The "play face" defuses the meaning of aggression-like elements in ape's and monkey's play. http://booksandjournals.brillonline.com/content/journals/10.1163/1568539x-00003327;jsessionid=159fytl7ydgdp.x-brill-live-03 …pic.twitter.com/d5jC5HsMvc
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@DegenRolf Laughter also signals "this isn't meant seriously", but acoustically, because play-fighting animals often can't see each other.