The joy of science communication

social systems, where the number of play partners of an individual is more restricted as only few can be trusted as play-mates. For bonobos, it seems it is safe to play with anyone. Play-partner diversity is important when thinking about adaptability: playing 100 times with the same individual requires less variation and adjustment of behaviour than playing 10 times with 10 different individuals. Playing with individuals of different sizes, personalities and sex requires learning about contextual-dependent behaviour: with whom and when a bite is appropriate, a chase over a push, a gentle tickle rather than a stomping slap, and so on. An example is selfhandicapping — when individuals self-adjust their strength in order to maintain playful interactions with animals of different size, status and ability. In short, because social play only happens when individuals learn to adjust to each other’s differences, diversity of play partners boosts experiential complexity and thus behavioural repertoires. There is another fundamental aspect to social play that is relevant to adaptability: positive emotion that is salient. Driven by positive emotion, play rewards engagement with the world, and in doing so it increases yet further the complexity to which adult brains are exposed. While this is of course very important for the development of immatures, we know that adult brains remain plastic and indeed match even slight variations in environmental enrichment. Complex systems (such as brains!) tend to show increased flexibility of responses with increased complexity. Interactions other than play can have the same result of course, but salient emotion makes play a particularly time-efficient mechanism of creating complexity — partly because it is so rewarding. Emotional salience has a strong effect in learning, memory and bonding intensity. So play — short in duration, diverse in expression — can serve as a ‘fast-track’ cognitive and bonding boost (which is especially useful given adults’ constrained time budgets). The human equivalent would be the efficacy that a good laugh or dance session has in bonding us with others compared to less salient activities such as plain conversation by the water cooler. Adult play can be an intense ‘fast-track’ creator of experiential complexity. The rewards of adult play Understanding the Peter Pan apes gives us insights as to why socially complex species may play into adulthood. Since the social world is necessarily unpredictable, cognitive flexibility is key for successful context-dependent strategies. This is true for immatures and I believe it to be particularly important for adult individuals that face decades of living in complex societies, where responding appropriately to changing contexts is crucial. I have termed this view of play ‘the Adaptive Joker hypothesis’ — play behaviour is the jester, a joyful shape-shifter, a biological wild card whose value and form changes according context. Play makes individuals more adaptable because it makes them more social; and more successful in their sociality as a result of being more adaptable. Life-long play is a bridge between sociality and adaptability. Play also rewards interaction with the world, driving cognitive complexity and adaptability. In the course of hominid evolution there has been an up-regulation of the reward system – which underlies positive motivational drives such as exploration and novelty-seeking. Thus humans and other highly social, big-brained animals are particularly fond of finding myriad ways to amuse themselves. Heightened positive emotion rewards seeking that complexity, and result in more adaptable brains — at all ages. Hence the biology of fun does not only belong in childhood.