The evolution of two types of play

Play is an important and understudied class of phenomena that likely serves a critical role in the ontogeny and maintenance of fitness-enhancing behaviors. Many species exhibit little or no play. Among those animals that do play, some exhibit only very simple forms, while others engage in complex play both solitarily and socially. Likewise, some animals play only as juveniles, while others continue to play as adults. We propose a general framework to explain interspecies variation in the evolution and emergence of simple vs. complex forms of play, supported by both a review of the empirical evidence and a novel mathematical model. The emergence of play requires that initial investment returns benefits that sufficiently compensate the opportunity costs associated with simple play. The subsequent evolution of complex play depends upon the interplay of several life-history factors related to the benefits, costs, and time course of play investment. We conclude with implications for understanding the evolution of play across the animal kingdom. Animals vary greatly in how much they play. Why? Some species do not appear to exhibit play at all. Some exhibit only simple, often solitary, play behavior when juvenile, while others engage in complex, often social, play behavior that extends into adulthood. We discuss the evolutionary benefits and costs of play, review interspecies variation, and discuss how that variation relates to the evolutionary tradeoffs during life history. We structure our discussion around a simple mathematical model.