Self-righting potential and the evolution of shell shape in Galápagos tortoises

Abstract

Self-righting, the capacity of an animal to self-turn after falling on its back, is a fitness-related trait. Delayed self-righting can result in loss of mating opportunities or death. Traits involved in self-righting may therefore be under selection. Galápagos giant tortoises have two main shell morphologies - saddleback and domed - that have been proposed to be adaptive. The more sloped shape on the sides of the shell and the longer extension of neck and legs of the saddlebacks could have evolved to optimize self-righting. The drier environments with more uneven surfaces where the saddleback tortoises occur increases their risk to fall on their back while walking. The ability to fast overturn could reduce the danger of dying. To test this hypothesis, we used 3D shell reconstructions of 89 Galápagos giant tortoises from three domed and two saddleback species to compare self-righting potential of the two shell morphotypes. Our results indicate that saddleback shells require higher energy input to self-right than domed ones. This suggests that several traits associated with the saddleback shell morphology could have evolved to facilitate self-righting. Studying the functional performances of fitness-related traits, as in this work, could provide important insight into the adaptive value of traits.

Figure 1

Saddleback (right) and domed (left) shell morphotypes in Galápagos giant tortoises. Photos by Y. Chiari.

Figure 2

Schematic of the experimental approach used to calculate the COM. (A) Frontal view of the platform placed on three force transducers. (B) Dorsal view of the tortoise on the platform. Positions of the supporting force transducers (a., b., c.) and the COM are indicated. (C) The platform was tilted to allow measurement of the vertical position of the COM. Gray lines indicate the tilted platform. (D) Dorsal view showing the position of the tilted platform in gray. (E) Detail of the displacement of the COM: a. indicates the total horizontal displacement of the COM, b. shows the horizontal displacement due to the vertical position of the COM relative to the platform.

Figure 3

Schematic view of the self-righting movement of a tortoise shell, from a stable position with the animal overturned (upside down) to a stable position on its four feet. h _min and h _max are indicated in the figure, as well as the center of mass (COM).