BLOB, R.W.*; FRALICK, M.S.; Clemson University, SC: Limb kinematics of swimming box turtles: is ancestral function retained despite terrestrial specialization?
Emydid turtles that regularly use aquatic habitats (e.g., sliders, painted turtles) move fore and hindlimbs very differently during swimming. Hindfoot digits are strongly webbed to form a broad paddle in these species. During retraction they extend the hindleg, retract it in a horizontal plane, and rotate the paddle perpendicular to flow to generate drag-based thrust. During recovery the paddle is feathered to minimize drag. In contrast, the forefeet of these species are not strongly webbed, and their movements do not minimize drag during recovery (e.g., forefoot feathering is poor), or optimize rearward thrust generation. Box turtles are derived, highly terrestrial emydids that have lost hindfoot webbing. To test if box turtles retain ancestral patterns of aquatic limb movements despite shifts in morphology and preferred habitat, we measured three-dimensional limb kinematics from high-speed video of three-toed box turtles (Terrapene carolina triunguis) swimming in a flow tank. Box turtles appear to minimize recovery-phase drag less effectively than more aquatic emydids, flexing the elbow 25° less than sliders and maintaining a high-drag orientation of the hindfoot (as well as the forefoot) during recovery. However, box turtles can swim at speeds similar to sliders (> 1BL/s), and display limb movements that could help maximize effective thrust generation. During the power stroke, box turtles show almost 30° more elbow extension and retract the femur further posteriorly than other emydids (which typically end retraction with the femur perpendicular to the direction of travel). Thus, although box turtles may have lost little of the swimming proficiency of their aquatic ancestors, these terrestrial specialists achieve swimming proficiency through substantially different limb kinematics.
