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If you want to be able to breathe underwater, you’re going to need to get creative. So some turtles, dragonfly nymphs, and sea cucumbers decided to use their butts.

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Sources:
http://www.straightdope.com/columns/read/2325/is-it-true-turtles-breathe-through-their-butts
http://onlinelibrary.wiley.com/doi/10.1002/jez.1055/abstract
http://news.discovery.com/animals/why-do-some-turtles-breathe-out-of-their-butt-140617.htm
http://caledonianrecord.com/main.asp?SectionID=5&SubSectionID=159&ArticleID=38602
http://illuminationstudios.com/archives/63
https://books.google.com/books?id=qKHwCAAAQBAJ&pg=PA102
http://jeb.biologists.org/content/44/2/317.full.pdf
https://books.google.com/books?id=DyyJIb2IvKAC&pg=PA19
There's something that the vast majority of animals need - oxygen. That one little molecule is exactly what our cells need to make energy, so it's kind of important. Luckily for us, there's a lot of it on this planet and to get some, all you need to do is take in a big nice gulp of air. But, that's a little harder to do if you happen to spend most of your time underwater, and animals have solved this problem in a few different ways, often by having gills - special flaps that filter oxygen out of the water - and some animals just so happen to keep their breathing organs inside their cloacas, a sort of multi-purpose hole in their rear ends. But it turns out they have a very good reason for it. 

Take Australia's Fitzroy river turtle for example. It has a normal pair of lungs but happens to like hanging out at the bottom of rivers and streams, which makes using those lungs kind of difficult. Some other animals like frogs solve this problem by using subcutaneous respiration, or they use their skin to get oxygen from the surrounding water. But turtle skin is all scaly, or covered by a shell, so they can't use it to breathe. So, like a few other species of water- loving turtles, the Fitzroy river turtle can get oxygen using what's known as cloacal respiration, the official term for breathing out of your butt.

They use cloacal bursae - two sacs inside its body on either side of the cloaca that act kind of like gills. This species of turtle is pretty good at it, getting about 70% of its oxygen just from drawing in water through its cloaca, filtering out the oxygen, and pumping it back out up to 60 times a minute.

And the system has added benefits for turtles that have to hibernate for the winter. If you tried to spend a few months underwater, you would die. At most you'd last a few minutes before you had to come up for air, even whales can only hold their breath for about 2 hours maximum. But turtles that use cloacal respiration don't really have to worry about that. In addition to helping them get oxygen underwater, it's a fantastic energy saver because breathing through your lungs is pretty challenging if you're a turtle. 

For most animals, including humans, take a breath, our rib cages expand. That creates a sort of vacuum in our lungs and air rushes in to equalize the pressure. Let go and the rib cage goes back to normal, forcing the air back out. The whole process doesn't use much energy. But turtles can't do that because their shells are their rib cages, and they can't really expand that way. Instead, their whole bodies have to move toward the edges of the shell to create that same negative pressure. It's a lot more work, and a lot more loss of energy just to breathe. Energy that a hibernating turtle can't really afford to lose. So, cloacal respiration may be kind of a gross way to get air, but it's an energy-efficient one. 

Baby dragonflies seem to think so too, though they've taken the idea one step further. Maybe you've seen dragonflies zipping around with their long thin tails. They start out looking very different though, with no wings and shorter, flatter rears. They also live in the water and breathe using gills and drum roll please - their cloacas. Like the turtles, they draw in water, pass it over their gills, and then pump it back out. But sometimes, a little baby dragonfly gets chased by a predator, like a duck, and that's when it turns its special abilities into a propulsion system. The young dragonfly takes in water the way it normally would to breathe, and then quickly pushes it back out again, propelling itself through the water and, hopefully, away from whatever's trying to eat it. 

Sea cucumbers handle things a little differently. They are echinoderms, marine animals related to starfish. Unlike starfish, they look like squishy cylinders instead of stars. Sea cucumbers live on the ocean floor and they can get oxygen in three different ways - through their feet, their skin, and their cloacas. The species that use cloacal respiration filter the water they draw in and pump out using respiratory trees. Basically just a series of tubes inside them that branch out from their cloacas. Startle or chase a sea cucumber and it'll just shoot some of those sticky tubes at you in the hope that you'll get stuck in them and stop chasing it. It's no big deal for the cucumber, it'll just grow them back, and it keeps enough of its internal organs to stay functional in the meantime.

So breathing through your butt might be a great way to get oxygen but it's also surprisingly useful if someone's trying to eat you. 

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