The shells of tumble-prone saddlebacks on the Galápagos Islands may not do them any favors, according to a new study.
The giant tortoises of the Galápagos Islands have no natural predators, but their shells represent a mortal danger of their own. When flipped over, the animals — who regularly weigh in at more than 90 pounds — often struggle to find their feet. If they fail, they eventually die.
And for a giant tortoise with one shell type, the saddleback, big spills are a regular part of life.
“The saddlebacks live in places where you have a lot of lava rocks, so they should fall more often,” said Ylenia Chiari, a biologist at the University of South Alabama, comparing them with domed tortoises, another type that lives on flatter terrain.
Although they are closely related, these two giant tortoises have very different shells. Domed tortoises have rounded shells, and saddleback tortoises have flatter shells with flared edges and a raised neck opening.
Dr. Chiari thought the shells on the saddlebacks, with their edges and corners, had evolved to make it easier for these tortoises to get back up, and set out to test her hypothesis in a study that was published Thursday in Scientific Reports. She was wrong, but her research offered additional insights into the anatomies of these endangered creatures and how they may have evolved to get back on their feet.
To test her idea, Dr. Chiari and her team first made digital 3D models of both types of shells using 89 tortoises, some in the wild and some at the California Academy of Sciences.
The researchers also determined centers of mass for the two different types of tortoises by placing them on an unstable platform and photographing them. The scientists were then able to calculate which shell would require a tortoise to expend more energy when rolling off its back.
The results suggested that a tortoise with a saddleback shell would have to work harder to get back on its feet. In general, the study found, the rounder the shell, the easier it is for the animal to right itself — seemingly an advantage for the domed tortoise.
But there is another significant anatomical difference between the saddleback and domed tortoises: the larger size of the saddleback’s neck opening. This allows the saddleback to extend its longer neck farther, which biologists long assumed was a trait that helped the tortoise reach food in a drier climate.
The shell’s larger front opening also allows the saddleback tortoises to use their long necks to help pick themselves up (they wiggle their feet to shift their balance, too). That hole and the longer necks “could have evolved to overcome the fact that self-righting would have been more difficult in saddlebacks,” Dr. Chiari said, although more research will be required to confirm that idea.