“Cold-blooded”: Winter Survival of our Amphibians and Reptiles
Written by Logan Parker, Director of Programming and Assistant Lake Science Manager
Those of us who remain in New England throughout the winter know all too well the struggle to stay warm. For me, a typical January day begins with pulling on many layers of clothing, driving to Belgrade Lakes in the tropical microclimate of my heated vehicle, and dashing from the car into the comfort of the climate-controlled Maine Lakes Resource Center. All through the day, I eat a variety of foods that keep my metabolism fueled and my internal temperature controls operational. My evenings are often spent studying in an armchair beside the wood stove. Both modern technology and my own biology have made it so that I can endure winter in such a climate. Without either, I’m in real peril up here in a short amount of time.
There are some animals, however, which get by with much less. Enter the ectothermic reptiles and amphibians: the “cold-blooded” inhabitants of our watershed. Unlike endotherms, such as you and me, these ectotherms are largely dependent on environmental sources of heat to regulate their body temperatures. They have neither dense fur nor insulating feathers and lack the appendages to fly to more favorable conditions. No, our resident frogs, snakes, turtles, and salamanders must endure the Maine winter by deploying a different set of adaptations.
The Common Snapping Turtle (Chelydra serpentina) is perhaps the least popular ectotherm in the watershed due to its aggressive behavior when encountered on land (although in water these turtles are quite meek and skittish). These large turtles have a long history of ambushing prey in the marsh, streams, and lakes of North America dating back some 60-100 million years (Coulter, 1999). To survive the rigors of winter, these turtles retreat to below the water’s surface to spend the season hibernating under ice in shallow marsh areas, stream inlets, and under submerged trees (Ultsch, 2006). Snapping turtles will enter this mud-mired hibernation state alone or sometimes in the company of other turtles. These turtles, which typically breathe air, are anoxia-tolerant in winter- meaning they can survive several months underwater in even poorly oxygenated lakes (Ulsch, 2006). They are suspected of being capable of absorbing oxygen through folds of their pharynx that are exposed to water (Ultsch, 2006).
Unlike the snapping turtle, a number of native frog species survive the coldest months on dry land. Wood Frogs (Lithobates sylvaticus), Spring Peepers (Pseudacris crucifer), and Gray Treefrogs (Hyla versicolor) spend the winter near the soil surface under leaf litter, tree roots, and woody debris (Davis, 1999). This means these animals are exposed to a more dynamic array of freezing conditions. What is a native ectotherm to do? Why, freeze of course! Extreme conditions call for some equally extreme survival measures. To survive undergoing a freeze, these frogs reduce the water content of their organs and accumulate either glucose or glycerol (depending on the species) as a “cryoprotectant”, a substance that prevents cell damage during freezing (Layne et al. 2001). As much as 80% of the frog’s body will freeze during particularly cold spells.
Snakes lack the anoxia and freeze tolerance of the preceding species and so deploy a different set of adaptations to persevere through months of freezing conditions. The Common Garter Snake (Thamnophis sirtalis) is the most widespread reptile in the region and stays active sometimes well into November (Haskins, 1999). When the winds shift and snows begin to fall, these snakes head for crevices, rock walls, and old foundations to pass the winter singly or en masse, sometimes by the thousands (Shine et al. 2003). By retreating below the frost line, these snakes are able to avoid potentially lethal low temperatures at the soil’s surface (Shine et al. 2003). These animals remain in the hibernaculum until spring when they emerge, mate, and disperse.
Through a variety of physiological and behavioral tactics, our watershed’s native ectotherms endure the harshest part of the year. As with all of the various life forms that inhabit our watershed, proper habitat must be fostered if they are to flourish here. Each of us can contribute to the success of these animals by preserving wild spaces and propagating habitat on our own land. Furthermore, the lakes, streams, and vernal pools in our watershed are of vital importance to wildlife. The needs of the biotic community must be maintained when considering the management of this resource.
Coulter, M. W. (1999). Common Snapping Turtle. Maine Amphibians and Reptiles. University of Maine Press. Orono, Maine, USA.
Davis, S. L. (1999). Gray Treefrog. Maine Amphibians and Reptiles. University of Maine Press. Orono, Maine, USA.
Haskins, J. J. (1999). Common Garter Snake. Maine Amphibians and Reptiles. University of Maine Press. Orono, Maine, USA.
Layne, J. R. & Jones, A. L. (2001). Freeze Tolerance in the Gray Treefrog: Cryoprotectant Mobilization and Organ Dehydration. Journal of Experimental Zoology. 290: 1-5.
Shine, R. & Mason, R. T. (2003). Patterns of mortality in a cold-climate population of garter snakes. Ecography. 12 (2): 81-86.
Ultsch, G. R. (2006). The ecology of overwintering among turtles: where turtles overwinter and its consequences. Biological Reviews. 83 (3): 339-367.