When getting back on their feet, shell shape matters for these reptiles
It would be a memorable sight. But it would also be so wrong to tip over Galápagos giant tortoises to see how shell shape affects their efforts to leg-pump, neck-stretch and rock right-side up again.
Shell shape matters, says evolutionary biologist Ylenia Chiari, though not the way she expected. It’s taken years, plus special insights from a coauthor who more typically studies scorpions, for Chiari and her team to measure and calculate their way to that conclusion. But no endangered species have been upended in the making of the study.
“They’re amazing,” says Chiari of the dozen or so species of Chelonoidis grazing over the Galápagos Islands. Hatchlings start not quite the size of a tennis ball and after decades, depending on species and sex, “could be like — a desk,” says Chiari, of the University of South Alabama in Mobile.
Two extremes among the species’ shell shapes intrigue Chiari: high-domed mountains versus mere hillocks called saddlebacks because of an upward flare saddling the neck. Researchers have dreamed up possible benefits for the shell differences, such as the saddleback flare letting tortoises stretch their necks higher upward in grazing on sparse plants.
At the dryer, lower altitudes where saddleback species tend to live, fields of lava chunks and cacti make walking treacherous. “I fell on a cactus once,” Chiari says. Tortoises tumble over, too, and she wondered whether saddleback shells might be easier to set right again.
She went paparazzi on 89 tortoise shells, taking images from multiple angles to create a 3-D computerized version of each shell. Many shells were century-old museum specimens from the California Academy of Sciences in San Francisco, but she stalked some in the wild, too. The domed tortoises tended to pull into their shells with a huffing noise during their time in front of the lens and just wait till the weirdness ended. A saddleback species plodded toward the interruption, though, butting and biting (toothless but emphatic) at her legs.
To calculate energy needed to rock and roll the two shell types, Chiari needed to know the animals’ centers of mass. No one, however, had measured it for any tortoise. Enter coauthor Arie van der Meijden of CIBIO, Research Center in Biodiversity and Genetic Resources at the University of Porto in Portugal. With expertise in biomechanics, he scaled up from the arthropods he often studies. For a novel test of tortoises, he arranged for a manufacturer to provide equipment measuring force exerted at three points under a tiltable platform. As the first giant tortoise, weighing in at about 100 kilograms, started to lumber aboard the platform at Rotterdam’s zoo, Chiari thought, “Oh my gosh, it’s going to crush everything.” For a gentler and more even landing, four men heaved the tortoise into position.
Calculating the centers of mass for Rotterdam tortoises, the researchers extrapolated to the 89 shells. The low, flattened saddleback shape actually made shells tougher to right, taking more energy, the team reports November 30 in Scientific Reports. Now Chiari muses over whether the saddle at the shell front might let freer neck movements compensate after a trip and a flip.
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