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Analysis of ecological constraints on juvenile mammals suggests that energy expended in play behavior does not reduce fitness, but actually increases it. When viewed as a promoter of adaptive energy loss, play can be considered an antipredator strategy. In addition, it may balance a low-protein diet in favor of growth, as well as increase resistance to pathogens and to cold exposure. These short-term benefits result from activation of the sympathetic nervous system (SNS), which is hypothesized to occur during play. SNS activation increases heat production in brown adipose tissue. The energy-regulation approach generates many predictions that are supported in the literature, and others that can be empirically tested.
Nigel Barber (Sat,) studied this question.