Play and energy regulation in mammals.

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.