PACE, C.M.*; BLOB, R.W.; WESTNEAT, M.W.: Comparative kinematics of the forelimb during swimming in red-eared slider and spiny softshell turtles.

In freshwater turtles the hindlimb typically has been viewed as the primary propulsive organ during swimming. However, in highly aquatic softshell turtles, extensive webbing is present between the digits of the hand as well as the foot, suggesting that the forelimb may also serve as an effective paddle for generating drag-based thrust in these species. To test whether forelimb movements in softshells also were consistent with a role in thrust production, we measured the three-dimensional swimming kinematics of the forelimb in spiny softshell turtles (Apalone spinifera) from digital video and compared these data with similar measurements from red-eared sliders (Trachemys scripta), turtles that frequently travel on land and do not possess webbed front feet. Several aspects of forelimb kinematics differ significantly between these species. In general, the limb segments move in a nearly horizontal plane throughout the stroke in softshells, but in sliders the humerus is raised as much as 30� above the horizontal during protraction before slicing down ventrally through the water during retraction. Peak elbow extension during the power stroke is nearly 50� greater in softshells than in sliders; in addition, the forefoot is oriented perpendicular to water flow through nearly all of forelimb retraction in softshells, but only through the middle third of retraction in sliders. As a result of these latter factors, softshells maintain a much larger surface area than sliders for drag-based propulsion with the forelimbs. Thus, the forelimbs could make much more substantial contributions to forward thrust during swimming in highly aquatic softshell turtles than in more generalized turtles like sliders. NIH 5F32NS1081302 to RB and ONR N000149910184 to MW.