Nature Note – Turtle Under Ice
Winter drives many creatures into hibernation, or at least states of severely reduced metabolic activity. Bears might curl up in a protected cave; chipmunks seek fur-lined holes in the ground; bats congregated in caves; but turtles face a particular challenge. How can a lung-breathing animal spend an entire winter underwater and still get enough oxygen to survive?
As anyone who has swallowed a mouthful of water while swimming knows, it is pretty difficult to move water in and out of lungs. Water has a much lower concentration of oxygen than air even when saturated. To make matters worse, lungs are not nearly as efficient at extracting oxygen from water as gills.
Turtles manage to tolerate low oxygen concentrations by breathing through their skin. In particular, they absorb oxygen through the lining of the mouth, the legs, and the cloaca (roughly equivalent to an anus), with uptake being most efficient through the lining of the mouth. Species that rely exclusively on skin breathing to survive the winter, like map and spiny soft-shell turtles, must hibernate in areas where oxygen is available in the water throughout the season.
A secondary strategy, utilized by painted and snapping turtles, is to switch in part to chemical pathways that don’t require oxygen. Humans also adopt this strategy for limited periods of time when engaged in strenuous exercise. The alternative pathways are less efficient and create yet another problem. While respiration with oxygen produces carbon dioxide, a relatively harmless gas, as a waste product, these alternate pathways produce acids. Build-up of acids in muscle tissue causes the familiar aches and stiffness we feel after a day of exercise. Imagine building up those acids over a few months of hibernation.
Painted turtles counter the build-up of acids by liberating calcium carbonate from their shells. The calcium carbonate acts like an antacid tablet and neutralizes the acids preventing them from accumulating in the blood and muscles. It may be that soft-shell turtles can’t utilize this strategy because they have less calcium in their shells.
A winter without ice is a bit easier to handle. Oxygen levels in the water can remain high, driven by wind and waves. But still the turtles will tap into alternate strategies for survival. The shells of turtles are helpful for more than just protection from predators.