Relaxed Open Mouth reciprocity favours playful contacts in South American sea lions (Otaria flavescens).

Fine-tuning communication is well documented in mammalian social play which relies on a large variety of specific and non-specific signals. Facial expressions are one of the most frequent patterns in play communication. The reciprocity of facial signals expressed by the players provides information on their reciprocal attentional state and on the correct perception/decoding of the signal itself. Here, for the first time, we explored the Relaxed Open Mouth (ROM), a widespread playful facial expression among mammals, in the South American sea lion (Otaria flavescens). In this species, like many others, ROM appears to be used as a playful signal as distinct from merely being a biting action. ROM was often reciprocated by players. Even though ROM did not vary in frequency of emission as a function of the number of players involved, it was reciprocated more often during dyadic encounters, in which the players had the highest probability to engage in a face-to-face interaction. Finally, we found that it was the reciprocation of ROMs, more than their frequency performance, that was effective in prolonging playful bouts. In conclusion, ROM is widespread in many social mammals and O. flavescens is not an exception. At least in those species for which quantitative data are available, ROM seems to be characterized by similar design features clearly indicating that the signal underwent to similar selective pressures.