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Pictures illustrating the two variants of the open mouth display (indicated by the blue arrows) performed during play in many non-human primates: (A) play face (PF), in which the mouth is opened and lower teeth are exposed, and (B) full play face (FPF), in which the mouth is opened and both lower and upper teeth are exposed. The group of bonobos (Pan paniscus) housed at La Vallée des Singes (Romagne, France). Photo by Giada Cordoni.
