Lake Look: When Shell Freezes Over

In stressful times, one may envy a turtle: built-in safety from predators with their shells and free from modern expectations of speed and efficiency. Spending summers lounging on logs and rocks warmed by the sun and winters in seemingly peaceful hibernation. However, the winter months present survival obstacles for all animals in Lake Champlain, and turtles don’t have it easy. While hibernation may seem appealing to those who don’t enjoy the long cold nights of the season, it poses challenges with some surprising solutions.

Winter Survival in the Lake Champlain Basin

Two of winter’s major hurdles for turtles are cold and lack of food. As temperatures fall below the freezing point, water in cells can form ice crystals and rupture cell membranes. Humans know the condition as frostbite, and it can happen to turtles too. Meanwhile, food is unavailable largely because plants and insects—their two primary sources of sustenance—react to the cold by entering their own states of dormancy or completing a seasonal lifecycle. Animals that haven’t built up a reserve of fat or stashed a winter supply of food can’t survive.

During winter, animals of the Lake Champlain region employ any of these three general strategies to cope: migrating, tolerating, and hibernating. Many birds, insects such as monarch butterflies, and some people, migrate and avoid winter altogether. However, migration requires tremendous amounts of energy, and when they arrive on wintering grounds, animals have to compete with local residents and weather parasites and other hazards not found in their breeding grounds. Most mammals and some non-migratory birds will stick around for winter. Their main challenge is finding and storing enough food to maintain high internal body temperatures. The third approach, employed by bats, bears, amphibians, and most reptiles (including turtles) is hibernation—a prolonged period of dormancy with reduced metabolic activity. This season-long “low-battery mode” helps animals evade the need for much food, but the task of keeping their bodies from freezing remains.

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. Some small mammals like groundhogs burrow underground where the combination of the earth and the snowpack provides insulation. Most turtles stay under the ice in rivers, lakes, or ponds where temperatures remain constant through the winter, but living underwater presents a separate problem.

Turtally Freezing

Many creatures living underwater--fish, invertebrates, and others besides turtles--still need oxygen to survive. The inactivity of hibernation reduces the need for oxygen, but it doesn’t eliminate it. When water freezes it’s less dense than in a liquid state, so it will always float atop the warmer water beneath. While this ice layer protects deep water from freezing, it also prevents oxygen from diffusing into the system. Compounding this, a host of mud-dwelling decomposers at the lake’s bottom quickly use up oxygen while recycling the year’s detritus, so even turtle species most tolerant of low oxygen conditions will seek springs or inlets where concentrations are somewhat higher.

To make matters worse for turtles, they have lungs whereas 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 quite difficult to move water in and out of the 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 them if the oxygen under the ice becomes depleted.

How do turtles manage this lack of oxygen? Their shells serve a purpose in addition to protection: they are mineral reserves. In the absence of oxygen, species such as painted and snapping turtles will switch in part to chemical pathways that don’t require oxygen, known as anaerobic respiration. Humans also adopt this strategy for limited periods 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 such as lactic acid. The buildup of acids in muscle tissue causes the familiar soreness we feel after a day of intense exercise. A whole season of winter acid accumulation wouldn’t just be painful, it would be fatal. That’s where the shell comes in. Turtles liberate calcium carbonate from their shells, which acts as an antacid tablet and neutralizes the acids, preventing them from accumulating in the blood and muscles.

The spiny softshell turtle—a designated rare and endangered species in both New York and Vermont—lacks this mineral reserve, as its shell is flattened and covered in leathery skin rather than bony plates. They must obtain oxygen solely by breathing through their skin in a process called cutaneous respiration. They absorb oxygen through the lining of the mouth (where uptake is most efficient), the legs, and the cloaca—turtle analog to the anus. Spiny softshell turtles must hibernate in areas where oxygen is available in the water throughout the season, so they often choose the same hibernation spots year after year that lack ice scour. They nestle in shallow muck within neck-reach of the water.

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 such relief 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, and spiny softshell turtles make do with cutaneous respiration. Winter becomes a marathon endurance trial, and only the hardiest survive it. Remember these harsh times for turtles next time you idealize the life of one basking in the summer sun.