Did you know that some turtles “breathe” through their rear ends? It’s true! Scientists have long known of this phenomenon, but lacked answers as to the hard-hitting question of “why.” Thankfully, that has changed. As it turns out, certain species of turtles (mostly those in cold climates where hibernation is essential to survival) are able to use an opening in their rears called the cloaca to as an effective respirator in low-oxygen environments.

“The cloaca of some kinds of freshwater turtles, such as our eastern painted turtle, has fingerlike extensions that are abundantly supplied with blood vessels,” writes Bill Amos, a retired biologist with Northern Woodlands magazine. “The muscular walls of the cloaca contract and relax forcing water in and out of the chamber. If a painted turtle hibernates in a pond where some dissolved oxygen is available, the cloaca will serve as a respiratory device.”

This mechanism can be so effective that a species of Australian river turtle breathes this way exclusively. In addition to providing oxygen where it’s otherwise unavailable, cloaca respiration has another benefit: The turtle, with it’s unique physiology, has muscles largely intended to move it in, out of, and around with its shell. They aren’t particularly well designed, however, for pumping oxygen in and out of lungs the way mammals do. Doing go for a turtle would cause an abundant buildup of lactic acid in the muscles and bloodstream.

Though this is never a desirable thing, it’s especially dangerous in a hibernation environment, where fresh air isn’t abundant. While the shell helps reduce blood acidity by absorbing some of it, the cloaca supplements enough oxygen so the turtle doesn’t have to do a lot of manual breathing.

And now you know how (and why) some turtles breathe through their butt parts.