Posthatching locomotor experience alters locomotor development in chicks.

We have previously demonstrated that, even though chicks are very precocial and can locomote within hours of hatching, they require a period of time to develop a mature stable walk. As an example, 1- to 2-day-old animals move with disproportionately small stride lengths compared with 10- to 14-day-old animals. The purpose of this study was to determine whether the maturation of walking, including the development of a mature stride length, depends on locomotor experience. We also investigated the development and experience-dependence nature of head bobbing, an optokinetic behavior that occurs during walking in birds. Chicks were randomly assigned to one of three groups receiving either increased locomotor experience (i.e., treadmill exercise), decreased locomotor experience (i.e., decreased housing space), or no alteration in locomotor experience. To assess the dependence of locomotor maturation on N-methyl-D-aspartate (NMDA)-type glutamate receptors, animals in each group were either given an NMDA antagonist (MK-801, 1 mg/kg intramuscularly daily) or saline control. Locomotor characteristics (stride length, leg support durations, horizontal head excursions) were quantified from videotaped recordings of chicks walking overground unrestrained on posthatching days 1, 2, 4, 6, 8, and 10. Animals subject to exercise restriction for at least 6 days moved with shortened stride lengths compared with age-matched treadmill-exercised or control animals, a change that was maintained for the duration of the study. NMDA antagonism also resulted in shortened stride lengths. Head bobbing behavior matured during the same posthatching time period. The rate of this maturation was also decreased by exercise restriction. Thus locomotor experience is required for normal development of locomotor behavior, even in very precocial animals. These results are discussed in terms of the possible neuroanatomical and neurophysiological mechanisms underlying experience- and activity-dependent changes during motor development.