The shells of tumble-prone saddlebacks on the
Galápagos-Islands may not do them any favors, according to a new study.
By DOUGLAS QUENQUA NOV. 30, 2017, New York
Times
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.
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