We appreciate the insightful article on the role of grooming and social behaviors in stress management and bonding within social species. We would like to extend this discussion by highlighting the importance of social play and related facial expressions and vocalizations (e.g., laughter, play face, 50 KHz sounds) which, like grooming, are crucial for stress regulation, relationship maintenance, and the formation of social bonds—yet are largely overlooked in the target article.

As noted in the article, grooming triggers endorphin release, fostering bonding and reducing stress. We want to emphasize that social play and laughter similarly activate endorphin responses and play key roles in social cohesion (Palagi, de Waal, & Caruana, Reference Palagi, de Waal and Caruana2022; Burghardt et al., Reference Burghardt, Pellis, Schank, Smaldino, Vanderschuren and Palagi2024). While laughter is briefly mentioned in relation to humans, social play is entirely omitted. We fear that the failure to address recent comparative data on these behaviors seriously undermines the target article and the evolutionary scalability of its model, depriving it of important empirical support. This is especially striking given the ubiquity and importance of play in humans, non-human primates, and indeed throughout the animal kingdom (Burghardt, Reference Burghardt2005; Burghardt & Pellis Reference Burghardt, Pellis, Smith and Roopnarine2019).

Social play activates the brain’s reward system, reducing stress and promoting cognitive development in both humans and other animals and, here, we provide recent studies empirically demonstrating such effects (e.g., Wang & Aamodt Reference Wang and Aamodt2012; Whitten et al., Reference Whitten, Kelly, Gillespie, Brooks, Hooker, Temple, Hennon, Kilgore, Singamaneni, Schulz, Burghardt and Cooper2025). In societies with high tolerance and a need for cooperation, social play serves as a mechanism for assessing partners’ willingness to invest in relationships while simultaneously allowing individuals to show vulnerability—key components of trust-building. Palagi (Reference Palagi2023) provide evidence on primates and social carnivores that play may reduce group dispersal by fostering cooperation through these social cues.

In relation to grooming, social play may, in fact, be more important in social bonding, especially in young animals. Shimada and Sueur (Reference Shimada and Sueur2018) studied a habituated troop of wild Japanese macaques and recorded grooming and play bouts as well as social proximity and concluded that “the hypothesis that the biological function of social play among immature Japanese macaques is to strengthen their social bonds in the near future and develop their social life appears to be correct. For juvenile macaques, social play, rather than grooming, functions as an important social mechanism to strengthen affiliative relationships.” They also showed the importance of social play in social bonding in wild chimpanzees (Shimada & Sueur Reference Shimada and Sueur2014). Regarding laughter, this behavior is barely mentioned in the target article and only in relation to humans, despite the emerging literature on the social bonding function of laughter across species (Caruana, Palagi, & De Waal, Reference Caruana, Palagi and De Waal2022). This is quite remarkable considering that the author himself has provided substantial evidence over the years regarding the significance of this multimodal communicative mechanism as a replacement for grooming behavior, which is extensively discussed in the target article (Dunbar, Reference Dunbar2012, Reference Dunbar2022).

Social connection mediated by laughter and play faces becomes evident through the process of motor resonance, which is manifested neurally through mechanisms grounded in the Mirror Neuron System (Caruana et al., Reference Caruana, Avanzini, Pelliccia, Mariani, Zauli, Sartori, Del Vecchio, Lo Russo and Rizzolatti2020; Billing, Cooper, & Scott, Reference Billing, Cooper and Scott2021; Del Vecchio et al., Reference Del Vecchio, Avanzini, Gerbella, Costa, Zauli, d’Orio, Focacci, Sartori and Caruana2024) and behaviorally through the phenomenon of rapid facial mimicry (Davila-Ross and Palagi, 2022). Rapid facial mimicry has now been demonstrated in numerous species of social mammals, including humans (Hess & Fischer Reference Hess and Fischer2013), non-human primates (Demuru, Ferrari, & Palagi, Reference Demuru, Ferrari and Palagi2015), carnivores (Nolfo, Casetta, & Palagi, Reference Nolfo, Casetta and Palagi2021), ungulates (Maglieri et al. Reference Maglieri, Bigozzi, Riccobono and Palagi2020), and cetaceans (Maglieri et al., Reference Maglieri, Vantaggio, Pilenga, Böye, Lemasson, Favaro and Palagi2024). Discussing the shared function of laughter and play faces in social bonding across species would strengthen the evolutionary scalability of the proposed model.

Finally, the connection between laughter and humor is worth mentioning. Humor is a complex cognitive faculty that alleviates tension and fosters social bonding through language—an ability with few counterparts in the animal kingdom, although teasing has been increasingly documented (Laumer et al., Reference Laumer, Winkler, Rossano and Cartmill2024). A recent theory suggests that humor primarily serves as a tool for social bonding via linguistic pragmatics, mirroring the role of play and laughter in social cohesion (Flamson & Barrett Reference Flamson and Barrett2008). This insight adds an important piece to the puzzle, reinforcing the evolutionary continuity of these behaviors.

In short, while grooming and other forms of physical bonding may indeed be important for stress management and the formation of social bonds, social play and laughter deserve equal recognition as grooming and other physical bonding as tools for both stress relief and the reinforcement of social relationships. They play a pivotal role in the development of motor and social skills, serving as foundational behaviors in species that rely on cooperation and alliance-building. Furthermore, social play is perhaps one of the best avenues for studying social cognition and communication and its underlying evolution (Palagi et al., Reference Palagi, Burghardt, Smuts, Cordoni, Dall’Olio, Fouts, Řeháková-Petrů, Siviy and Pellis2016).