Meeting Abstract
11.2 Monday, Jan. 4 No slip locomotion of hatchling Loggerhead Sea turtles on granular media MAZOUCHOVA, Nicole*; GRAVISH, Nick; SAVU, Andrei; GOLDMAN, Daniel; Georgia Institute of Technology, Atlanta; Georgia Institute of Technology, Atlanta; Georgia Institute of Technology, Atlanta; Georgia Institute of Technology, Atlanta nmazouch@gatech.edu
Sea turtle locomotion occurs predominantly in aquatic environments. However, upon hatching, juvenile turtles (hatchlings) must cross an expanse of unconsolidated particles (beach) to reach the sea. They achieve speeds up to 3 BL/sec on sand, a granular medium that exhibits both solid and fluid-like dynamics. To discover how sea turtles locomote effectively on sand with aquatically adapted limbs, we studied the kinematics of 31 hatchling Loggerhead sea turtles (Caretta caretta) in the field (Jekyll Island, GA, USA) and thrust generation of model limbs in the lab. We challenged sea turtles to traverse a sand filled trackway of controlled packing and incline angle θ, tracking body and limb position using high speed infrared video. We characterized locomotion at θ=0° while flowing air up through the sand to reduce the granular yield stress. For all tested angles and sand preparations the turtles employed a diagonal gait, similar to terrestrial organisms moving on solid ground. Average forward speed v increased linearly with limb frequency f with slope, dv/df, significantly greater than zero (p<0.0001) for all treatments. Compared to the slope at θ=0° with no air flow, dv/df was 2/3 smaller (p=0.049) at θ=20°, while at θ=0° with air flow dv/df shows no statistical difference (p>0.05). We explain the results by balancing frictional and gravitational forces against flipper thrust. Whereas the frictional and gravitational forces are roughly constant, our lab experiments reveal that flipper thrust increases with relative displacement of the limb through the sand (slip). Limb slip without body motion occurs until thrust equals the resisting forces; dv/df is then determined by the effective stride length (stride length in the body frame minus total limb slip).
