Turning turtle: scaling relationships and self-righting ability in Chelydra serpentina.

Ilan M Ruhr,Dane A Crossley,Kayleigh A R Rose,Jonathan R Codd,William I Sellers

doi:10.1098/rspb.2021.0213

Abstract

Testudines are susceptible to inversion and self-righting 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.

Highlights

Predator–prey dynamics drive adaptations in animals, including the evolution of protective armour
Isometry) across body mass (Mb), all linear dimensions were expected to scale to Mb1/3; force was expected to scale to Mb2/3; kinetic-energy equivalent (KEE) should scale as Mb4/3; self-righting time should scale as Mb1/2; and mean power-output equivalent (PE) as Mb5/6
We show that a simple geometric model, based on body mass, predicts shell shape and selfrighting time when neck force is applied, in C. serpentina

Summary

Introduction

Predator–prey dynamics drive adaptations in animals, including the evolution of protective armour. The aim of this study, was to examine the influence of body mass, shell shape and neck length on neck-powered self-righting ability and the accompanying biomechanical costs, in a freshwater turtle species, C. serpentina. We would expect scaling of mass4/3 for the selfrighting effort of geometrically similar shell shapes, we predicted that, due to selection against the possible increase in risks associated with being inverted, self-righting should be easier in younger/smaller turtles, which would be reflected in the speed and biomechanical cost. Isometry) across body mass (Mb), all linear dimensions were expected to scale to Mb1/3; force was expected to scale to Mb2/3; KEE should scale as Mb4/3; self-righting time should scale as Mb1/2; and mean PE as Mb5/6 Formal derivations of these predicted relationships are in the electronic supplementary material. For the derivation of these scaling relationships, see Derivation of Scaling Predictions in the electronic supplementary material

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Perspectives