shared motivation and emotional state towards the upcoming playful context. Moreover, playmates exchanged mutual gaze more often when ending a joint action compared with a control condition of nonjoint spatial proximity. MG was exchanged independently of the dominance ranks of the young playmates, but it was particularly prevalent in male–male dyads, suggesting that MG may serve as a low-cost play signal in riskier interactions. Notably, we found an association between MG opening and closing the joint action and events of rapid facial mimicry of play faces during the same joint action. This suggests a pivotal role for rapid facial mimicry in the maintenance and reaffirmation of shared engagement, with a similar function of human collaughter. Our study shows that monkeys exchange cognitively low-demanding signals to start, maintain and dissolve shared engagements in joint actions, highlighting the importance of visual communication in play fighting. Furthermore, we provide insights into the potentially ancestral origins of joint commitment as a process, contributing to a deeper understanding of the evolution of the social minds of primates.

© 2025 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Living in groups implies the ability to daily manage social interactions. Both human and nonhuman animals are capable of engaging in joint actions, defined as all those interactions in which the agents sequentially and reciprocally adjust their motor patterns (e.g. motor resonance, task sharing, Sebanz et al., 2006) to coordinate their actions in space and time (Godman, 2013). Although joint actions can sometimes be driven by shared intentionality and common goals, this is not a prerequisite for their definition. Instead, they are

often driven by shared socioemotional motivations, linking them to emotional state sharing and the reinforcement of social bonds (Godman, 2013). Already before engaging in a joint action, subjects need to share the motivation to interact (Miss et al., 2022) and establish some kind of joint commitment towards the interaction they are about to have (Heesen et al., 2017). Joint commitments can be seen as processes through which different signals (e.g. gestures, gaze and vocalizations, Holler, 2022) are exchanged to create frameworks for mutual engagement to fulfil a joint action (Heesen et al., 2021). The phenomenon is not always easy to be discerned from other related phenomena (joint attention and shared intentionality, Miss et al., 2022; Siposova & Carpenter, 2019) and is indeed generally ascribed to the greater field of joint action, crucial to understanding the evolution of social mind (Miss et al., 2022). In

<footnote><sup>a</sup> Unit of Ethology, Department of Biology, University of Pisa, Pisa, Italy</footnote>

<footnote><sup>b</sup> Université de Rennes, Université de Normandie, CNRS, EthoS (Ethologie animale et humaine) - U.M.R 6552, Rennes, France</footnote>

<footnote><sup>c</sup> Centre de Recherche et d'Etude pour l'Animal Sauvage, Planète Sauvage, Port Saint Pere, France</footnote>

<footnote><sup>d</sup> Institut Universitaire de France 1 rue Descartes, Cedex 05, Paris, France</footnote>

<footnote><sup>e</sup> Natural History Museum, University of Pisa, Calci, Pisa, Italy</footnote>

<footnote>* Corresponding author.
E-mail addresses: luca.pedruzzi@phd.unipi.it, luca.pedruzzi@etudiant.univrennes.fr (L. Pedruzzi).</footnote>

<footnote n="1">¹ Luca Pedruzzi and Giulia Facondini share the first authorship.</footnote>

<footnote n="2">² Alban Lemasson and Elisabetta Palagi share the senior authorship.</footnote>
human social world, joint commitments are continuously established, ultimately leading to a deeper understanding of the collective sense of unity and connection (Gilbert, 2015).

The capacity to mutually engaging in joint actions is more ancestral than previously thought, possibly evolutionary antecedent to the appearance of the *Homo* genus (Heesen et al., 2017), also considering the recent evidence extending these capacities to great apes (Heesen et al., 2020, 2021). Indeed, naturally occurring cooperative behaviours requiring cognitive skills such as monitoring conspecific attention and engaging in behavioural patterns to reach a common goal (Tomasello, 2022) have also been observed in nonhuman animals (e.g. hunting in chimpanzees, *Pan troglodytes*, Mine et al., 2022, or in some social carnivores, Marshall-Pescini et al., 2017). Also, non-ape species such as cooperative breeding marmosets have been suggested to have convergently developed skills to engage in joint actions analogous to those of humans (mutual gaze in joint actions, turn-taking and prosocial behaviours, Miss et al., 2022; Miss & Burkart, 2018). A more parsimonious viewpoint is in line with recent evidence, suggesting that the capacity of sharing commitment might be accomplished through rather automatic and cognitively low-demanding mechanisms (Miss et al., 2022), such as facial expressions (Pellis & Pellis, 1997), eye gaze (Iki & Hasegawa, 2020), vocal or gestural cues (Heesen et al., 2021) and body postures (Adriaense et al., 2024; Maglieri et al., 2023; Yanagi & Berman, 2014). Moreover, in humans, commitments are established since early childhood (Kachel et al., 2019) and can be formed implicitly and nonverbally through sensorimotor communication (Michael & Székely, 2018; Vesper et al., 2010).

An ideal behavioural sphere to naturalistically study the communicative and cognitive scaffolds at the basis of joint actions in nonhuman animals is social play (Heesen et al., 2017). Play fighting naturally occurs at a high frequency in primates, specifically among young animals (Palagi, 2023), making it a good behavioural model to comparatively study joint commitment by applying a bottom-up approach (de Waal & Ferrari, 2010). Indeed, albeit differing from experimental cooperative tasks (Duguid et al., 2014; Miss & Burkart, 2018), naturally occurring play fighting similarly requires effective communication (e.g. signals, Palagi, 2008; Pellis & Pellis, 1996; Šinka et al., 2016, facial mimicry, Davila-Ross & Palagi, 2022), self-inhibition and physical calibration from both playmates (Palagi et al., 2016; Palagi, 2018). As has been demonstrated in chimpanzees, *Pan troglodytes* and bonobos, *Pan paniscus*, the use of signals exchanged anticipating and closing a playful interaction might be a good strategy to establish a joint commitment (Heesen et al., 2020, 2021), with bonobos expressing more human-like coordination patterns. Moreover, contrary to chimpanzees, bonobos make greater use of communicative efforts to interact with low-bonded or higher-ranking subjects. This is in line with the Politeness Theory developed for humans, predicting a large investment in signal exchange towards more unfamiliar and high-status individuals (Brown & Levinson, 1987), possibly representing novel and more unpredictable social situations.

In nonhuman primates, eye contact can be linked to positive emotions (Ferrari et al., 2009) and cooperative interactions (Ballesta & Duhamel, 2015), but it can also act as a threatening signal and be linked to agonistic interactions, possibly depending on the age and rank of the interacting subjects, the context and the social structure of the species (Harrod et al., 2020; Kano et al., 2015). In monkeys, mutual gaze often occurs before joint actions (cooperative tasks, Miss & Burkart, 2018; play fighting, Iki & Hasegawa, 2020). Importantly, although so far interest has been specifically put on the opening or closing phase of joint actions (Heesen et al., 2021), it has been hypothesized by Iki and Hasegawa (2020) that the shared engagement established before an

interaction should be continuously reinforced by exchanging other signals during the main body of the interaction. Although systematic data on the role of gaze in joint commitment in monkeys are missing, mutual gaze could be used as a low-demanding strategy to favour mutual engagement (Bangerter et al., 2022).

Here, by focusing on play fighting, we analyse mutual gaze (MG) in rhesus macaques, *Macaca mulatta*, to test whether it can act as a means of interventional establishment of a joint commitment. Given the communicative strategies used by rhesus macaques during play fighting (Facondini et al., 2024; Yanagi & Berman, 2014) and the despotic social style of rhesus macaques (Thierry, 2007), the species is an ideal model for studying the exchange of facial signals in such a risky (yet playful) joint action.

If MG anticipating play can inform partners of their engagement (hypothesis 1a, Table 1; Heesen et al., 2021; Iki & Hasegawa, 2020), the presence of MG should lead to successful playful interactions compared with the absence of MG being linked to shorter playful attempts (prediction 1a). If MG is exchanged between players to signal their mutual motivation to stop the ongoing joint action (hypothesis 1b, Table 1; Heesen et al., 2021) and not as a means to monitor the activity of spatially close conspecifics, especially in such a despotic-intolerant species (Curtin et al., 2011; Gaynor & Cords, 2012), MG should occur more often after joint play fighting compared with after instances of simple prolonged spatial proximity between two subjects not socially interacting (prediction 1b). Mutual signal exchanges are crucial not only during the initiation and maintenance of play fighting but also during its closure (Heesen et al., 2021). These signals help ensure a smooth transition out of play, reducing the risk of misunderstandings or negative emotional states that could arise from an abrupt termination. By facilitating a consensual and coordinated end to the interaction, such signals may also prevent potential aggressive escalation, thereby preserving positive social bonds between the participants.

According to the motor training hypothesis (that is, social play as physical and social training useful for the neuromuscular development to improve competitive skills, Byers & Walker, 1995), male-male play fighting is more frequent and rougher in those species in which males need to compete in the future to increase their reproductive opportunities (for a review on sex-specific differences in rough and tumble play, see Marley et al., 2022; *Macaca* genus, Berghanel et al., 2015; Brown & Dixson, 2000; Ciani et al., 2012; Petit et al., 2008; Wang et al., 2021; Wooddell et al., 2017; Western lowland gorilla, *Gorilla gorilla gorilla*, Palagi et al., 2007; capuchin monkey, *Cebus apella*, Paukner & Suomi, 2008). Therefore, effective communicative strategies should have been positively selected especially in males (hypothesis 2, Table 1). For this reason, MG both anticipating and closing play should be more frequent before and after male-male play sessions compared with the other sex combinations (prediction 2). Moreover, the human Politeness Theory (Brown & Levinson, 1987) posits that social interactions are shaped by efforts to manage and mitigate potential threats to an interlocutor's social image, particularly in contexts where social distance or power asymmetries are present, where communicative efforts will be higher. In human joint action coordination (e.g. during greetings), politeness strategies help navigate hierarchical differences and unfamiliarity, facilitating smoother interactions (Heesen et al., 2021). According to such Theory (hypothesis 3, Table 1), the probability of MG both anticipating and closing play should be higher between hierarchically distant subjects, especially when they share weak social bonds (that is, immature subjects who rarely engage in play fighting together; prediction 3). Although the Politeness Theory (Brown & Levinson, 1987) has been predominantly applied to humans and, more recently, to great apes (Heesen et al., 2021), evidence indeed suggests that non-ape primates also invest greater communicative effort in situations characterized by
<table><caption>**Table 1**
Summary of the hypotheses, predictions and outcomes of the study</caption><thead><tr><th>Hypothesis</th><th>Prediction</th><th>Outcome</th></tr></thead><tbody><tr><td>Hypothesis 1a</td><td>The presence of MG leads to successful joint play (&gt; 5 s) compared to when MG is absent (prediction 1a)</td><td>Supported</td></tr><tr><td>Hypothesis 1b</td><td>MG occurs more often after joint play fighting compared to after instances of spatial proximity between subjects not interacting (prediction 1b)</td><td>Supported</td></tr><tr><td>Hypothesis 2</td><td>MG both opening and closing play is more frequent in M-M play compared to the other sex combinations (prediction 2)</td><td>Supported</td></tr><tr><td>Hypothesis 3</td><td>MG is more frequent between hierarchically distant subjects, especially when they share weak social bonds (prediction 3)</td><td>Not supported</td></tr><tr><td>Hypothesis 4</td><td>MG anticipating play increases the sensitivity to rapidly mimic each other's playful facial expressions (prediction 4)</td><td>Supported</td></tr><tr><td>Hypothesis 5</td><td>RFM during joint play leads to a higher concordance between playmates to stop play, increasing the likelihood of MG closing play (prediction 5)</td><td>Supported</td></tr></tbody></table>

MG: mutual gaze; M-M: male-male; RFM: rapid facial mimicry.

higher uncertainty or risk, such as interactions with less familiar subjects or in more risky conditions (Grampp et al., 2023; Rebout et al., 2022).

In many species, playmates replicate each other's playful facial expressions through rapid facial mimicry (RFM, Davila-Ross & Palagi, 2022), a phenomenon recently found also in our study group, where RFM possibly reflects a higher arousal of playmates and is shaped by risk-mitigating factors such as playmates ranks (Facondini et al., 2024). If MG before joint play enhances playmate engagement (hypothesis 4, Table 1; Iki & Hasegawa, 2020), we expect that during the main body of interaction, MG increases the sensitivity of playmates to rapidly replicate each other's playful facial expressions RFM (prediction 4). Finally, RFM may contribute to maintaining and reinforcing the motivation and commitment to play, with its presence serving as an indicator of efficient communication between playmates (Davila-Ross & Palagi, 2022; Diana & Kret, 2025; hypothesis 5, Table 1). Within this framework, the occurrence of facial mimicry during an ongoing interaction should thus be reflected in a higher concordance between playmates during the closure of the interaction, possibly achieved by the exchange of MG at the end of play (prediction 5). See Table 1 for a summary of hypotheses and predictions of the present study.

# METHODS

## Ethical Note

The study only implied that the recording at distance of macaques was purely observational and conducted without any contact or manipulation of animals. Therefore, the ethics committee of the University of Pisa waived the need for a permit.

## Study Group and Data Collection

The study group is housed at Planète Sauvage (Port-Saint-Père, France) in a 6110 m² enclosure, composed of a grassy area with high trees connected through a system of ropes, rope nets and wood/ metal shelters. Animals are fed at 0930 hours with commercial primate pellet and at 1430 hours with fruits, vegetables and bread. Water is available ad libitum. Data were collected (August to October 2022) on a stable group (no new animals introduced in the

previous 2 years) of 122 captive-born animals (59 males and 63 females, see Table S1). We video-recorded animals (Panasonic full-HD HC-V785 25x, HC-V180 200x, Panasonic Corporation, Osaka, Japan; https://www.panasonic.com) both in the morning (0900–1300 hours) and afternoon (1300–1900 hours). Before the data collection, the researchers underwent a training period to become skilled in animal and behaviour recognition. All the subjects were identified, thanks to facial and body characteristics (e.g. hair/skin colour, scars, tail shape and size). Kin relationships (e.g. matrilines) among group members are unknown. The age categories, based on partial birth data provided by the zoo staff and on the literature on rhesus macaques, were characterized as follows (Berghänel et al., 2016; Hudson et al., 1996; Liao et al., 2018): (1) adults: individuals with developed and clearly visible sexual traits (≥6 years old for males, >5 years old for females; (2) subadults: fully developed individuals never observed breastfed by mothers, smaller in size than adults (3 ≤ years old < 6 for males, 3 ≤ years old ≤ 5 for females) and (3) juveniles: individuals spending most of their time close to the mother and observed in nipple contact with the mother (0–2 years old). Via all-occurrences sampling (Altmann, 1974), all the social interactions and their durations were recorded in continuum (e.g. social play, aggression, affiliation, proximity and resting periods) by following visible subgroups of subjects. Importantly, the observers recorded at the same time from different parts of the enclosure distinct subgroups of animals. These subgroups frequently and fluidly changed their composition, allowing to collect data on several playful dyads of subjects. At the end of the fieldwork, videos had a relatively uniform distribution among subjects. Via scan animal sampling, we calculated the observation time for each macaque. Every 5 minutes, the video was stopped and the IDs of the visible subjects were coded (e.g. 2–15 animals; Altmann, 1974). We collected about 120 h of videos (mean individual observation time ± SE was 3.47 ± 0.16 h).

## Video Coding and Operational Definitions

A frame-by-frame video analysis (accuracy: 0.02 s) of dyadic playful interactions was done via PotPlayer and the exact occurrence and duration of each behavioural pattern were codified (see Table S2). A playful bout started when an individual directed a playful pattern towards a groupmate that reciprocated the playful
action (Gallo et al., 2022). We defined a playful session as successful joint play when it overcome the threshold of 5 s; this threshold was chosen based on previous literature on social play generally discarding playful interactions or attempts lasting less than 5 s (Carter et al., 2019; Facondini et al., 2024; Gallo et al., 2022; Romero et al., 2015). If the play bout lasted less than 5 s, it was coded as unsuccessful joint play. A play session contains pauses not longer than 10 s: when a pause longer than 10 s occurred, the following playful interaction was coded as a new one. This threshold is in line with the literature on social play in monkeys (Tonkean macaques, Macaca tonkeana, Ciani et al., 2012; M. mulatta, Facondini et al., 2024; geladas, Theropithecus gelada, Gallo et al., 2022; Japanese macaques, Macaca fuscata, Iki & Hasegawa, 2021; Tibetan macaques, Macaca thibetana, Wright et al., 2018), although slightly different thresholds are seldomly used (e.g. 5 s, crested macaques, Macaca nigra, Petit et al., 2008; 30 s, M. mulatta, Yanagi & Berman, 2014). Following Yanagi and Berman (2014), we verified whether a play bout had ended by observing whether the two players stopped interacting with each other, ceased mutual gaze and began engaging in separate activities, such as foraging or interacting with other individuals. This criterion was consistently met when applying the 10 s lag rule. Notably, Heesen et al. (2021) investigated the use of signals to initiate and close join actions using a longer threshold of 120-s lag. In our case, such a threshold could not be applied due to the speediness and fast playmate changes typically characterizing play fighting in our study group. For each play session, we coded the playmates' identity (ID, sex and age category) and the exact sequence of behavioural patterns and play faces (or relaxed open mouth, Symons, 1974). Only play sessions involving two subjects (dyadic play) were included in the analyses.

## Mutual gaze

MG occurred when both partners simultaneously looked at each other's faces (Heesen et al., 2021). Notably, mutual gaze was inferred using head direction as a proxy for actual gaze direction. For parsimony, we excluded all the events in which the face of one playmate was not clearly visible (only when both playmates were completely visible it was possible coding the presence/absence of MG), as well as when also a facial expression was emitted during mutual gaze (e.g. play face, Iki & Kutsukake, 2023). We coded for the presence/absence of MG anticipating play whether MG sequentially preceded the first playful pattern (no other pattern between MG and the first playful action, within 2.0 s before the first playful pattern, see also Video S1). We coded for the presence/ absence of MG closing play whether MG sequentially followed the last playful pattern (no other playful pattern after MG, within 2.0 s after the last playful pattern, see also Video S1).

To demonstrate that the presence of the MG closing play was actually related to joint commitment and not to the simple monitoring of conspecifics, we also coded for the presence/absence of MG in a control condition during resting context: one subject left the other after being in close proximity (that is, subjects at a distance of one macaque body length from each other) for more than 10 s without engaging in any active social interaction. Another suitable control condition could have involved situations where two animals engaged in individual goal-directed actions while sitting next to each other (e.g. cofeeding). However, the feeding procedures of the zoo did not allow for those cases to be comparable to the closing phase of a play fighting interaction, as feeding was provided in few clumped areas with the co-presence of 20–40 animals in close proximity, a high-tension condition seldomly observed during play fighting, where animals mostly played in dyads (Facondini et al., 2024). The proximity and the presence of few other conspecifics thus makes our control condition suitable. Moreover, the despotic nature of the group social dynamics may act

as a factor increasing the baseline need for social monitoring also in a noninteractive control condition (Gaynor & Cords, 2012).

## Rapid facial mimicry

A play face was coded as detected when the sender displayed a facial expression within the stereoscopic view of the receiver (Facondini et al., 2024). Instances of uncertainty were discarded. RFM was coded when play faces were detected and replicated within 1 s (Palagi et al., 2020). To ensure the reliability of assessing whether the observer's play face was genuinely prompted by the trigger's play face, we did not code as instances of RFM those cases in which the observer already showed a play face within the 1 s period preceding the detection of the trigger's stimulus (Facondini et al., 2024).

## Familiarity in play

For each dyad, we calculated the index of familiarity in play to assess the social bond strength for each dyad based on social play. This index is derived from the ratio of the time (s) two macaques engaged in play together to the total time (s, obtained via scan sampling) they are recorded together (that is, in the same video scan). This index serves as an approximation of the probability for two subjects to engage in play when they have the opportunity to interact. It offers a metric for assessing ‘social bonding’ based on social play, a behaviour that is more common for immature individuals compared with grooming (Facondini et al., 2024; Palagi, 2023), which was rarely observed among nonadult subjects of our study group.

## Dominance hierarchy

The individual dominance rank was calculated through the average dominance index (ADI, Saccà et al., 2022), which was extracted from a matrix of all the agonistic interactions coded. The ADI (going from 0 to 1) is a highly suitable measure for hierarchies of large groups, where many dyads may lack interactions (Saccà et al., 2022). For all agonistic encounters, we coded the victim and aggressor identity as well as their behavioural patterns and time of occurrence (that is, avoidance, bite, chase, displacement, grab, half-open mouth, jump, jump away, open mouth bared teeth, presenting, push away, run after, run away, silent bared teeth, scalp retraction, slap, silent stare and open mouth, stamp and threat) using published ethograms on macaques (Partan, 2002; Thierry, 1985; Thierry et al., 2000). The outcome of the interaction was also coded (decided conflicts were characterized by the absence of any sort of retaliation/counterattack and by the presence of flee behaviour/ submission by the victim). All cases with doubtful valence or not clear winner/loser roles were not computed in the matrix. A total of 1330 agonistic events were recorded and used for extrapolating the dominance hierarchy by Aere et al. (2025); here, the authors found a steep and linear dominance hierarchy in the group.

## Interobserver agreement

Interobserver reliability was calculated on 15% of the videos (Facondini et al., 2024) coded independently by two observers. Cohen's Kappa coefficient (Cohen, 1968) was always higher than 0.80 (subject identification, k = 0.94; playful patterns k<sub>min</sub> = 0.82, k<sub>max</sub> = 0.87; PFs, detection, RFM k<sub>min</sub> = 0.81, k<sub>max</sub> = 0.91; presence of MG: k<sub>min</sub> = 0.84, k<sub>max</sub> = 0.88; agonistic patterns and winner/loser roles: k<sub>min</sub> = 0.87, k<sub>max</sub> = 0.96).

## Statistical Analyses

### Model 1

MG anticipated play and successful joint play (prediction 1a). To understand if MG anticipating play affects the occurrence of successful joint play (interaction ≤ 5.00 s = unsuccessful;
interaction > 5.00 s = unsuccessful; binomial variable), we ran a generalized linear mixed model (GLMM, glmmTMB 1.2.5042). The interaction between the identities of Player 1*Player 2 was included as a random factor. The predictor variable included in the full model, but not in the null one, was MG anticipating play (presence/absence), whereas the predictor variables included in both full and null models were as follows: (1) index familiarity in play, (2) age combination (juvenile–juvenile/juvenile–subadult/subadult–subadult), (3) sex combination (male–male/male–female/female–female) and (4) the absolute value of the difference in ADI of the two subjects (|diffADI|). Unsuccessful playful interactions were excluded from subsequent analyses to avoid potential biases or confounding effects arising from the inclusion of such failed ‘playful’ attempts.

### Model 2

MG closes joint play versus close proximity (prediction 1b). To understand if the condition (joint play versus close proximity) affects the occurrence of MG closing play/proximity (binomial variable), we ran a GLMM. The interaction Player 1*Player 2 was included as a random factor. The predictor variable included in the full model but not in the null one was the condition, whereas the predictor variables included in both full and null models were as follows: (1) index familiarity in play, (2) age combination, (3) sex combination and (4) |diffADI|.

### Model 3

Factors affecting MG anticipating joint play (prediction 2, prediction 3). To understand which factors affected the occurrence of MG anticipating play (binomial variable), we ran a GLMM. The interaction Player 1*Player 2 was included as a random factor. The predictor variables included in the full model, but not in the null one, were as follows: (1) index familiarity in play, (2) age combination, (3) sex combination, (4) |diffADI| and (5) the interaction familiarity in play*|diffADI| (to account for possible variations in the effect of familiarity in play according to the ranking distance between playmates, see hypothesis 3).

### Model 4

MG anticipating play and RFM during play (prediction 4). To understand if MG anticipating play enhances playful engagement, we ran a GLMM with the presence of RFM during the interaction as a response variable (presence/absence, binomial error distribution). The interaction Player 1*Player 2 was included as a random factor. The predictor variable included in the full model, but not in the null one, was the presence/absence of MG anticipating play, whereas the predictor variables included in both full and null models were as follows: (1) the length of play session (s), (2) familiarity in play, (3) age combination, (4) sex combination and (5) |diffADI|. To ensure equal opportunities to engage in RFM, we excluded cases where neither of the players could perceive at least one play face from the other.

### Model 5

RFM during play and MG closing joint play (prediction 5). To understand that factors affect the occurrence of MG at the end of play, we ran a GLMM with MG closing play (presence/absence, binomial error distribution) as response variable. The interaction Player 1*Player 2 was included as a random factor. The predictor variables included in the full model but not in the null one was the presence of RFM in the main body of the play session, whereas the predictor variables included in both full and null models were the (1) familiarity in play, (2) age combination, (3) sex combination, (4) |diffADI| and (5) the length of play session (s).

Multicollinearity (Bolker et al., 2009) in the model was checked with the check_collinearity function from the R package performance 0.4.4 (by means of variation inflation factors, VIFs): ‘low correlation’ was found for all the fixed factors in the models (VIF range 1.05–1.38). The significance of the models was evaluated by comparing the full model against a null model consisting only of random effects (and potential control factors). This comparison was carried out using the likelihood ratio test with the χ² test argument (Dobson & Barnett, 2018). To determine the *P* value for each predictor, separate likelihood ratio tests were conducted between the full model and a model lacking that specific predictor, using the ANOVA function (Barr, 2013). Relative odds ratios (OR) were used to illustrate the actual impact of estimated effects using the *confint*() function. For conducting pairwise comparisons involving factors with more than two levels in the models, we used the R package emmeans and performed the Tukey test (Bretz et al., 2016; Lenth et al., 2021). All the aforementioned analyses were carried out using RStudio (http://www.r-project.org).

## RESULTS

A total of 954 playful interactions were coded. Play fighting involved 72 subjects and was mostly displayed by juveniles and subadults (N = 63), on which we focused our analyses (mean frequency of social play ± SE: 2.18 ± 0.26 min/h). We found evidence that playmates exchanged MG anticipating play in 72.2% play sessions (304/421) and MG closing play in 32.2% play sessions (148/459).

### Model 1

MG anticipating play and successful joint play (prediction 1a supported, Table 1). The full model investigating if MG anticipating play affected the probability of successful joint play differed from the control model (χ²₁ = 9.63, *P* = 0.002, Table 2). The presence of MG anticipating play increased more than twice (OR = 2.40) the likelihood of the two players having successful joint play (Fig. 1a, χ² = 9.80, *P* = 0.002) compared with instances where mutual gaze was not present.

### Model 2

MG closing joint play close proximity (prediction 1b supported, Table 1). The full model investigating if the condition affected the probability of MG closing play/proximity differed from the control model (χ²₁ = 59.35, *P* < 0.0001, Table 2). The presence of joint play increased more than nine times (OR = 9.11) the likelihood of the two players exchanging MG when closing the interaction (Fig. 1b, χ² = 40.59, *P* < 0.0001) compared with after control conditions of simpler spatial proximity.

### Model 3

Factors affecting MG anticipating joint play (prediction 2 supported, prediction 3 not supported, Table 1). The full model investigating the factors affecting the likelihood of MG anticipating play significantly differed from the null one (χ²₆ = 21.97, *P* = 0.003, Table 3). The variable sex combination had a significant effect on the probability of MG anticipating play. Specifically, male–male dyads exchanged MG before their play fighting more than the other sex combinations (χ² = 10.77, *P* = 0.005). Pairwise comparison results are reported in the legend of Fig. 2.
<caption>**Table 2**</caption>

Estimated parameters (Coeff), SE and results of the likelihood ratio tests ($\chi^2$) of Model 1 and Model 2

<table><thead><tr><th>Fixed effects</th><th>Coeff</th><th>SE</th><th>χ<sup>2</sup></th><th>df</th><th>P</th></tr></thead><tbody><tr><td colspan="6"><strong>Model 1: MG anticipating play and successful joint play (prediction 1a supported).</strong> Response variable: Successful joint play (yes/no)</td></tr><tr><td>Intercept</td><td>0.853</td><td>0.557</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Tested variable</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MG anticipating play<sup>a</sup> (presence)</td><td>0.878</td><td>0.280</td><td>9.804</td><td>1</td><td><strong>0.002</strong></td></tr><tr><td>Control variables</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sex combination<sup>a</sup></td><td>—</td><td>—</td><td>5.340</td><td>2</td><td>0.069</td></tr><tr><td>Sex combination (M-F)</td><td>0.090</td><td>0.632</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Sex combination (M-M)</td><td>-0.829</td><td>0.541</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Age combination<sup>a</sup></td><td>—</td><td>—</td><td>3.331</td><td>2</td><td>0.189</td></tr><tr><td>Age combination (sj)</td><td>0.656</td><td>0.377</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Age combination (ss)</td><td>0.538</td><td>0.370</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Familiarity in play</td><td>1.636</td><td>2.450</td><td>0.446</td><td>1</td><td>0.504</td></tr><tr><td>|diffADI|</td><td>1.622</td><td>0.801</td><td>4.097</td><td>1</td><td><strong>0.043</strong></td></tr><tr><td colspan="6"><strong>Model 2: MG closing joint play versus close proximity (prediction 1b supported).</strong> Response variable: MG closing play (yes/no)</td></tr><tr><td>Intercept</td><td>-3.319</td><td>0.506</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Tested variable</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Condition<sup>a</sup> (joint play)</td><td>2.209</td><td>0.506</td><td>40.597</td><td>1</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Control variables</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sex combination<sup>a</sup></td><td>—</td><td>—</td><td>10.584</td><td>2</td><td><strong>0.005</strong></td></tr><tr><td>Sex combination (M-F)</td><td>-0.149</td><td>0.471</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Sex combination (M-M)</td><td>0.821</td><td>0.420</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Age combination<sup>a</sup></td><td>—</td><td>—</td><td>3.066</td><td>2</td><td>0.216</td></tr><tr><td>Age combination (subadult-juvenile)</td><td>-0.218</td><td>0.292</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Age combination (subadult-subadult)</td><td>0.192</td><td>0.297</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Familiarity in play</td><td>-1.610</td><td>1.760</td><td>0.837</td><td>1</td><td>0.360</td></tr><tr><td>|diffADI|</td><td>-0.704</td><td>0.546</td><td>1.666</td><td>1</td><td>0.197</td></tr><tr><td colspan="6"><strong>N<sub>observations</sub></strong> = 701, <em>N</em><sub>player1</sub> = 67, <em>N</em><sub>player2</sub> = 64. Random factors: Player 1, Variance = 1.12 × 10<sup>-8</sup>, SD = 0.000106; Player 2, Variance = 5.36 × 10<sup>-2</sup>, SD = 0.231</td></tr></tbody></table>

Significant *P* values are in bold; * = not applicable. Estimate = SE refers to the difference of the response between the reported level of this categorical predictor and the reference category of the same predictor. |diffADI|: absolute value of the difference in the ADI of the two subjects; F: female; M: male; MG: mutual gaze.

ª These predictors were dummy coded, with 'MG anticipating play (absence)', 'Sex combination (female-female)', 'Age combination (juvenile-juvenile)', 'Condition (spatial proximity)' being the reference categories.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Effect plot showing (a) the effect of the presence of mutual gaze anticipating play on the probability of successful joint play and (b) the effect of the condition on the presence of mutual gaze closing play/spatial proximity. Plotted circles represent the different dyads of subjects and their predicted proportion of successful joint play interactions. As indicated by the legend, the larger the circle, the higher the number of observations per dyad.</figcaption></figure>

**Model 4**

MG anticipating play and RFM during play (prediction 4 supported, Table 1). The full model investigating the factors affecting the likelihood of RFM significantly differed from the control model

($\chi^2_1$ = 4.33, *P* = 0.037, Table 3). The presence of MG before play increased more than twice (OR = 2.24) the probability of RFM during the ongoing interaction (MG anticipating play: $\chi^2$ = 4.33, *P* = 0.037, Fig. 3a) compared with instances where mutual gaze was not present.
<table><caption>**Table 3**
Estimated parameters (Coeff), SE and results of the likelihood ratio tests (χ²) of Model 3 and Model 4</caption><thead><tr><th>Fixed effects</th><th>Coeff</th><th>SE</th><th>χ<sup>2</sup></th><th>df</th><th>P</th></tr></thead><tbody><tr><td colspan="6"><strong>Model 3: Factors affecting MG anticipating joint play (prediction 2 supported, prediction 3 not supported).</strong> Response variable: MG anticipating play (yes/no)</td></tr><tr><td>Intercept</td><td>0.247</td><td>0.472</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Tested variables</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sex combination<sup>a</sup></td><td>-</td><td>-</td><td>10.774</td><td>2</td><td><strong>0.005</strong></td></tr><tr><td>Sex combination (M-F)</td><td>-0.322</td><td>0.445</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Sex combination (M-M)</td><td>0.779</td><td>0.426</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Age combination<sup>a</sup></td><td>-</td><td>-</td><td>1.524</td><td>2</td><td>0.467</td></tr><tr><td>Age combination (subadult-juvenile)</td><td>-0.296</td><td>0.321</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Age combination (subadult-subadult)</td><td>0.021</td><td>0.353</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Familiarity in play</td><td>5.388</td><td>3.744</td><td>3.070</td><td>1</td><td>0.080</td></tr><tr><td>|diffADI|</td><td>0.018</td><td>1.036</td><td>0.3758</td><td>1</td><td>0.540</td></tr><tr><td>Familiarity in play × |diffADI|</td><td>-5.199</td><td>10.053</td><td>0.2675</td><td>1</td><td>0.605</td></tr><tr><td colspan="6">N<sub>Observations</sub> = 421, N<sub>player1</sub> = 53, N<sub>player2</sub> = 54. Random factors: Player 1, Variance = 0.033, SD = 0.183; Player 2, Variance = 0.0575, SD = 0.240</td></tr><tr><td colspan="6"><strong>Model 4: MG anticipating play and RFM during joint play (prediction 4 supported).</strong> Response variable: Presence of RFM</td></tr><tr><td>Intercept</td><td>-1.660</td><td>0.480</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Tested variable</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>MG anticipating play<sup>a</sup> (presence)</td><td>0.500</td><td>0.241</td><td>7.88</td><td>1</td><td><strong>0.038</strong></td></tr><tr><td>Control variables</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sex combination<sup>a</sup></td><td>-</td><td>-</td><td>19.034</td><td>2</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Sex combination (M-F)</td><td>0.430</td><td>0.475</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Sex combination (M-M)</td><td>1.574</td><td>0.439</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Age combination<sup>a</sup></td><td>-</td><td>-</td><td>1.436</td><td>2</td><td>0.488</td></tr><tr><td>Age combination (subadult-juvenile)</td><td>-0.188</td><td>0.299</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Age combination (subadult-subadult)</td><td>0.117</td><td>0.312</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Familiarity in play</td><td>1.930</td><td>1.915</td><td>1.015</td><td>1</td><td>0.314</td></tr><tr><td>|diffADI|</td><td>-0.666</td><td>0.582</td><td>1.310</td><td>1</td><td>0.252</td></tr><tr><td>Length of play session</td><td>0.020</td><td>0.0065</td><td>9.056</td><td>1</td><td><strong>0.003</strong></td></tr></tbody></table>

Significant *P* values are in bold; * - not applicable. Estimate ± SE refers to the difference of the response between the reported level of this categorical predictor and the reference category of the same predictor. |diffADI|: absolute value of the difference in the ADI of the two subjects; F: female; M: male; MG: mutual gaze; RFM,:rapid facial mimicry.

<sup>a</sup> These predictors were dummy coded, with 'MG anticipating play (absence)', 'Sex combination (female-female)', 'Age combination (juvenile-juvenile)', 'Presence of RFM (absence)' being the reference categories.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Effect plot showing the effect of the sex combination of playmates on the probability of mutual gaze anticipating play. Plotted circles represent the different dyads of subjects and their predicted proportion of successful joint play interactions. As indicated by the legend, the larger the circle, the higher the number of observations per dyad.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Effect plot showing (a) the effect of the presence of mutual gaze anticipating play on the probability of rapid facial mimicry (RFM) during the play session, (b) the effect of the presence of RFM during a play session on the probability of mutual gaze closing play. Plotted circles represent the different dyads of subjects and their predicted proportion of successful joint play interactions. As indicated by the legend, the larger the circle, the higher the number of observations per dyad. Below, a graphic scheme of the three proposed possible steps for processual joint commitment ((i) establishing joint commitment, (ii) maintaining/reinforcing joint commitment and (iii) dissolving joint commitment).</figcaption></figure>

## Model 5

RFM during play and MG closing joint play (prediction 5 supported, Table 1). The full model investigating the factors affecting the likelihood of MG closing play significantly differed from the null

model ($\chi^2_1 = 5.26$, *P* = 0.022, Table 4). The presence of RFM increased almost twice (OR = 1.96) the probability of MG occurring at the end of the play session (presence of RFM: $\chi^2 = 5.09$, *P* = 0.024; Fig. 3b) compared with instances where RFM did not occur during joint play.

## Table 4

Estimated parameters (Coeff), SE and results of the likelihood ratio tests ($\chi^2$) of Model 5

<table><thead><tr><th>Fixed effects</th><th>Coeff</th><th>SE</th><th>χ<sup>2</sup></th><th>df</th><th>P</th></tr></thead><tbody><tr><td colspan="6"><strong>Model 5: Factors affecting MG closing joint play (prediction 5 supported). Response variable: MG closing play (yes/no)</strong></td></tr><tr><td>Intercept</td><td>-0.620</td><td>0.594</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Tested variable</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Presence of RFM<sup>a</sup> (presence)</td><td>0.675</td><td>0.299</td><td>5.087</td><td>1</td><td><strong>0.024</strong></td></tr><tr><td>Control variables</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sex combination<sup>a</sup></td><td>—</td><td>—</td><td>4.493</td><td>2</td><td>0.106</td></tr><tr><td>Sex combination (M-F)</td><td>-0.942</td><td>0.652</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Sex combination (M-M)</td><td>0.073</td><td>0.586</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Age combination<sup>a</sup></td><td>—</td><td>—</td><td>2.529</td><td>2</td><td>0.282</td></tr><tr><td>Age combination (subadult–juvenile)</td><td>-0.353</td><td>0.401</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Age combination (subadult–subadult)</td><td>0.181</td><td>0.424</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Familiarity in play</td><td>-2.821</td><td>2.581</td><td>1.194</td><td>1</td><td>0.275</td></tr><tr><td>|diffADI|</td><td>-1.179</td><td>0.749</td><td>2.473</td><td>1</td><td>0.116</td></tr><tr><td>Length of play session</td><td>0.001</td><td>0.007</td><td>0.035</td><td>1</td><td>0.852</td></tr><tr><td colspan="6"><strong>Model 5: Factors affecting MG closing joint play (prediction 5 supported). Response variable: MG closing play (yes/no)</strong></td></tr><tr><td>N<sub>observations</sub> = 298, N<sub>player1</sub> = 50, N<sub>player2</sub> = 51. Random factors: Player 1, Variance = 3.43 × 10<sup>-9</sup>, SD = 5.86 × 10<sup>-5</sup>; Player 2, Variance = 0.303, SD = 0.550</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Significant *P* values are in bold; — = not applicable. Estimate ± SE refers to the difference of the response between the reported level of this categorical predictor and the reference category of the same predictor. |diffADI|, absolute value of the difference in the ADI of the two subjects; F: female; M: male; MG, mutual gaze; RFM, rapid facial mimicry.

<footnote><sup>a</sup> These predictors were dummy coded, with 'Sex combination (female–female)', 'Age combination (juvenile–juvenile)', 'Presence of RFM (absence)' being the reference categories.</footnote>
DISCUSSION

The capacity to use low-cost facial signals to engage and coor-
dinate in complex joint actions does not seem to be a prerogative of
Homininae species. Using the same analytical approach previously
applied to great apes (Heesen et al., 2021), we demonstrated that in
rhesus macaques, mutual gaze (MG) plays a role in establishing and
dissolving shared engagement during naturally occurring joint
actions, such as social play. This finding suggests that joint
commitment may also operate as a process in non-ape species.
What emerged from our data is that MG not only anticipates and
closes the joint action, but it also affects and is affected by a motor
resonance phenomenon occurring in the main body of the inter-
action known to foster motor and possibly affective synchroniza-
tion, RFM of play faces. Our findings fit with recent theoretical
models, suggesting that joint commitments develop along with
step-by-step communicative efforts throughout the opening, main
body and closing phases of joint actions (Bangerter et al., 2022). See
Table 1 for an overview of the hypotheses, predictions and out-
comes of the present study.

The first research effort was to demonstrate that the exchange of MG could be actually linked to shared engagements towards joint action. The presence of MG anticipating play fighting increased the probability of success of the forthcoming social interaction (overcoming the 5-s threshold). This proximate effect suggests that exchanging mutual gaze can be an actual low-cost signal, influencing the subsequent play and possibly indicating the establishment of shared attention and internal motivation to play between playmates (Adriaense et al., 2024; Iki & Hasegawa, 2020; Palagi, 2008). Additionally, when macaques physically separated from each other and closed an interaction (joint play or spatial proximity), they engaged in MG more often after a joint play session (joint action condition) rather than when getting apart from each other after close proximity (control condition). Thus, MG does not seem the mere byproduct of a simple reciprocal social monitoring, particularly considering the despotism of rhesus macaque societies. These first results suggest that mutual gaze exchanged before and after a joint action can help convey the shared social motivation and engagement to fulfil a joint action. It must also be noted that other signals might be used together or instead of mutual gaze when engaging in joint actions. Yanagi and Berman (2014) analysed body signals used by rhesus macaque youngsters to invite to play fight and interestingly found that six of seven candidate signals (that is, crouch and stare, dangle-and-stare, leg-peak, look-back, roll-onto-back-and-stare) involved staring and looking between the playmates. This further suggests that gazing fulfil an important function to solicit, maintain and close social play in monkeys.

Due to the risky nature of play fighting and the despotic nature
of our study species, it is expected that communicative strategies
evolved to assess the partner's motivation to engage in joint action.
Such signals can help clarify the playful context of the upcoming
interaction (Palagi, 2008). In rhesus macaques, MG anticipated
more than 70% of successful playful interactions and concluded
about 30% of them. Interestingly, a similar rate was found for the
opening phase of joint play in chimpanzees, where bilaterally
exchanged signals anticipated 82% and closed 61% of playful in-
teractions. We need to underline that a strict comparison is difficult
because the authors considered not only mutual gaze but also other
communicative patterns (e.g. exchange of facial expressions, vo-
calizations and gestures) to open and close play (Heesen et al.,
2021). As detailed in the Methods section, Heesen et al. (2021)
used a 120-s threshold to separate consecutive play bouts,
whereas we used 5 s. This shorter timeframe may explain the lower
percentage of signals observed in our study, as closely timed in-
teractions could be interdependent, reducing the occurrence of MG

during exits. Despite such different methodological criteria, the
similarities highlight possible homologies in the ways nonhuman
primates enter or exit joint actions.

MG was more frequently exchanged in male–male dyads, possibly reflecting a higher need for communicative signals to be exchanged by young males (Yanagi & Berman, 2017). This can be explained by the roughness enacted by males during their play fighting (Ciani et al., 2012; Marley et al., 2022; Pellis, 2002; Symons, 1974; Wang et al., 2021), which can imply a certain level of risk when a fine-tune communication is lacking. Male play vigorous-ness is particularly adaptive in those species characterized by male exogamy because players need to test and increase their reciprocal knowledge that will be useful to manage their future competitive interactions (Motor training hypothesis, Byers & Walker, 1995). Therefore, it could be even more important for males to establish an agreement sealing the intention to engage in a joint action. This finding is also in line with the human Politeness Theory (Brown & Levinson, 1987), stating that people tend to initiate and terminate their interactions by investing in more communicative exchanges during uncertain situations, when more caution is needed (O'Leary & Gallois, 1985). Our data on monkeys show that neither familiarity nor rank difference did influence the exchange of MG. This is in line with what was found in chimpanzees where, differently from bonobos, the likelihood of producing signals was not affected by social or hierarchical rules (Heesen et al., 2020, 2021). However, the use of MG here seems to contrast with recent results on facial mimicry during the main body of the interaction in the study group, where playmates replicate more often the facial expressions of dominant subjects, possibly as a risk-mitigating strategy (Facondini et al., 2024). Whatever the social closeness or dominance relationship shared by the two agents, in less tolerant species, establishing mutual engagement through low-demanding MG might always be an effective strategy due to a higher baseline risk during play fighting. A further, not mutually exclusive, the explanation could relate to the behavioural domain studied here, social play between immature subjects, where dominance rules get (relatively) less important even in despotic species (Palagi, 2023).

The present study only involved juvenile subjects, as adult play was absent in the study group (Facondini et al., 2024) and is generally rarely reported for rhesus macaques (Yanagi & Berman, 2017), consistently with the despotic nature of the species social groups (Palagi, 2023). Possible differences may arise when examining how adults engage and coordinate in joint actions, as recent evidence suggests that the ability to participate in complex communication and joint commitments increases with age (Heesen et al., 2023). Given the despotic social structure of rhesus macaques, focusing on other joint actions performed by adults (e.g. grooming) may reveal a stronger influence of hierarchical dynamics and the use of risk-mitigation strategies when engaging in such interactions.

Exchanging signals to ensure a shared motivation and affective
state when opening a joint action is not always sufficient to make
the interaction successful per se. Indeed, mutual engagement in
joint actions often does not consist of a unique, all-encompassing
agreement, but are instead composed of a variety of small, step-
by-step commitments that are created, upheld and terminated
throughout the course of interactions (Bangerter et al., 2022).
During human conversations, laughter seems to be one of the main
effective engines in prolonging positive engagement in the inter-
action (Vettin & Todt, 2004). Specifically, collaughter punctuates,
regulates and prolongs joint conversations and activities (Rathcke
& Fuchs, 2022; Wood et al., 2022). Similarly, for nonhuman pri-
mates, it is reasonable that other strategies concur in increasing
coordination during the main body of the interaction (e.g. RFM,
Facondini et al., 2024). Here, we found that exchanging MG at the
beginning of a playful session led macaques to engage in RFM at high frequency while playing together (Fig. 3a). The agreement reached through MG between monkeys before a joint action reflects in the occurrence of facial mimicry, linked to smoother interactions (Davila-Ross & Palagi, 2022; Diana & Kret, 2025) and possibly higher emotional arousal (Facondini et al., 2024) between interacting subjects. More in detail, MG may help to establish a shared intentionality between interacting subjects, thus fostering and reflecting in the occurrence of RFM, which might help reconfirm and maintain joint commitment by rapidly mimicking positive valence play faces (Fig. 3) and increase the emotional arousal of playmates (Facondini et al., 2024). However, it is important to acknowledge that we cannot draw conclusions on the causal relationships between MG and RFM, as alternative explanations might account for the temporal link found between the exchange of mutual gaze before play and the occurrence of RFM during the joint action. Future studies should use a more fine-grained temporal analysis of the sequence of MG and RFM occurrences to address this possible relationship. Here, the role of RFM was not restricted to the management of the main body of the play session but also extended to the closing phase of the joint action by increasing the mutual agreement to dissolve a joint commitment (Fig. 3b). Considering that the research fields of joint action and emotional expressions had been approached separately (Bienkiewicz et al., 2021), understanding the linkage between shared engagement and the emotional states experienced by interacting subjects could open new scenarios on how emotions permeate our embodied social interactions (Bienkiewicz et al., 2021). Indeed, data on humans demonstrate that when individuals experience positive emotions during their joint actions, they also increase their behavioural synchronization (Smykovskyi et al., 2022).

In summary, although from the present data, we cannot draw conclusions on rhesus macaques' capacity of mental state attributions and high-order intentionality, our study suggests that monkeys exchange low-demanding visual signals to start, maintain and dissolve shared engagements during naturally occurring joint actions requiring joint commitment (Heesen et al., 2017; Lewis & Krupenye, 2022). Overall, our findings underscore the pivotal role of social play in revealing effective communicative strategies and reinforce gradualist perspectives that trace the evolution of primate social cognition back to more ancient origins.

## Author Contributions

**Alban Lemasson:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation, Conceptualization.
**Elisabetta Palagi:** Writing – review & editing, Writing – original draft, Project administration, Investigation, Formal analysis, Data curation, Conceptualization.
**Giulia Facondini:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation.
**Luca Pedruzzi:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Formal analysis, Data curation, Conceptualization.
**Martin Böye:** Methodology, Investigation, Formal analysis, Data curation.

## Data Availability

The full data set used for the analyses is provided as Supplementary material.

## Declaration of Interest

The authors declare no conflict of interest of any kind.

## Acknowledgments

We wish to thank the keepers at Planète Sauvage and especially Lucie Durand for their support in the research, and Jacopo Ballerini for the help during video analysis. We extend our heartfelt thanks to Raphaela Heesen and an anonymous referee for their insightful suggestions and valuable contributions to improving our work. None of the authors received funding for the present study.

## Supplementary Material

Supplementary material associated with this article is available at https://doi.org/10.1016/j.anbehav.2025.123215.

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