The different shell shapes of Galapagos giant tortoises affect the amount of energy the animals require to self-right after falling on their backs, according to a study in Scientific Reports. The authors suggest that the evolution of shell shapes may have been partly driven by pressures to improve the ability to self-right.
Galapagos giant tortoises have two main shell types, saddleback and domed, that have evolved multiple times on the archipelago. The different shell types generally occur in different habitats, with saddleback tortoises occupying drier and lower elevation environments, whereas domed tortoises are found in more humid, colder and higher elevation habitats. It has been proposed that the shapes are adaptive for distinct feeding niches; however, varying self-righting performances may also be a driver of shell-shape variation. Galapagos giant tortoises commonly walk on irregular surfaces and often fall on their backs or in crevices between lava rocks and delays in self-righting may therefore increase their chance of mortality.
Ylenia Chiari and colleagues created 3D reconstructions of the shells of 89 Galapagos giant tortoises from three domed and two saddleback species. They then determined the centre of mass for two live, domed, Galapagos giant tortoises (assuming the internal anatomy of saddleback and domed tortoises is not different) to compare self-righting potential. The authors found that, in their models based on shell shape, saddleback tortoises require a higher energy input than domed species to successfully self-right. Saddleback tortoises self-right by vertically pushing their head on the ground and then bobbing their feet, whereas domed tortoises rely on moving their feet and head to gain sufficient momentum to self-right. The authors suggest that several traits associated with the saddleback shell shape could have evolved to facilitate self-righting.