by Whit Gibbons

February 15, 2009

Questions about how various animals survive winter come across my desk (or rather my computer screen) rather frequently. Every species living in the temperate zone has to cope in some way with winter cold. Birds fly south as winter approaches. Mammals add a layer of body fat when cold weather arrives. Trees lose their leaves before they freeze. Turtles, one of the most conspicuous animals in warm weather, have special ways to deal with winter.

What happened to the turtles you saw basking on logs or sun-warmed rocks during spring, summer, and fall? They have disappeared. Where did they go, and why? Turtles are reptiles, so their surroundings determine their body temperature. At body temperatures of about 40 to 50 degrees F, most reptiles become sluggish, stop eating, and seek hiding places to get safely through the winter.

Many aquatic turtles go into the bottom mud or under the bank where the water is cold but does not freeze. An advantage reptiles have over most mammals is that their metabolism drops with their body temperature, meaning that they require less oxygen. Some turtles can stay underwater for days at a time without taking a breath, as long as the water stays cold.

Recently born baby turtles have a different strategy. Turtles lay their eggs on land, usually by digging a hole in dirt or sand and then covering the nest. Most turtle eggs hatch in autumn, but the hatchlings often do not leave the nest until the following spring, a year or more after the eggs are laid. This phenomenon, known as overwintering in the nest, occurs worldwide among many different kinds of turtles.

Overwintering may sound like a reasonable way for a helpless baby turtle in mild-wintered Alabama or Florida to pass its first cold spells and avoid predators. But what do baby turtles do in Canada, Michigan, and Minnesota, where painted turtle hatchlings are entombed only a few inches beneath the soil for the winter months? Even in an underground nest, soil temperatures drop as low as 25 degrees F. Most animals deal with these extremely low winter temperatures by seeking a warmer place. Not so baby painted turtles.

In Michigan, hatchling turtles that overwinter on land differ in body composition from those that leave the nest during late summer. The eggs of overwintering hatchlings have proportionally more body fat and oils than do the eggs of turtles that leave the nest early. The overwintering baby turtles can survive from late summer to the following spring on their own fat reserves, without eating. This added energy easily gets them through a long, cold winter.

Some hatchling turtles are also believed to be capable of producing antifreeze compounds. Hatchling painted turtles exposed to subfreezing temperatures produce significantly higher levels of glucose in the blood than do those kept at normal temperatures. The glucose and other body products may function as a form of antifreeze, although how the process works is unknown.

An even more important discovery is that some baby turtles can survive when more than half their internal body water freezes. The painted turtle is one of the highest vertebrate life forms known in which the freezing of body fluids is tolerated during hibernation. This does not mean that other animals are incapable of surviving such an assault, only that scientists have not yet documented the phenomenon.

If you go for a walk around the edge of a lake this winter, consider that adult turtles are lying dormant beneath the lake's surface and that baby turtles may be on land beneath your feet. Both the adults and hatchlings have a good chance of enduring anything winter has to offer, in the South as well as the North.

Hibernating underwater by adult reptiles is not particularly uncommon, but the phenomenon of hatchlings overwintering in the nest is unusual behavior. Both methods of surviving winter are indicative of just how versatile and endlessly fascinating the natural history of native wildlife is. And we still have much to learn about how even the most common of animals around us survive in the natural world.

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