Certain turtles, among them the Australian Fitzroy river turtle and the North American eastern painted turtle, breathe through the back end. YES! They breathe through their butts. These turtles can breathe through their mouths if they so chose.
How do they know that turtles are breathing with their butts and not out, like the rest of us? 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 a butt or anus, it’s a cloaca. This is the opening through which the turtle excretes, urinates, and lays its eggs.
Why? If the turtle can use its cloaca to breathe, why doesn’t it just use its mouth to breathe?
Predation is the reason and the secret lies in the turtle’s shell. The shell, which evolved from ribs and vertebrae that flattened out and fused, does more than keeping the turtle safe from bites. When a turtle hibernates, it conceals itself in cold water for up to five months.
To survive, it has to change 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 into the turtle’s body.
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.
Then other muscles 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.
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. Because turtles do not have much to spare.
Cloacal respiration is not so much breathing as just diffusing oxygen in and carbon dioxide out, but the fact remains: when turtles hibernate, their main source of oxygen is through their butt.
This unusual technique, shared by a handful of other turtle and fish species, gave the turtles an evolutionary advantage for millennia, allowing them to hide from predators underwater for days at a time.
As cold-blooded animals, when the temperature drops in the winter, a turtle’s internal temperature drops with it, and its metabolism slows down to match.
While they are in this slowed-metabolism hibernation period, their oxygen needs are quite low, and the oxygen diffused from the water running over them is enough to sustain them until spring.
If times get tough, they can always switch to anaerobic respiration: powering their metabolism without oxygen, but this mode comes with a time limit due to the buildup of a respiratory byproduct, lactic acid.
This breathing process is fairly common amongst amphibians and reptiles and is properly called cutaneous respiration. Besides the turtle butt-breathers, notable users of cutaneous respiration include frogs, salamanders, and sea snakes.
Turtles are ectotherms and rely on an external source of heat because of a turtle’s body temperature tracks that of its environment. If the pond water is 1℃, so is the turtle’s body.
But turtles have lungs and they breathe air. So, how is it conceivable for them to survive in a frigid pond with a lid of ice that prevents them from coming up for air? The answer lies in the connection between body temperature and metabolism.
A cold turtle in cold water has a slow metabolism. The colder it gets, the slower its metabolism, which translates into lower energy and oxygen demands.
When turtles hibernate, they rely on stored energy and uptake oxygen from the pond water by moving it across body surfaces that are flush with blood vessels. In this way, they can get enough oxygen to support their minimal needs without using their lungs. Turtles have one area that is especially well vascularized. And that is their butts.