Acid-Buffering, Bum-Breathing Turtles

The turtles are buried under the mud for the winter. How do they survive down there?

While walking around Mud Pond on Monday, I knew very well I would not see any turtles. I know that by this late in autumn, turtles have buried themselves deep in mud to hibernate until spring. What I did not know, however, was just how they manage to do that, since the rest of the year they breathe with their lungs and have to come up for air. And just as I was pondering this mystery, my Winter issue of Northern Woodlands magazine arrived in the mail, with a fascinating description of just how turtles do this. I wish I could offer a link to the article, but since Northern Woodlands won't show the Winter issue's articles on its website until next season, I'm reproducing much of the article here.

Bum breathers

by Bill Amos

As northerners, our turtles must contend with frigid winters. Unable to maintain the necessary body temperature to carry out essential metabolic activities, a turtle settles down into the mud for a long winter's sleep. Its heart rate slows to almost nothing, and its organ systems more or less shut down.

Finding food is not a problem -- like all hibernators, a turtle lives on the rich body fat that was stored in the previous summer's season of plenty. But finding oxygen with which to metabolize this fat is a challenge. Unlike a terrestrial hibernator, a turtle winters in an oxygen-deficient environment for four or five months. When fat is metabolized in the absence of oxygen, the process creates lactic acid, and when too much of this acid builds up, the resulting condition -- known as acidosis -- can be fatal to an animal.

A pond turtle finds a remedy to this problem in its shell and skeleton, which together account for over a third of the animal's weight. The shell of a turtle is not just for protection -- being heavy, it also helps with buoyancy control by counteracting the air-filled lungs, and it acts as a hydrofoil when the animal swims.

For hibernating turtles, the shell is also vitally important in a chemical sense. Just as we take buffering tablets, like Tums, to relieve an acid stomach, a hibernating turtle uses its shell and bone, composed of limy compounds, to release carbonates into its increasingly acidic body fluids. This buffers the acid buildup in a non-breathing turtle, when it snoozes down in the oxygenless mud of a winter pond, and deadly acidosis in the animal's tissues is prevented. Furthermore, the shell stores accumulations of this buffered lactic acid where it can do no harm.

In effect, a hibernating turtle is a closed system independent of the environment, an extraordinary feat of biochemical juggling that allows metabolism to continue throughout winter.

But wait. There's more.

All reptiles (also amphibians and birds) have a sizeable rear opening called a cloaca. It is a common, muscular chamber into which the rectum, urinary, and reproductive systems empty. It serves as a mating and egg-laying opening as well as one through which urinary and digestive wastes are excreted. It's a good design that has served these animals well for millions of years.

The cloaca in some kinds of freshwater turtles, such as our Eastern Painted Turtle, has fingerlike extensions that are abundantly supplied with blood vessels. 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 respiratory device. As a substitue "lung," the cloaca allows for the transfer of oxygen and carbon dioxide between the animal and the surrounding water.

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Imagine that! A turtle can breathe through its butt! Isn't Nature amazing? And so is that magazine Northern Woodlands, which is just full of fascinating information for nature lovers. You can check it out at northernwoodlands.org and even read whole articles from past issues there. Even better, you can subscribe.

Another good site for nature lovers to visit is "Ask a Naturalist," where you can find out anything you ever wanted to know about the natural world. Check it out here.