Picture by: Harold_5056@flickr
Because it’s still cold around these parts. At least, still cold for a Floridian living in Texas. Here is a comforting article about how these little guys survive the winter:
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, 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.