Nature has a juvenile sense of humor. That, at first, seems like the only explanation for why certain turtles, among them the Australian Fitzroy river turtle and the North American eastern painted turtle, breathe through the back end. Both turtles can breathe through their mouths if they so chose.
And yet, when scientists placed a small amount of food coloring in the water near these turtles, they found that the turtles were drawing in water from both ends (and sometimes just the hind end.) Technically, this hind end isn't an anus, it's a cloaca — an opening through which the turtle excretes, urinates, and lays its eggs. Still, the entire situation begs the question: why? If the turtle can use its anus like a mouth to breathe, why doesn't it just use its mouth to breathe?
The possible answer to the question lies in the turtle's shell. The shell, which evolved from ribs and vertebrae that flattened out and fused together, does more than keep the turtle safe from bites. When a turtle hibernates, it buries itself in cold water for up to five months. To survive, it has to change a lot of things about the way its body works. Some processes, such as fat burning, go anaerobic — or without oxygen — in a hibernating turtle. Anaerobic processes result in the build up of lactic acid, and anyone who has seen Aliens knows that too much acid isn't good for a body. The turtle's shell can not only store some lactic acid, but release bicarbonates (baking soda to the acid's vinegar) into the turtle's body. It's not just armor plating, it's a chemistry set.
It is, however, a fairly restrictive chemistry set. Without ribs that expand and contract, the turtle has no use for the lung and muscle set-up that most mammals have. Instead it has muscles that pull the body outwards, towards the openings of the shell, to allow it to inhale, and more muscles to squish the turtle's guts against its lungs to make it exhale. The combination makes for a lot of work, which is especially costly if every time you use a muscle your body's acid levels go up and oxygen levels go down.
Compare this to the relatively cheap butt breathing. Sacs next to the cloaca, called bursa, easily expand. The walls of these sacs are lined with blood vessels. Oxygen diffuses through the blood vessels, and the sacs are squeezed out. The entire procedure uses little energy for a turtle that doesn't have a lot to spare. Dignity has to play second-fiddle to survival sometimes.
Top Image: Geoff Gallice.