Development of hindlimb locomotor behavior in the frog

Hindlimb motor behavior of the larval frog (tadpole) begins during midlarval life and occurs with increasing frequency until the tail degenerates during metamorphosis. The threshold for hindlimb withdrawal in response to tactile stimulation is low during premetamorphic stages and rises dramatically during metamorphosis. Testing tadpoles in different environments altered the stage of development at which different hindlimb behaviors were first observed but did not change the ontogenetic sequence of behavioral development. However, even under conditions most favorable to hindlimb locomotion, behavioral expression lagged behind electrophysiological expression. The rates of tail beating, hindlimb stepping, and frog kicks are similar to the rate of bursting of tail and hindlimb motoneurons of the isolated nervous system, but their coordination is variable, whereas that recorded from the isolated CNS is fixed. Because neural mechanisms of hindlimb locomotion are functional prior to their behavioral use, the basic hindlimb locomotor circuits must develop without benefit of practice or sensory feedback. However, sensory activity modulates coordination and alters the probability that particular behaviors will be expressed. Implications of these results for studies of early behavior in other species, and the problem of inferring neural maturity from behavioral observations, are discussed.