Most vertebrates have a flexible rib cage that allows the lungs to expand and contract during breathing. Not so the turtles, who long ago traded away flexible ribs in favor of a fixed, protective shell. Various species of turtles have evolved distinct means of drawing air in their lungs. Turtles have also developed indirect methods for obtaining oxygen during times when they are sealed from contact with the air, as when hibernating or staying underwater.
In turtles, the lungs lie just beneath the carapace and above the other internal organs. The upper surface of the lungs attaches to the carapace itself, while the lower part is connected to the viscera (heart, liver, stomach, and intestinal tract) by a skin of connective tissue called diaphragmaticus. The viscera themselves can also be included within a membrane that attaches to the diaphragmaticus. Groups of muscles rhythmically change the amount of the abdominal cavity. 1 set of muscles moves the viscera upward, pushing air out of the lungs.
When turtles walk about, the motions of their forelimbs foster the suction and compression activities that encircle the lungs. A turtle can alter its lung volume simply by drawing its limbs , then extending them outward again: Turtles floating along with the water often can be seen moving their legs in and out, which helps them breathe. A turtle pulled back within its shell has no room in its lungs for air. At these and sometimes, turtles utilize different approaches to obtain oxygen.
Raising and lowering the hyoid apparatus causes a turtle’s neck to grow and drop, pulling in air. (Along with boosting ventilation, this air motion enables a turtle to better use its sense of smell.) From the highly aquatic soft-shell turtles, the throat is lined with fingerlike projections of skin called villi, which are richly supplied with blood. The villi work like gills, expelling carbon dioxide and taking in oxygen from the water. To process oxygen rich water, a soft shell uses its hyoid apparatus to fill and empty its own throat in a process known as buccopharyngeal breathing. When submerged, a soft shell typically pushes water out and in about sixteen times per minute. Turtles that hibernate underwater also exchange gases through the throat lining, cycling the water inside the throat cavity several times each minute. Many turtles practice this method of breathing, and a few turtles even take in oxygen through the cloaca.
Many of the specifics of turtles breathing remain unidentified. What is clear however is that different kinds of turtles have evolved different methods of fulfilling their oxygen needs. Through evolution, they have gotten very good at obtaining this essential gas. As Ronald Orenstein notes in Turtles, Tortoises, and Terrapins: Survivors in Armor, turtles seem able to breathe”with the least amount of effort regardless of what their circumstances.”