Swinging hips help turtles take greater strides
CLEMSON, South Carolina — Turtles have a reputation.
“They’re slow, they’re clumsy and the shell just gets in the way of everything,” said Richard Blob, a biologist at Clemson University who specializes in studying how animals have evolved to move the way they do. But, Blob adds quickly, “I don’t think that’s the case anymore.”
Fueling the pokey reputation is a long-held belief that a turtle can’t move its pelvis or hips. Until recently, however, nobody has been able to see under, or through, a turtle’s shell to confirm that notion.
Thanks to technology developed at Brown University and research by Blob; graduate student Christopher Mayerl; and Elizabeth Brainerd, a Brown biologist, the myth has been busted. At least, in part. In a paper published by the Journal of Experimental Biology, Mayerl, Blob and Brainerd describe seeing the pelvis of a cryptodire turtle — a river cooter — roll, pitch and yaw, much like a pelvis in any other vertebrate. It was the first time anyone had seen a turtle pelvis in motion.
The study helps explain why turtles live where they live and do what they do: form and function are complimentary. It also helps inform conservationists about what the animals could be at risk of if their environments change, Blob said. And, because the turtle spine is fused, it has fewer moving parts, which makes it easier to study how important the pelvic girdle is to locomotion.
“The way that animals move is important for almost everything they do in every species: to escape, to find food and to find safe places to live,” said Mayerl, a doctoral student in Blob’s Evolutional Morphology and Biomechanics Lab at Clemson.
Almost every animal in the lineage known as tetrapods, which includes all reptiles, amphibians, birds and mammals (including humans), rotates its hips to take longer strides, which helps them move faster, Mayerl said.
“Previous research suggested that turtles couldn’t move their pelvis in any way,” said Mayerl, the study’s first author. “Unlike with a lizard or a human or a mouse, you can’t just film a turtle and watch the hips move.”
Because, you know, that whole shell thing.
Brainerd provided a way to see through the shell with XROMM, which stands for X-ray Reconstruction of Moving Morphology. The technology combines CT and X-ray images to create a 3-D video of an animal’s movement.
“XROMM is like having X-ray glasses to look inside animals and see their skeletons in motion,” Brainerd said. “XROMM uses animation techniques from the entertainment industry, but XROMM animations are not cartoons or simulations. They are precise and accurate reconstructions of how the bones of a specific animal moved in real life.”
The XROMM technology creates new opportunities for research, Blob said.
“For example, now we can actually see if bones that differ in shape across species also move differently and provide a different performance — longer steps, bigger bites and so on. Likewise, if differently shaped bones all moved in the same way, we can look for different reasons why some species take longer steps or bigger bites than others,” he said.
For the study, Mayerl, Blob and Brainerd compared two very different groups of turtles: cryptodires, represented by river cooters, and, pleurodires, represented by pink bellied sliders (not related to red eared sliders). The separation of the groups resulted from an evolutionary split some 140 million years ago (turtles predate dinosaurs), when the pelvis of the pleurodires fused with the top and bottom of its shell. Pleurodires remained mostly aquatic animals, Mayerl said, while cryptodires split off into numerous different species, including most tortoises and turtles, even terrestrial ones.
A 1971 imaging study of a painted turtle, part of the cryptodire group, tried to determine if the pelvis moved and no movement was detected. But the study had several technical limitations and there has been speculation that a cryptodire might be able to move its pelvis.
The pelvis of pleurodires, because it’s fused, was never considered to be moveable.
Mayerl and Blob transported their turtles from the Clemson labs to Brainerd’s lab in Providence, Rhode Island, and put them through their paces in water and on a treadmill with cameras trained on the pelvis.
“What was shocking was how much the cryptodire’s pelvis moved,” Blob said. “If you were to pick up a turtle skeleton, you could imagine some of the bones are sliding around in there. But you still think, the turtles are inside a box, all the bones are inside a box. That’s got to limit what it can do,” Blob said.
“It doesn’t limit it at all,” he said. “Those hips are swinging around just like they are in just about any other kind of animal.” The turtles were able to extend their strides to move faster.
The pleurodires’ pelvis stayed fixed and its range of motion was less than the cryptodires’, but that also creates new questions. Animal morphology, or body design, helps explain behaviors.
“There’s a reason why Michael Phelps is such a good swimmer,” Mayerl said. “Sure, he practices a lot, but he also has certain morphological traits that enable him to swim great and be the best.” Phelps’ disproportionately long hands, arms and torso are said to give him an advantage in the water.
Likewise, a cryptodire can move faster across land, but a pleurodire, with its evolved, fused pelvis, may be more stable in water, where they live.
The study, Blob said, “gives us a new appreciation for the way turtles move.”
“We imagine turtles being limited, and instead maybe we need to think about them as actually having specializations, just for a different way of life than us,” he said. “They’re not restricted, they just do something else well.”
The research was funded by the Company of Biologists (publishers of the Journal of Experimental Biology), Clemson University and the National Science Foundation.
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