Q. You recently explained how alligators survive cold winters. What about turtles, which even live in cold northern states and Canada. How do they do it?
A. Like alligators, turtles are reptiles, and their surroundings determine their body temperature. At body temperatures below 50 degrees F, they become sluggish, stop eating, and seek hiding places to get safely through the winter. In the South, turtles can be seen basking on logs year round, even on cold days if it is sunny. Turtles living in regions where lakes stay frozen during winter will disappear till spring. Where do they go? How do they survive?
Terrestrial box turtles can tolerate cold but will bury themselves beneath dirt and leaves below the freeze line. 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 and birds is their metabolism, which drops along with their body temperature, meaning they require less oxygen. Some turtles can stay underwater for days or weeks 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 of some species 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 and Wisconsin, 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. Baby painted turtles have a different plan.
A study in Michigan revealed that hatchling turtles overwintering 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 without eating, relying solely on their own fat reserves. This added energy easily gets them through a long, cold winter.
In addition, hatchling painted turtles exposed to subfreezing temperatures produce higher levels of glucose in the blood than do those kept at normal temperatures. The glucose and other body products function as a kind of antifreeze, although how the process works is not fully understood. 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.
Adults hibernating underwater is not particularly unusual behavior for reptiles. If you go for a walk around the edge of a lake this winter, consider that adult turtles may be lying dormant beneath the lake’s surface or under the bank. And baby turtles on land may be concealed beneath your feet. The adults and the hatchlings have a good chance of withstanding anything winter has to offer, in the North as well as the South. That some turtle hatchlings overwinter in the nest while others leave the nest in summer underscores the versatility and complexity of native wildlife. We still have much to learn about the world around us.
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