The rough-and-tumble play of rats as a natural behavior suitable for studying the social brain

social play, rats, social dynamics, social skills, individual differences Play fighting, the most commonly reported form of social play, involves competition to gain an advantage (Aldis, 1975), but there are two features that make it different from serious fighting.First, it is highly pleasurable and associated with a positive affective state (Vanderschuren et al., 2016).Second, the competition is moderated by cooperation, ensuring that the interactions have a degree of reciprocity or turn taking between partners (Palagi et al., 2016a).That is, the player with the advantage may voluntarily relinquish it, thus allowing a role reversal to occur, with the original defender gaining the opportunity to become the attacker (Pellis and Pellis, 2017).Thus, although actions performed during play fighting can be accurately described as involving attack, defense, and counterattack, the context of their use should not be confused with that of aggression.For this reason, we will refer to this kind of play as rough-and-tumble play (RTP), to highlight the cooperative aspect of these interactions.Nonetheless, competition during RTP can create ambiguity as to whether a partner may be taking unfair advantage of the situation, with effective communication being important to avoid the risk of escalating to aggression or to partners being ostracized if they play too roughly (Palagi et al., 2016b).Creating and resolving ambiguity, which requires balancing competition and cooperation, also provides a vehicle by which juveniles and adolescents can train socio-cognitive skills (Pellis and Pellis, 2009, 2017).Laboratory rats have been an important model species with which to study the neurobiology of RTP (Siviy and Panksepp, 2011;Siviy, 2016;Vanderschuren et al., 2016).As shown in Figure 1, RTP in rats involves competition to gain access to the partner's nape of the neck, which is nuzzled with the snout if contacted (Pellis and Pellis, 1987;Siviy and Panksepp, 2011).A variety of tactics are used to attack and defend the nape, including launching counterattacks following a successful defense (Himmler et al., 2013;Pellis et al., 2022).Rats are a particularly good model species for studying RTP, as this behavior not only differs from serious fighting in the ways described above, but also because serious fighting involves attacking other body targets, namely the flanks and rump, which are bitten if contacted (Blanchard et al., 1977;Pellis and Pellis, 1987).Consequently, it can be readily discerned when a playful encounter escalates to serious fighting, as the aggressor switches from attacking the nape to biting the partner's posterior (Stark and Pellis, 2020).

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