The Essential Turtle
OVERLEAF
OPPOSITE
I ONCE ASKED a professor of mine,
a noted authority on the anatomy and
biology of turtles, why he had devoted
his life to their study.
“I have great respect,” he replied solemnly, “for any animal that can
get its shoulder girdle inside its rib cage.”
A Green Sea Turtle
over the reefs of Sipadan
Island, Sabah, Malaysia,
beneath a school of
The carapace and plastron
of these Steindachner’s
Snake-necked Turtles
(Chelodina steindachneri)
are joined together by a
bony bridge.
My professor, Dr. Thomas S. Parsons, was only half joking. He had,
in fact, put his finger on one of the things that makes turtles so puzzling
and fascinating: the fact that almost everything about them, from their
inside out, represents a tremendous evolutionary distortion of what we
expect from the body of a land vertebrate. That distortion—including
the weird relationship between ribs and shoulder girdle that so fasci-
nated Dr. Parsons—is the product of a marvelous series of evolution-
ary adjustments that have refitted turtles for life encased in their single
most notable and distinctive feature: the shell. The shell does not merely
encase a turtle; to a great extent, it defines what a turtle is. It has meant
redesigning much of a turtle’s internal anatomy simply to allow it to
carry out normal, everyday functions—functions as basic as breathing.
How does a turtle, locked inside its shell, take a breath? Other
armored vertebrates, such as armadillos, have soft bellies that can move
as they inhale or, like the trunkfishes puttering around coral reefs, do
not have lungs at all. Only turtles face the challenge of filling their lungs
while sealed, above and below, in a case of armor (though admittedly
the plastron, the portion of the armor covering the belly, may be flexible,
especially in sea turtles). They cannot expand their ribs to breathe, as we
do, because their ribs are fused to the shell itself.