Until recently, however, scientists didn't really know why some turtles -- most notably Australian Fitzroy river turtle and the North American eastern painted turtle -- took in air through the back end. Chalk it up to another one of nature's cruel but hilarious jokes.
But now, scientists have an explanation. As always, the answer is evolutionary problem solving.
The turtles in question hibernate for an extended part of the winter in frigid waters, sometimes for as long as five months. That requires a lot of breathing underwater. Unfortunately, a turtle's shell -- the product of ribs and vertebrae that slowly flattened out and fused together over time -- is built for protection, not to support the muscle system that enables the robust pulmonary setup gifted to so many mammals.
A turtle's muscles are built to help it emerge from the gaps in its shell, not to contract and expand lungs, inhaling and exhaling oxygen. Thus, breathing in and out in the normal fashion requires a lot of work for the turtle -- muscle exertion that causes a buildup of acid. And too much acid in the body is a bad thing.
Luckily, the turtle's cloaca -- the rear end hole (not an anus) that allows the reptile to excrete, urinate, and lay its eggs -- features two sacs, or bursa, which more efficiently absorb oxygen. Though the Australian Fitzroy river turtle, North American eastern painted turtle, and other rear-breathing turtles can breathe through their mouths if they feel so inclined, the bursa help them take in oxygen without expending as much energy and producing as much acid byproduct.
In related news, other turtles pee through their mouths.