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.

Scutation asymmetries in red-footed tortoise Chelonoidis carbonaria Spix, 1824 (Testudines: Testudinidae)

“…The differences in the scutes from the left and right sides are consistent with the data obtained by many other researches performed in turtles and tortoises (Davis & Grosse, 2008;Macchi et al, 2008;Buică & Cogălniceanu, 2013;Cherepanov, 2014;Claude, 2016). Self-righting, the capacity of an animal to self-turn after falling on its back, which is a fitness-related trait (Domokos & Varkonyi, 2008;Chiari et al, 2017), is rejected, as no correlation was observed between extracted size and residual shape variables, as it would be logical if the FA had a this functional sense. The possibility of being a secondary sexual characters is also rejected, although being different between males and females, as it would need to appear with a negative relationship (larger sexual traits are presumably more costly to produce, which should lead to greater developmental stress and corresponding increases in asymmetry) (Jennions, 1996).…”

Performance in three shell functions predicts the phenotypic distribution of hard‐shelled turtles

“…But so too do island giant species such as Aldabrachelys or members of the Chelonoidis nigra complex (which can experience selection for self‐righting as adults (Chiari et al. )), and they do not occur particularly close to the unoccupied optima. These species are all closer to peaks with higher weights for strength–perhaps juvenile predation places selective pressure on juvenile shell shape, constraining potential adult shapes.…”

Biomechanical factors influencing successful self-righting in the pleurodire turtle,Emydura subglobosa

“…Certain shell shapes are thought to facilitate self-righting and reduce the energy required to successfully flip (Ashe, 1970), but the ability to self-right is important for all species of turtle. By integrating biomechanical data with morphological comparisons, it may be possible to identify traits that enable turtles to self-right despite morphological constraints (Chiari et al, 2017).…”

“…Turtles use a variety of strategies that are thought to be primarily dependent on shell morphology, which itself is strongly correlated with the habitat in which a species lives. In terrestrial taxa such as tortoises, the carapace (top of the shell) is typically domed, and turtles use a strategy primarily involving movements of the limbs to shift the center of mass and induce rolling of the body (Ashe, 1970;Chiari et al, 2017). However, in aquatic species with a flatter, more streamlined carapace, structures such as the limbs and head (though typically not the tail, which is reduced in most turtles) may be able to reach the ground and act as levers to flip the body (Ashe, 1970).…”

“…Previous studies have evaluated how various morphological factors influence self-righting in turtles, often through comparisons of how long it takes until an attempt is made (latency) or how long it takes until an attempt is successful (duration) (Mann et al, 2006;Delmas et al, 2007;Domokos and Varkonyi, 2008;Golubovicé t al., 2013;Mitchell et al, 2016;Chiari et al, 2017). However, this approach only provides insight into the correlation between morphological variation and the end-product of the self-righting behavior.…”