When Darwin was being inspired by the giant tortoises of the Galapagos Islands he only considered a subsection of the ways in which natural selection operates. Followers in his giant footprints have discovered the shape of these great reptiles' shells may depend not just on the optimum shape for reaching food, but on the tortoise's capacity to get back on its feet when it has fallen on its back.
Nature has few sights more pitiful than a tortoise struggling to get shell-side up. As Dr Ylenia Chiari of the University of Southern Alabama wrote in Scientific Reports; “Delayed self-righting can result in loss of mating opportunities or death”. So the tortoise that is better at rolling over is more likely to be the one to pass on its genes.
Chiari and her co-authors wondered if this might have contributed to the famous differences in shell shape among Galapagos tortoises. As the vice-governor of the islands at the time famously told Darwin, it is possible to determine the climate of the island from which a tortoise comes based on its shell. Domed shells are associated with wetter islands. Islands with dry lowlands have smaller (although still huge) tortoises with longer necks and “saddlebacked” shells. This shape is thought to allow tortoises to access food beyond the reach of their domed counterparts, but it has never been proven that this is the prime adaptive advantage.
The saddleback shape is less protective against predators, but for most of the tortoises' time on the islands, no such threat existed. A much bigger danger, particularly on steeper islands, was falling down a mountainside and landing the wrong way up.
One might test which shape is more suited to self-righting by pushing a large sample of tortoises onto their backs and timing their restoration, but this would be cruel and perhaps dangerous to the threatened genus. Instead, the team made virtual reconstructions of the shells of 57 domed and 32 saddleback tortoises from five species and modeled the prospects of rolling over if something went wrong. Since the drier islands have more treacherous surfaces, the authors thought saddlebacks would be under greater selection pressure in this regard, and the shape might be optimized for self-righting.
However, after carefully weighing two tortoises on scales containing three force sensors (to determine center of mass), the authors found it takes more energy to turn a saddleback tortoise over than a domed tortoise. To compensate, saddlebacks push with their head on the ground, while the shorter-necked domed tortoises have to swing their appendages until they can get enough momentum to flip over.
We think we hear the IgNobel committee calling.