Testudines are susceptible to inversion and self-right using their necks, limbs, or both, to generate enough mechanical force to flip over. We investigated how shell morphology, neck length, and self-righting biomechanics scale with body mass during ontogeny in Chelydra serpentina, which uses neck-powered self-righting. We found that younger turtles flipped over twice as fast as older individuals. A simple geometric model predicted the relationships of shell shape and self-righting time with body mass. Conversely, neck force, power output, and kinetic energy increase with body mass at rates greater than predicted. These findings were correlated with relatively longer necks in younger turtles than would be predicted by geometric similarity. Therefore, younger turtles self-right with lower biomechanical costs than predicted by simple scaling theory. Considering younger turtles are more prone to inverting and their shells offer less protection, faster and less costly self-righting would be advantageous in overcoming the detriments of inversion.

The experimental setup consisted of a force plate, with a pressure pad on top, covered by a thin rubber mat. The force plate (3D Force Plate Type 9286B, Kistler® Instruments Ltd, Hook, Hampshire, UK) was used to measure the vertical reaction force exerted by a turtle during self-righting. Force data were recorded (at 420 Hz), using the BioWare® data-acquisition software (type 2812A, Kistler®). To measure the relative contributions of the neck and body to the vertical force, a pressure pad (Pressure Mapping Sensor 7101, Tekscan, Inc, South Boston, MA, USA) was placed on top of the force plate. The pressure pad data were recorded (at 100 Hz) using the FootMat® Research software (v 7.1, Tekscan). A camera (Sony® Cyber-shot RX10 III) was used to record videos (at 100 fps) of the self-righting movements. The self-righting times were calculated from videos, using Tracker (https://physlets.org/tracker; The Open Source Physics Project). The self-righting time is defined as the duration from the moment a turtle’s head first contacted the pressure pad/force plate until the head was no longer in contact with the setup (see Fig. 1A-D).