Painted Turtle (Chrysemys picta)

Breathless Turtles ...

In early January, I went skating on a small pond in western Massachusetts. A week or so of cold, dry weather had produced ice that was smooth, more than six inches thick, and so clear I could see to the bottom of the pond.

Unfortunately, it wasn't until a few weeks after that clear ice had been clouded by melting and refreezing that Al Richmond, curator of herpetology at the University of Massachusetts told me he remembers seeing painted turtles moving under the ice when he was a kid.

Painted turtleWhen I heard this, my first thought was to wish I had known to look for them back in January. My second thought was "How do turtles under the ice get air?"

Turtles are air breathing vertebrates, after all. How would painted turtles breathe under the ice?

One possibility is that they find pockets of trapped air. That seems pretty straightforward. But is it enough?

Radio-tracking studies have shown that Dr. Richmond was pretty lucky to see turtles moving under the ice when he was a kid, an omen, maybe, of his future career.

Though painted turtles do occasionally move under the ice, mostly what they do is stay very still. In the northern parts of their range, which extends into Canada, they sit on the bottom or buried in mud at the edges of lakes, ponds, and streams, often under ice, from late October to March.

The problem for a such a northerly turtle is this: if you stay out of the water during the winter, you'll freeze and/or dehydrate and/or be eaten by every hungry predator. If instead, you retreat to a watery refuge for the cold months, you'll get very cold, but you won't actually freeze. And though you may still be susceptible to underwater predators such as otters, you are much safer than on land.

Unfortunately, ice is impermeable to air. And if you live far enough north where ice covers lakes, ponds, and rivers for months at a time, you can't come to the surface to breathe.

So what's a turtle to do?

First of all, stop doing whatever you're doing. In fact, stop doing anything at all.

Painted turtles sitting on the bottom of a lake don't breathe. For the most part, they don't move. They don't eat.

Turtles are ectothermic, or what used to be called cold-blooded. Their temperature depends on their environment, so as the water temperature drops, the turtle temperature does, too. In an ice-covered pond, the temperature of the water at the bottom of the pond -- and of the turtles sitting there -- is often just a few degrees above freezing.

As the temperature drops so does the turtle's metabolism, by as much as 99.4% -- far greater than the metabolic drop of any hibernating mammal. A cold painted turtle's heart may beat only once every 5 or 10 minutes.

They are just about as close to being dead as they can get.

Still, they are alive. And if an organism is to stay alive, it must use energy. Normally, this is done by metabolizing sugars through what is called aerobic respiration.

Aerobic respiration, however, requires oxygen. In water that has oxygen in it, a resting, cold-slowed turtle can absorb enough oxygen through its skin to continue to use aerobic respiration. But in water that becomes oxygen depleted over the winter, that's no longer possible.

Painted turtles can stay alive even in those waters. How?

There is another way to produce energy, called anaerobic respiration, that takes place without oxygen. Our muscle cells use this form of energy production when we are working so hard that we can't get enough oxygen to supply the muscles.

For a short time, our muscles can operate without oxygen. Turtles use the same trick when they are under water without oxygen all winter. The problem with anaerobic respiration is the byproduct lactic acid. When lactic acid builds up in the blood stream, it lowers the Ph and disrupts blood chemistry.

That's what makes our muscles feel tired after we work them hard. Our body restores the normal chemistry once we rest. Using oxygen again, it metabolizes all that built up lactic acid.

But turtles under water can't do this if there's no oxygen.

Instead of using oxygen to get rid of the lactic acid, turtles have buffers in their blood that neutralize lactic acid and keep it from turning the blood acidic.

But this only works up to a point. As the winter wears on, lactic acid begins to overwhelm the blood buffers.

This is the point where the painted turtle separates from the crowd.

Animal Physiologists Gordon Ultsch at the University of Alabama and Donald Jackson at Brown University, have shown that painted turtles release a substance called carbonate from their bone into the bloodstream. Carbonate neutralizes the lactic acid and keeps the turtle's body chemistry in an acceptable range.

Essentially, a painted turtle dissolves its own bones over the winter.

This can only go so far, obviously, or come springtime, a turtle would find itself without a skeleton. And if the ice cover stays all winter, and the turtle has to use anaerobic respiration for months at a time, there just isn't enough carbonate in the bone to neutralize all the lactic acid that is produced.

A turtle's shell, however, is bone fused between the ribs and vertebrae, covered with a thin layer of skin and scales. A painted turtle's bone and shell together constitute one third of its total body weight. The turtle uses its bony shell as an additional source of carbonate to buffer the lactic acid. It's also able to pull lactic acid out of its blood and trap it within the bone of its shell.

Using these tricks, painted turtles can live without breathing for five months or more in water that has absolutely no oxygen -- longer than any other known air-breathing vertebrate. The only other vertebrate that comes close is, another turtle, the Snapping Turtle.

That's why, once you get far enough north that ponds and lakes freeze over for months at a time, the only turtles you'll find everywhere are painted turtles and snapping turtles.

Other turtles are limited to water that remains oxygenated all winter. Generally, this means large lakes, rivers, and some smaller lakes and ponds that havePainted turtle steadily flowing water.

Being able to overwinter in virtually any body of water that doesn't freeze solid gives painted turtles and snapping Turtles a big advantage.

So if you see a painted turtle early in spring in a northern climate, stand back and give the poor thing some air. It may be taking it’s first real breaths – and getting its first oxygen – since November.




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