feeding, self-maintenance and social contexts) but also frequently engage in object play. Although tool use is a relatively rare phenomenon in the primate order, one of our closest living cousins, chimpanzees ( Pan troglodytes), not only regularly engage in tool use and manufacture in a variety of contexts (e.g. Evolutionary explanations for play behaviour focused on its function to provide practice or training for specific behaviours needed as adult individuals, such as for instance food processing or tool use. Importantly, first object play occurs in human children at the age of 12–18 months, when they also start to combine their first spoken words with gestures. Play behaviour consists of solitary and social play and often involves the use of objects. It represents an essential building block to the development of physical and social tactics in life, by which immature individuals learn to explore and manipulate their physical and social worlds. Play has been defined as repetitious behaviour out of ‘serious' contexts that does not serve an immediate purpose. The high impact of play on evolutionary and ontogenetic development has further been emphasized by studies on humans' closest living relatives, the non-human primates (hereafter primates), showing that investments in play can take ontogenetic priority over growth with persisting consequences for life history. Humans' unique creativity and innovation skills have been suggested to be longer-term outcomes and benefits of playfulness. Our study thus strengthens the view that gestures are mutually constructed communicative means, which are flexibly adjusted to social circumstances and individual matrices of interactants. Object-associated and self-handicapping gestures were frequently used to initiate play with same-aged and younger play partners, respectively. Our results demonstrated that the usage of (i) audible and visual gestures increased significantly with infant age, (ii) tactile gestures differed between the sexes, and (iii) audible and visual gestures were higher in interactions with conspecifics than with mothers. We examined the influence of age, sex and kin relationships of the play partners on gestural play solicitations, including object-associated and self-handicapping gestures. We thus carried out the first systematic study on the exchange of play-soliciting gestures in two chimpanzee ( Pan troglodytes) communities of different subspecies. While it is well established that great apes test and practise the majority of their gestural signals during play interactions, the influence of demographic factors and kin relationships between the interactants on the form and variability of gestures are relatively little understood. At a stretch, they speculate, this evolution may have caused the human face “to become more behind-like”.Social play is a frequent behaviour in great apes and involves sophisticated forms of communicative exchange. This evolution, they continued, suggests that the face took over important properties shared with primate buttocks and became the go-to area for “socio-sexual signalling”. “The present study demonstrates that chimpanzees, unlike humans, show a ‘behind inversion effect’ and suggests that identity recognition ‘moved up’ from the bottom to the face in our uprightly walking species,” the team concluded. The results, the researchers speculate, suggest that our reliance on recognising faces to know who’s who could have evolved from an earlier reliance on buttocks. The so-called inversion effect means that humans process information about faces in a totally different way to how they process information about other objects. While the importance of buttocks in chimpanzee society is well-known – the anogenital region of chimpanzee buttocks swells and reddens around the time of ovulation, for example – it wasn’t known if they processed characteristics of individual buttocks in the same way as humans process faces. The image in the top-right shows distinguishing features of chimpanzee buttocks (left) and human faces (right) Mariska E. A chimpanzee is asked to match upright and inverted images of faces, buttocks and feet.
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