Smiling underwater: Exploring playful signals and rapid mimicry in bottlenose dolphins

Play is a widespread behavior present in phylogenetically distant taxa that, in its social form, relies on complex communication. Playful communication has been largely neglected in marine mammals. We focus on playful visual communication in bottlenose dolphins. The open mouth (OM) display was mainly emitted during social than during solitary play and occurred more frequently when the sender was in the receiver's field of view, suggesting that animals are attentive to the playmate's attentional state. Detecting an OM evoked the same facial display in the receiver, a result that strikingly matches with those obtained on cooperative social primates and carnivores. It is difficult to know whether such similarities derive from shared evolutionary pathways (homology) or from evolutionary convergence (homoplasy), as both have been suggested for play behavior. The pervasive presence of OM and rapid mimicry in the mammal phylogenetic tree indicates the relevance of visual mechanisms in shaping complex communication.