Featured Image: A garter snake (Thamnophis sirtalis parietalis) in northern Minnesota. Photo by author.
When days become shorter and the temperature outside begin to drop, our home interiors become warm, welcoming refuges from the rain and snow outside. We see the trees enter dormancy as they drop their leaves and wildlife become busy preparing for winter: Many birds migrate to warmer climates; the central population of Monarch butterflies begin their famous journey to Mexico. Some fish species, like swordfish and great white sharks, move to warmer ocean regions. Soil invertebrates, including earthworms and termites, move as far as six feet below the soil surface. Most of us see squirrels and chipmunks scurrying to and fro, preparing for the colder months ahead.
But where do the smooth and scaly things go? The frogs? The snakes? The turtles? And without a fur coat and thick layer of blubber, it makes one wonder how they survive in prolonged freezing temperatures. As it turns out, behavioral and physiological adaptations – such as brumation and supercooling – allow many amphibians and reptiles to withstand some of our planet’s most extreme winter conditions.
Brumation is the term used for hibernating amphibians and reptiles. Brumation is similar to hibernation in that animals take their cue from shortening day lengths and dropping temperatures; prior to dormancy they are feeding more, building up fat reserves, their metabolism slows and body temperature drops. But there are some distinct differences between brumation and hibernation. Hibernating mammals enter a true deep sleep period, but brumating amphibians and reptiles enter a period of dormancy that may be punctuated by activity. Hibernating mammals maintain a minimal level of body heat and require sufficient oxygen levels. Because amphibians and reptiles are ectothermic, their body can only stay as warm as its surrounding environment, including during brumation. Amphibians and reptiles can also survive low-oxygen environments due to glycogen in their blood. Alligators, turtles, and many frog species bury themselves deep in mud or underwater in low-oxygen places during brumation. Some of these species, such as turtles, are able to absorb oxygen in the water through skin tissue. But for those species that do not brumate in water, dehydration can be a risk. This is one main reason amphibians and reptiles cannot always remain dormant throughout the entire winter. They may temporarily break dormancy to drink water or find a sunny spot to bask.
In the middle of a frosty January, it is difficult to imagine any snakes and frogs still alive outside when stinging cold wind screams outside your window. Indeed, amphibians and reptiles are sensitive to freezing temperatures too! But they are skillful at locating protective hibernacula. Hibernacula is a term for refuge sites used by animals during hibernation or brumation. Amphibian and reptile hibernacula are often in belowground substrate – in mud, underwater mud, burrows, or cracks in the soil. Some species, like wood frogs, can survive winter under just leaf litter. Other hibernacula include spots under logs or rocks. Some species, like garter snakes, will den together which adds further insulation to each individual. Increased protection comes with snowfall. Snow actually helps insulate hibernacula and further shields the amphibians and reptiles from freezing air.
Nevertheless, a good hibernaculum is sometimes not good enough. Extreme cold snaps can still affect a brumating animal if it is not insulated well enough. In one winter, over 60,000 garter snakes (Thamnophis sirtalis parietalis) were estimated to have died in a midwinter freeze event in central Manitoba, Canada. This population study attributed mortality to unusually light snow cover at the time of the freeze event (Shine and Mason 2004).
Many amphibian and reptile species have evolved tolerances to freezing temperatures for short durations. The tolerance level varies by species and is often referred to as supercooling – the ability to remain unfrozen when temperatures drop below freezing. Most species that have been studied can remain unfrozen just below freezing (0° C or 32° F) for a limited time. But many of them perish if frozen for longer than 24 hours. Blanchard’s cricket frogs (Acris crepitans blanchardi) in South Dakota express better supercooling abilities (~6 hours) compared to cricket frogs living at lower latitudes. But they cannot survive 24 hours of freezing. Thus, winter survivors of this species are probably those individuals that found hibernacula buffered from freezing temperatures. Overwinter mortality is common among Blanchard’s cricket frogs in South Dakota and is one reason this species has not expanded farther north (Swanson and Burdick 2010).
Other species such as the gray tree frog (Hyla versicolor), spring peeper (Hyla crucifer), and western chorus frog (Pseudacris triseriata), are able to survive up to two weeks at mild freezing temperatures (25° to 28° F or -2° to -4° C; Storey and Storey 1986). Garter snakes, painted turtles (Chrysemys picta), red-backed salamanders (Plethodon cinereus), blue-spotted salamanders (Ambystoma laterale), and American toads (Anaxyrus americanus) are some examples of other species that exhibit limited supercooling abilities (Storey and Storey 1986, Paukstis et al. 1989).
Wood frogs (Lithobates sylvaticus or Rana sylvatica) are one of the most well-studied species when it comes to freeze-tolerance. These frogs have demonstrated the ability to survive more severe temperatures for longer durations than any other studied amphibian or reptile. Wood frogs can freeze solid and are the only frog species found north of the Arctic Circle. Scientists in Alaska followed 18 wood frogs for a year and documented full survival after experiencing temperatures as low as 0° F (-18° C) for up to seven months (Larson et al. 2014). Unlike other frogs, wood frogs produce an exceptionally high level of glucose in their blood prior to winter. The glucose, along with high levels of urea, act like antifreeze in a wood frog’s cells, preventing them from freezing. While this protects the cells themselves, water between cells and between tissue layers freezes solid. Over half the frog’s body water turns to ice. The animal stops breathing and its heart stops beating; by most metrics any other organism would be considered dead. But come spring, the frog’s heart begins beating, its lungs thaw out and resume operating, and the rest of its tissue follows suit. The body immediately addresses and repairs any cell damage sustained from water crystals. But otherwise, the frog resumes its life cycle, unscathed.
The amazing ability of wood frogs to sustain freezing for extended durations is of interest to researchers, not only because of the unique adaptation, but the potential to learn from this phenomenon and apply it to human organs. Safely freezing and thawing human organs would add longevity to organ transplant potential. These frogs also tolerate extremely high glucose levels in their blood without causing cell damage; if scientists can better understand this ability, they may be able to reduce risks associated with diabetes.
Many amphibian and reptile species are known for living lives spanning over a decade. (There are instances of American toads living for 30 years or more in captivity.) One interesting hypothesis for their life longevity is their ability to turn off bodily functions when they enter brumation. This is analogous to the winter-readying of a vehicle before letting it sit for 6 months, thereby elongating its overall working life. However, like most hypotheses, more research is needed to explore this idea.
Next time you are feeling sorry for yourself during the cold, tortuous months of winter, just appreciate that you are not outside in a hole that may or may not freeze you to death. And when you see your first frog, toad, turtle, or snake of spring, you can better appreciate what that critter endured (and how its species adapted!) to be there.
Larson, D.J., L. Middle, H. Vu, W. Zhang, A.S. Serianni, J. Duman, and B.M. Barnes. 2014. Wood frog adaptations to overwintering in Alaska: new limits to freezing tolerance. Journal of Experimental Biology 217(Pt 12):2193-200.
Paukstis, G.L., R.D. Shuman, and F.J. Janzen. 1989. Supercooling and freeze tolerance in hatchling painted turtles (Chrysemys picta). Canadian Journal of Zoology 67(4):1082-1084.
Shine, r. and R.T. Mason. 2004. Patterns of mortality in a cold-climate population of garter snakes (Thamnophis sirtalis parietalis). Biological Conservation 120(2):201-210.
Storey, K.B. 2006. Reptile freeze tolerance: metabolism and gene expression. Cryobiology 52(1):1-16 DOI:10.1016/j.cryobiol.2005.09.005
Storey, K.B and J.M. Storey. 1986. Freeze tolerance and intolerance as strategies of winter survival in terrestrially-hibernating amphibians. Comparative Biochemistry and Physiology. Comparative Physiology 83(4):613-617.
Swanson, D.L. and S.L. Burdick. 2010. Overwintering physiology and hibernacula microclimates of Blanchard’s cricket frogs at their northwestern range boundary. Copeia 2010(2):247-253.