These behaviors are not exclusive to the play context and can be found in contexts that are more aggressive. Many expressive body behaviors are observed during chimpanzee play including hitting and kicking, raised arms, ground slaps, foot stomps, pokes, head bobs, hand claps, and throwing (Tomasello et al., 1994 McCarthy et al., 2013). Play faces can play a role in initiating and maintaining play (Tomasello, 2008), and matching of play faces and laughter by social partners prolongs the duration of play bouts (Waller and Dunbar, 2005 Davila-Ross et al., 2011). Play faces (relaxed open mouth displays with the teeth either covered by the lips or exposed to varying degrees) and the laughter-like vocalizations which sometimes accompany play faces (soft, breathy pants or grunts) appear almost exclusively during play (van Hooff, 1973 Parr et al., 2005 Davila-Ross et al., under review). These expressions convey information about an individual's motivations, intentions, and emotions, which may influence the recipient's behavior (see Owren et al., 2010 Seyfarth et al., 2010, for debate on the importance of information vs. Here we explore the rate of playful facial and body expressions in solitary and social play, and the extent to which social partners match expressions, which may illuminate a route through which context influences expression.Ĭhimpanzee play is punctuated by a variety of facial, vocal, and body expressions. Moreover, social partners sometimes match playful expressions, which prolongs play bouts (Davila-Ross et al., 2011). Comparisons across contexts are essential in evaluating the extent to which these expressions function as social signals, expressions of individuals emotional state, or some combination (Seyfarth and Cheney, 2003 Gaspar, 2006). Therefore, the study of playful expressions is incomplete without considering their occurrence in a variety of social and solitary contexts. However, solitary play is a distinctive feature of chimpanzee infancy with playful expressions being reported during solitary play (Cordoni and Palagi, 2011). Given that playful expressions emerge early in life and continue to occur in solitary contexts through the second year of life, we suggest that the play face and certain body behaviors are emotional expressions of joy, and that such expressions develop additional social functions through interactions with peers and older social partners.Ĭhimpanzee playful expressions have typically been studied within social contexts, driven primarily by an interest in communicative function. Matched expression rates were highest when playing with peers despite infant expressiveness being highest when playing with older chimpanzees. Social partners matched both infant play faces and infant body expressions, but play faces were matched at a significantly higher rate that increased with age. Among the most frequent types of play, mild contact social play had the highest rates of play faces and multi-modal expressions (often play faces with hitting). However, modalities of playful expression varied with type of play: in social play, the rate of play faces was high, whereas in solitary play, the rate of body expressions was high. No group or age differences were found in the rate of infant playful expressions. Naturalistic observations of seven chimpanzee infants ( Pan troglodytes) were conducted at Chester Zoo, UK ( n = 4), and Primate Research Institute, Japan ( n = 3), and at two ages, 12 months and 15 months. Here we explore the rate of playful facial and body expressions in solitary and social play, changes from 12- to 15-months of age, and the extent to which social partners match expressions, which may illuminate a route through which context influences expression. Knowledge of the context and development of playful expressions in chimpanzees is limited because research has tended to focus on social play, on older subjects, and on the communicative signaling function of expressions.
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