Shell suits

It’s a problem not widely faced amongst animals: but, for those such as beetles and turtles with hard, round shells, it is serious. Ending upside down presents a major, lifethreatening problem if they are unable to right themselves pretty quickly. A new study of this convex problem is reported by Gabor Domokos at the Budapest University of Technology and Economics and Peter Varkonyi at Princeton University, in the Proceedings of the Royal Society (series B) published online. A common strategy for beetles and turtles involves active mechanisms, using the legs, and for beetles, wings, to right themselves. Aquatic turtles, with flattened shells, face a particular problem and use their long muscular neck to flip back. But terrestrial turtles with high-domed shells and much shorter necks have no apparent active means to right themselves. So the researchers looked at the properties of the shells themselves in these species. Based on field data gathered on a broad range of aquatic and terrestrial turtle species, the authors have developed a geometric model of the shell. They find that the geometry of the shells in highly domed terrestrial species is almost optimal in shape for the animals to get back on their feet when they find themselves upside down. Many factors have been identified to affect the shape of turtles. The authors note that in some species there is a sexual dimorphism in shell shape where males are often overturned during combats and the male shell has adapted to this outcome. Flat shells with sharp and smooth edges are advantageous for swimming and for digging, respectively. On the other hand, increased shell height was found to offer better protection against the snapping jaws of predators; it also protects from dessication and improves thermoregulation. While these factors indicate that higher shells might be an advantage for these animals, they do not predict this, nor do they determine the optimal height/­width ratio. The new work on shell structures not only shows that there exists a narrow optimal range of height/­width ratio for self-righting in a terrestrial environment, but also predicts the optimal geometry. The researchers found that the height/­width ratios of highly domed species, such as Geochelone elegans and G. radiata, are close to the optimum for the trade-off between self-righting and factors penalising increased height, such as decreased stability. These animals also showed a low centre of gravity, which also helps. Through limb and neck movements, the animals are more easily able to get the base of their shell to vertical and then right themselves. So being able to get from being on their back to being on their side helps them get back on their feet.