The evolutionary origins of play revisited: lessons from turtles / Gordon M. Burghardt -- Play in common ravens (Corvus corax) / Bernard Heinrich & Rachel Smolker -- Object play by adult animals / Sarah L. Hall -- Kangaroos at play: play behaviour in the Macropodoidea / Duncan M. Watson -- Intentional communication and social play: how and why animals negotiate and agree to play / Marc Bekoff & Colin Allen -- Structure-function interface in the analysis of play fighting / Sergio M. Pellis & Vivien C. Pellis -- Sparring as play in young pronghorn males / Michelle N. Miller & John A. Byers -- Squirrel monkey play fighting: making the case for a cognitive training function for play / Maxeen Biben -- Self assessment in juvenile play / Katerina V. Thompson -- Biological effects of locomotor play: getting into shape, or something more specific? / John A. Byers -- Neurobiological substrates of play behavior: glimpses into the structure and function of mammalian playfulness / Stephen M. Siviy -- Play as an organizing principle: clinical evidence and personal observations / Stuart Brown.