REFINING THE MOTOR TRAINING HYPOTHESIS FOR THE EVOLUTION OF PLAY

The motor training hypothesis, proposed in its first form nearly half a century ago and broadened subsequently, states that the function of play is adaptive modification of the developing neuromuscular system. Evidence from many mammalian species indirectly supports the motor tra ining hypothesis, but the exact nature of developmental change prompte d by play remains unknown. We reviewed literature on the anatomical an d physiological effects of exercise in mammals and categorized these a s effects available to individuals at any age, versus effects availabl e only during a discrete period of postnatal development, and transito ry effects, which decay soon after exercise ends, versus permanent eff ects. We found that most effects are available at any age and are tran sitory; we argue that they are not likely primary benefits of play. Ho wever, two effects that influence motor performance-modification of ce rebellar synaptogenesis and modification of skeletal muscle fiber type differentiation-are available only during a short period of postnatal development and appear to be permanent. In three species for which bo th kinds of data were available, the age distribution of play closely matched the age distribution of these two types of experience-modifiab le development. We propose that play may not be motor training in the broad sense, but rather it may be behavior designed to influence speci fic types of development.