The relaxed open mouth is a true signal in dogs: demonstrating Tinbergen's ritualisation process

Play fighting, the most iconic form of social play, is often punctuated by specific signals, such as the relaxed open mouth (ROM) display, limiting the risk of misunderstanding between playmates. Although there is general consensus that the ROM of dogs is a ritualized version of play biting, the empirical demonstration of the actual ritualization of ROM has been lacking. We videorecorded and analysed 118 playful sessions involving 24 Czechoslovakian wolfdogs (12 females; 12 males), which is a breed of domestic dog, Canis lupus familiaris, showing wolf-like behavioural traits. By using an integrated approach of different techniques (dog facial action coding system, an unsupervised cluster analysis and the Levenshtein distance), we empirically demonstrate that the ROM is intrinsically different from the play biting action in this breed of dog. Contrary to the play bite, during ROM, the recruitment of muscular action units for each facial display was more consistent, conspicuous and intra- and interindividually stereotyped. Moreover, a sequential analysis revealed that the ROM usually preceded playful offensive patterns, thus underlining the real metacommunicative function of the signal. Finally, by running a linear mixed model, we found that the most balanced sessions were punctuated by the most prolonged performance of ROM, thus revealing the efficiency of the facial signal in maintaining a balanced session. In conclusion, through the processes of formalization, simplification and emphasis, an ordinary precursor behaviour (i.e. play biting) has been taken out of context and transformed into an extraordinary, derived behaviour (i.e. ROM) specifically designed to attract receivers' attention and modulate playful social interactions in dogs.