DO BABY TURTLES SURVIVE COLD WINTERS?
by Whit Gibbons
January 18, 2004
is the most obvious and easily measured environmental factor influencing
the lives of plants and animals and is therefore of great interest to
ecologists. Every species living in the temperate zone deals 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.
to the turtles you saw basking on logs or rocks in the sun during spring,
summer, and fall? They have disappeared, but where did they go, and why?
Turtles are reptiles and their surroundings determine their body temperature.
At body temperatures of about 40 to 50°F, most reptiles become sluggish,
stop eating, and seek hiding places to get safely through the winter.
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.
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.
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°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.
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.
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 possibly
baby turtles are on land beneath your feet. Both the adults and hatchlings
have a good chance of enduring anything winter has to offer, in the North
as well as the South. Hibernating underwater by the adults is not particularly
unusual for a reptile, but the phenomenon of overwintering in the nest
by the hatchlings demonstrates how intricate the natural history and versatility
of native wildlife can be. We still have a lot to learn about how the
most common of animals around us survive in the natural world.
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