 In the other facial display, the lips are relaxed and the mouth open, and breathing is shallow and staccato, like panting. This vocalization in chimpanzees is on the in-breath: “Ahh ahh ahh.” According to van Hooff, this “relaxed open-mouth display” or “play face” evolved into human laughter. The relaxed mouth in laughter contrasts with the mouth in real aggression that is tense and prepared to bite hard. That difference, combined with the distinctive shallow, staccato breathing pattern, allows laughter to serve as a play signal, announcing that “This is just for fun; it’s not real fighting.” Chimps and gorillas show that face and vocalization during rough-and-tumble play, and it can be elicited in them by the playful grabbing and poking we call tickling (Andrew 1963). 