Meeting Abstract
Many legged animals regularly transition between aquatic and terrestrial environments. While on land, limbs must bear the entire weight of the body; however, when moving at different water depths, body weight is buoyed by the surrounding fluid. In simulated reduced gravity conditions, during which the limbs support only a portion of the body weight, humans do not move in a dynamically similar manner. Here we test whether dynamic similarity principles apply across size and water depths in turtles. We hypothesize that similarly-sized turtles will not move dynamically similarly across different submersion levels, but that turtles of different size will move in a dynamically similar manner at similar water depths. To address these hypotheses, we filmed eleven semi-aquatic red-eared slider turtles (Trachemys elegant scripta) grouped into three size classes: small, medium and large (mass: 6.2 – 106.8 g, carapace size: 3.0 – 9.8 cm) while moving in different water depths: 0% (terrestrial), 25% (below plastron), 75% (half of carapace) and 100% water (fully submerged). Turtles within the same size class did not move in a dynamically similar fashion at different water depths (duty factor: p<0.001: mixed-model ANCOVA). At similar Froude numbers only the medium and large turtles moved dynamically similarly for 0 and 25% submergence, whereas at 75% submergence all size classes moved in a similar fashion. At 100% submergence, differences in stride frequency and duty factor revealed three distinct locomotor modes: swimming, bottom walking, and hindlimb pushing. These results show that dynamic similarity is largely preserved between different size classes within each water depth, except when the turtles are fully submerged.
