For most vertebrates, locomotor activity begins at the time of hatching or birth. Although handicapped by small size, rapidly growing tissues, and naivete juveniles of most species must maneuver in the same environment and avoid the same predators as adults. Thus, it is not surprising that some ectothermic and precocial endothermic tetrapods undergo ontogenetic changes that allow juveniles to sprint almost as fast and jump almost as far as adults Allometric changes that have been shown or suggested to enhance performance in juveniles include relatively longer limbs, relatively greater muscular forces and contractile velocities, and higher muscular mechanical advantage. Compensation for rapid growth has been shown to occur in the bones of precocial birds and mammals The limb bones of these animals have relatively greater cross-sectional diameters and areas than those of adults. This maintains a parity of bone and muscular strength during periods of rapid growth, when hones ave composed of weaker more flexible tissue. In contrast to their sprinting and jumping performance, young animals appear to have significantly less locomotor stamina and agility than adults. The lower stamina may, in large part, simply, be a consequence of juveniles being smaller than their parents. The awkwardness of youth appears to result from a conflict between the process of growth and the effective integration of the sensory, neural control, and motor systems. Because juveniles often suffer higher rates of mortality from predation, selection for improved locomotor performance is likely to be strong. Consequently, as a possible result of ontogenetic canalization, the adult phenotype may be determined as much or more by selection on the locomotor performance of juveniles as by direct selection on the locomotor abilities of adults.