Turtles are well known for a morphological trait more frequently seen in insects: a hard external shell. In turtles, internal skeletal elements such as the ribs are fused to thick dermal bone to form a stiff protective covering. While this shell provides +10 in defense, it also presents an interesting challenge for locomotion, especially in the water. Most swimming vertebrates like fish and aquatic snakes have soft and flexible bodies that they can bend and twist to effectively propel themselves through the water. Because of their stiff shells, turtles must use their limbs as oars, rowing their arms and legs to swim. But how does their unique morphology affect swimming performance? And do species of turtles with different ecological niches behave differently?
Christopher Mayerl and colleagues at Clemson University investigated these questions by measuring both limb morphology, swimming kinematics, and performance in two species of primarily aquatic turtles from different evolutionary lineages. The Pink-Bellied Sideneck turtle (Emydura subglobosa) is from the suborder Pleurodira, a group with species that tend to be highly aquatic. The Painted turtle (Chrysemys picta) is from the suborder Cryptodira, a group with species that can be semi to fully terrestrial. Mayerl and his co-authors trained the turtles to swim through a tank at full speed, chasing a prey stimulus. At the end of the track was a sharp 90 degree turn. This setup allowed the investigators to measure both stability (how much the turtle resists disturbances) and turning performance (how quickly and sharply the turtle could turn). These measures of performance are often at odds: animals that are more stable are often worse at turning. However, measuring a wide array of performance variables such as heave, pitch, yaw, turning angle, and turning speed, they found surprising performance differences between the two species: the Pink-bellied turtle was both more stable, and more maneuverable, than the Painted turtle.
Mayerl suggests that these performance differences are attributable to both morphology and behavior. The Pink-bellied turtle has longer forelimbs than the Painted turtle, and the distal ends of both the fore and hind limbs have larger surface areas. This increase in surface area improves control. The Pink-bellied turtle used smaller limb movements during steady swimming, which likely helps it to keep stable. However, it also also used faster limb movements and increased limb excursions during turns, elevating its maneuverability. Mayerl suggests that although the Painted turtle itself is primarily aquatic, it may have a disadvantage in swimming performance as a result of its ancestry. Since Cryptodires can be both aquatic and terrestrial, the Painted turtle may have inherited traits that are actually suited for terrestrial locomotion, such as a pelvic girdle that is not fused to their shell. Studies like this one emphasize the importance of examining the interplay between morphology, behavior, and phylogeny when investigating performance.
Cassandra Donatelli is a PhD candidate at Tufts University interested in manouverability and locomotion in fishes and robots.