How Does a Turtle Breathe Inside Its Shell? Not With Its Ribs

The shell has two parts, a domed upper section and a flat lower section covering the belly, joined at the sides by a bony bridge. The lower part is formed from bones of the shoulder girdle and the abdominal ribs fused into a single plate, and the upper part incorporates the vertebrae and the ribs themselves, which are fused to the inside of it. The result is a rigid box containing the animal's body, with the limbs, neck and tail emerging through openings at the front and back, and the whole thing covered by horny plates that grow and are shed or worn.

Fusing ribs into a shell removes the usual breathing mechanism:

• •Most reptiles and mammals breathe by moving the ribs
• •That expands the chest and draws air in
• •Fused ribs cannot move at all
• •Turtles have no diaphragm of the mammalian kind
• •So the lungs must be squeezed and released by something else
• •Muscle sheets attached to the shell and the limb girdles do it

Two opposing sheets of muscle inside the shell do the job, one pulling the abdominal contents backward and downward to enlarge the space around the lungs and draw air in, and the other pulling them forward and upward to push air out. Limb movement affects it too, since drawing the legs into the shell compresses the body cavity and forces air out, which is why a turtle withdrawing rapidly can be heard to hiss. Several species supplement lung breathing by absorbing oxygen through thin-walled sacs near the tail or through the lining of the throat, which lets some of them remain underwater for months in cold water.

The origin of the whole structure was argued about for a century and fossils found since 2008 have largely settled it. A form from China preserves a fully developed lower plate with a normal ribbed back and no upper shell at all, which shows the lower half came first. An older South African form has broadened ribs and a broadened backbone with no shell of either kind, which looks like a burrowing animal using a stiffened trunk for digging. The sequence therefore runs from broadened ribs stiffening the body, to a fused lower plate, and only then to the upper shell closing over, which reverses what most people assume.

The lower shell varies between species in ways that are readable. A hinged one, present in box turtles and several others, lets the animal close itself completely, which is a defence no tortoise without the hinge can manage. A reduced or cross-shaped one, seen in softshells and sea turtles, saves weight and permits flexibility, at the cost of protection, and suits animals that escape by swimming. A concave one in males of many tortoise species accommodates the curve of the female's upper shell during mating, which makes it one of the few external ways of telling the sexes apart. And growth rings on the plates record seasons of growth.

The lower shell is built from shoulder girdle bones and abdominal ribs fused into one plate, while the upper incorporates the vertebrae and true ribs, so the ribs cannot move and the usual breathing mechanism is unavailable. Opposing muscle sheets pull the abdominal contents back and forth instead, and withdrawing the limbs forces air out. A hinge lets some species close completely, and a concave plate marks males.