Ah, to be a turtle… At first blush, they appear to have very comfortable lives. Their hard shells protect them from most predators; no one expects them to move very quickly; but best of all, they get to spend all summer swimming around the lake and all winter sleeping. However, as idyllic and restful as a turtle’s winter might sound, hibernation is no easy task.
Two of winter’s big challenges are cold and lack of food. As temperatures fall below the freezing point, water in cells turns to ice. Since ice is less dense than cold water, it also expands and may rupture cell walls. Humans know the condition as frostbite. Food is unavailable largely because plants and insects, two prime food sources, react to the cold by entering their own states of dormancy. Animals that haven’t built up a reserve of fat or stashed a winter’s supply of food won’t survive.
There are three general strategies that animals employ to cope with winter’s challenges. Many birds, humans and a few insects like monarch butterflies migrate to warmer climates. The migrations require tremendous amounts of energy and once they arrive on the wintering grounds they must deal with competition from the local residents and a multitude of parasites not found on their breeding grounds. Other birds and most mammals stick around and try to weather the season. Their challenge is finding enough food to maintain high internal body temperatures. The third strategy, and the one employed by most turtles, is to enter a prolonged period of dormancy and reduced metabolic activity, and thus avoid the need for large quantities of food. The challenge to these hibernators is to avoid (or in some cases tolerate) freezing.
Hibernators seek out areas insulated from the bitterest cold of winter to avoid freezing. Many bats head to caves where the average temperature is fairly constant year round. Chipmunks burrow underground where the combination of the earth and the snow pack provides insulation. Most turtles stay under the ice cover of rivers, lakes, or ponds where temperatures remain constant through the winter, but living under water presents a separate challenge.
The ice that protects deep water from freezing also prevents any oxygen from diffusing into the system, yet many creatures living under water - fish, invertebrates, and others besides turtles - still need it. At the bottom of the lake, a host of mud-dwelling decomposers quickly uses up the oxygen while recycling the year’s detritus, making conditions even more challenging. The inactivity of hibernation reduces, but does not eliminate, the need for oxygen. Even those turtle species most tolerant of low oxygen conditions will seek springs or inlets that keep concentrations somewhat higher.
To make matters worse for turtles, they have lungs; most animals that live below the water for extended periods have gills instead. Water flows across the gills rather than in and out as with lungs. 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. Lungs are not nearly as efficient at extracting oxygen from water as gills. Even though fish have gills, hard winters will kill many of them if the oxygen under the ice becomes depleted. Many a March one can see eagles and gulls feasting on piles of dead carp at Dead Creek in Addison, Vermont.
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 intense exercise. Imagine building up those acids over a few months of hibernation.
Painted turtles counter the acid build-up 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 cannot utilize this strategy because they have less calcium in their shells.
A January thaw can provide a respite for animals hibernating on land; a chance to awaken and replenish diminished stores of fat. Under the water there is no respite until the spring thaw. Oxygen continues to be depleted with no replenishment while painted and snapping turtles continue to mobilize calcium reserves in their shells to buffer acids. Winter becomes a marathon endurance trial, and only the hardiest survive it. Turtles actually have it pretty rough.