The face never lies: facial expressions and mimicry modulate playful interactions in wild geladas

Abstract

Play fighting, the most common form of social play in mammals, is a fertile field to investigate the use of visual signals in animals’ communication systems. Visual signals can be exclusively emitted during play (e.g. play faces, PF, context-dependent signals), or they can be released under several behavioural domains (e.g. lip-smacking, LS, context-independent signals). Rapid facial mimicry (RFM) is the involuntary rapid facial congruent response produced after perceiving others’ facial expressions. RFM leads to behavioural and emotional synchronisation that often translates into the most balanced and longest playful interactions. Here, we investigate the role of playful communicative signals in geladas (Theropithecus gelada). We analysed the role of PF and LS produced by wild immature geladas during play fighting. We found that PFs, but not LS, were particularly frequent during the riskiest interactions such as those including individuals from different groups. Furthermore, we found that RFM (PF→PF) was highest when playful offensive patterns were not biased towards one of the players and when the session was punctuated by LS. Under this perspective, the presence of context-independent signals such as LS may be useful in creating an affiliative mood that enhances communication and facilitates most cooperative interactions. Indeed, we found that sessions punctuated by the highest frequency of RFM and LS were also the longest ones. Whether the complementary use of PF and LS is strategically guided by the audience or is the result of the emotional arousal experienced by players remains to be investigated. Significance Statement Facial expressions and their rapid replication by an observer are fundamental communicative tools during social contacts in human and non-human animals. Play fighting is one of the most complex forms of social interactions that can easily lead to misunderstanding if not modulated through an accurate use of social signals. Wild immature geladas are able to manage their play sessions thus limiting the risk of aggressive escalation. While playing with unfamiliar subjects belonging to other groups, they make use of a high number of play faces. Moreover, geladas frequently replicate others’ play faces and emit facial expressions of positive intent (i.e. lip-smacking) when engaging in well-balanced long play sessions. In this perspective, this “playful facial chattering” creates an affiliative mood that enhances communication and facilitates most cooperative interactions.

A convergent interaction engine: vocal communication among marmoset monkeys

“…others for social play [26][27][28] and grooming [29] or during mating [30] (see [27] for overview of other species). Intriguingly, marmosets and other callitrichids show a striking lack of gaze aversion in almost all contexts.…”

“…The extent of gaze aversion varies across primate species and contexts. For instance, bonobos make more eye contact than chimpanzees [25] and gaze is used as a social tool to start and end social interactions by several primates, in particular when recruiting others for social play [26–28] and grooming [29] or during mating [30] (see [27] for overview of other species).…”

Laughter, play faces and mimicry in animals: evolution and social functions

“…Mimicking and other types of behavioural matching within the context of play are also likely to be important for socially learning and practising a wide range of behaviours in humans and nonhuman animals [ 4 , 5 , 136 ]. In support of this notion, there is evidence that animals match the exact variant of the same expression of their playmates [ 58 , 112 ] and that the matching of play expressions may differ in form and function between social groups [ 86 , 137 , 138 ]. This brings us back to the Power Asymmetry Hypothesis [ 59 ], which could be extended to colony differences.…”

Rapid facial mimicry in Platyrrhini: Play face replication in spider monkeys (Ateles fusciceps, Ateles hybridus, and Ateles paniscus)

“…Facial expressions are more likely to be mimicked during face‐to‐face interactions (Annicchiarico et al, 2020; Herrando & Constantinides, 2021). Consequently, for RFM to occur, it is essential that the potential receiver perceives the facial stimulus (Bertini et al, 2021; Gallo et al, 2022; Herrando & Constantinides, 2021; Palagi et al, 2019, 2020). According to the PAM, observing another's facial expression activates shared neural areas that enable replication not only of the expression but also of the emotion it conveys (de Waal & Preston, 2017).…”

“…In primates, RFM has been demonstrated primarily in Catharrini including humans (Chartrand & van Baaren, 2009; Olszanowski et al, 2019), bonobos (Bertini et al, 2021), chimpanzees (Palagi et al, 2019), lowland gorillas (Bresciani et al, 2021; Palagi et al, 2019), Bornean orangutans (Davila‐Ross et al, 2008), geladas (captive, Mancini et al, 2013; wild, Gallo et al, 2022), Tonkean and Japanese macaques (Scopa & Palagi, 2016).…”