Shell shape does not accurately predict self-righting ability in hatchling freshwater turtles

The different degrees of hydrodynamic shell shape in freshwater turtles
likely represent an evolutionary trade-off between adaptation to an
aquatic lifestyle and the movement capabilities of a larger and more
rounded shell. Trade-offs often result in compromises, and this is
particularly true when these turtles must self-right to avoid the negative
effects of inverting. With flatter carapaces aquatic turtles,
theoretically, must invest more biomechanical effort than their
terrestrial counterparts with their more rounded carapaces, to achieve
successful and timely self-righting. This places the hatchlings of
freshwater species in a precarious position; more prone to inversion and
predation than adults and with shells seemingly maladapted to the act of
self-righting. Here, we examine the self-righting performance in three
morphologically distinct freshwater turtle species (Apalone spinifera,
Chelydra serpentina and Trachemys scripta scripta) that inhabit similar
environmental niches. We demonstrate that hatchlings of these species were
capable of rapid self-righting and used considerably less biomechanical
effort relative to adult turtles. Despite differences in shell morphology
the energetic efficiency of self-righting remained remarkably low and
uniform between the three species of freshwater hatchling. Our results
confound theoretical predictions of self-righting ability based solely on
shell shape metrics and indicate that other morphological characteristics
like neck or tail morphology and shell material properties must be
considered to better understand the biomechanical nuances of Testudine
self-righting.