Winter can be full of surprises. There is the natural beauty that comes from the frozen fog, or rime ice, that can coat every surface creating a wonderland that may last for only a few hours before melting under the sun’s low-angle rays. The plethora of animal tracks on a new fallen snow testifying to the abundant nocturnal animal life never fails to amaze me. Bear tracks are not uncommon, with some bear choosing to remain active throughout the winter season, while others may arouse briefly for a midwinters’ stroll.
And perhaps nothing is so surprising as the explosion from the snow of a ruffed grouse that had found refuge deep in the snow next to the hiker’s trail.
Most of our reptiles (snakes, turtles, lizards and skinks) burrow below the frost line to avoid freezing. As temperatures drop, they will burrow deeper into the soil, staying beyond the frost line. However, you should not be surprised to find some reptiles out and about during the winter months. Garter snakes, box turtles, painted turtles and ground skinks are some of the reptiles that have evolved strategies that enable them to endure sub-freezing temperatures without harm.
There are two primary strategies available to the reptiles that allow them to survive in sub-freezing conditions. One is freeze tolerance (limited extracellular water freezing) and the other is freeze avoidance (supercooling).
One January day, I found a garter snake along the Berry Hollow trail near Old Rag Mountain. Garter snakes, like most snakes, overwinter in rock crevices, under rocks deep in soil, or in basements. However, their ability of both freeze avoidance and freeze tolerance enables them to overwinter in shallow leaf litter and arouse themselves in mild winter episodes. They do this by flooding the cells with glucose (a sugar) and glycerol (an alcohol), which act as anti-freeze to prevent the rupturing of the cells (freeze avoidance). At the same time, water is osmotically removed from the cells to the extracellular space, where it can freeze without damaging the cells themselves (freeze tolerance). Tests have found garter snakes can tolerate 6 hours of -26° F, resulting in body ice contents of 18-36%.
I had the opportunity to observe the fall activities of a box turtle that made its hibernaculum in my shrub border. Over the month of October, as the temperatures dropped, he would dig a depression with his rear feet and partially submerge himself only to vacate the site when the weather warmed again. Finally, by early November, he had dug into the ground so that only his head was still above ground. Eventually, he turned around in the hole and, by the next day, he was out of sight.
Box turtles also exhibit freeze tolerance. Box turtles from the upper Midwest have been shown to tolerate temperatures as low as -25°F for at least three days while 7 to 58% of their total body water froze solid. Like the garter snake, this enables the box turtle to survive shallow overwintering sites despite freezing temperatures.
Painted turtles range farther north than any other land or freshwater turtle. Hatchlings have been found to survive 24 hours of freezing at -24°F with 52-53% of their total body water as ice. Other tests found them able to withstand 25°F for at least eleven days. Under these conditions, there is no muscle movement, no breathing, heartbeat, or blood flow. They survive in true metabolic arrest.
Painted turtles can exhibit freeze tolerance or freeze avoidance, in the form of supercooling. This means the liquids in the body remain in the liquid state even at temperatures below freezing. In fact, supercooling has been documented at temperatures down to 10°F! Whether a painted turtle utilizes freeze tolerance versus freeze avoidance depends on its environmental conditions. Freeze tolerance only works if temperatures stay above 25°F throughout the winter. If colder temperatures occur, further freezing of the turtle will cause its demise. Thus, in colder habitats, most turtles will adopt the freeze avoidance strategy of supercooling.
Interestingly, the painted turtles’ ability to survive such low temperatures exists only as hatchlings. Unlike most turtles that, upon hatching, will emerge from the summer nest and crawl to the nearest pond, the painted turtle remains in the nest, thus, making it susceptible to more severe low temperatures. After the first winter, the painted turtle will hibernate in pond muds. Thus, not surprisingly, this freeze tolerance is not found beyond the painted turtles’ first year.
But the winter life of the painted turtle is not necessarily a breeze after the first winter. Like the snapping turtle, the painted turtle overwinters in shallow pond muds. Preference for mud submersion is to avoid predation by raccoons and shallow water to apparently better feel the warmth of spring. The problem with this habitat is that it is common that such muds become anaerobic. So, how does nature accommodate the needs of these air-breathing creatures for survival; not only underwater, but in the complete absence of any oxygen?
Hibernating aquatic turtles have a much-reduced metabolism, with only one heartbeat in ten minutes (compared to 30-40 beats per minute in summer), thus having a very low oxygen demand. In the early stages of winter, the limited oxygen in the muds can be obtained through mucous membranes and skin to meet the needs of the turtle. When the oxygen in the muds is used up, the turtle must switch to the much less efficient method of anaerobic respiration. This process, known as glycolosis, derives energy from the breakdown of glycogen carbohydrates. Unfortunately, the byproduct of this process, lactic acid, can build up to lethal concentrations within the turtle. This is the same lactic acid that makes human muscles ache after long exercises. Turtles deal with this excess acid in two manners. First, calcium carbonate from their shells is used to neutralize the acid, much like Rolaids. Second, turtles will emerge from the muds and exercise their muscles. Thus, it is possible at such times to see turtles swimming under ice in mid-winter.
If the thought of an air-breathing turtle living under water for five months doesn’t surprise you, how would the idea of garter snakes spending the winter underwater affect you? It has been documented. Like turtles, the 35 degree water supplies all the oxygen a snake needs with a drastically reduced metabolism including a heart rate of one beat a minute.
One final comment. Please note that the statistics presented here have been taken from northern populations. As noted with the painted turtles, regional variations have evolved to meet the specific needs of their habitat. So, don’t take a hatchling painted turtle from our forests and stick it into your refrigerator for the winter. Nature knows best.