BLOB, R.W.*; SCANGA, E.L.; WESTNEAT, M.W.: Locomotion in juvenile turtles: effects of size on hindlimb kinematics and muscle function

Juvenile animals may experience different locomotor forces than adults because they are smaller and can have different body proportions. Consequently, limb kinematics and motor patterns might change dramatically as animals grow. We tested for size-correlated changes in locomotor function in turtles and found significant differences between juveniles and adults in both swimming and walking. For example, juvenile softshell turtles (genus Apalone) depress the femur twice as much as adults during swimming (16 degrees vs 8 degrees). By directing their hindlimbs more ventrally than adults do, swimming juveniles can produce a more dorsally oriented net thrust vector that may help compensate for lower buoyancy or the relatively larger juvenile head. Adult and juvenile softshells flex the knee twice during walking steps, once in swing phase and once in stance. In juveniles, strong EMG bursts from the flexor tibialis internus (FTI) accompany both phases of knee flexion, but in adults FTI is active mainly during limb retraction in stance, suggesting that this muscle shifts function as turtles grow, from knee flexor to limb retractor. Like adults, juvenile softshells hold the knee straighter in walking than in swimming. This suggests that, when these highly aquatic turtles move on land, the knee extensor muscles of juveniles and adults begin contraction at a short length and contract over a range of lengths that may be poor for generating force. However, adults hold the knee less straight than juveniles do, so terrestrial force generation by the knee extensors may improve with growth. These results show that despite the appearance of several features of adult locomotion in juvenile turtles, many aspects of limb function change substantially as turtles grow. NIH 2F32NS10813-03 (RB), ONR N000149910184 (MW).