Guest Blog: Freezing Wood Frogs

Dr. Jon Costanzo, Senior Research Scholar and Adjunct Professor in the Department of Zoology at Miami University in Oxford, OH was invited to tell us about his amazing research on wood frogs; animals that can actually survive freezing! Here is his post:

In a recent post, Dr. Dolittle commented on our poster.pdf presented at the Ohio Physiological Society's meeting in Cincinnati, Ohio. We reported on seasonal variation in the abundance of proteins within the liver of the wood frog (Rana sylvatica), the most cold adapted of all North American amphibians. The wood frog is one of about a dozen known species of amphibians and reptiles that are naturally "freeze tolerant," meaning that they tolerate the freezing of their tissues under naturalistic thermal and temporal conditions.

Wood frogs inhabit forests from the southern Appalachian Mountains north to Labrador
and west to Alaska, where they occur within the Arctic Circle (Figure 1). Throughout their range they hibernate beneath the forest duff, enduring dehydration and hypothermia, and passing many months without feeding.

Figure 1: Geographic range of the wood frog, Rana sylvatica. http://en.wikipedia.org/wiki/Wood_Frog

Depending on the weather and other factors, frogs may become frozen for periods lasting from hours to several weeks, and at high latitudes, perhaps months. While frozen they appear inanimate and have no heartbeat or pulmonary breathing, yet they completely recover upon thawing.

Figure 2: Fully-frozen adult wood frog. Photo credit: Evelyn Dietz.

The wood frog can survive the freezing of up to 65-70% of its body water so long as it cools slowly (and ice accumulates gradually) and body temperature remains above some critical temperature. This temperature is -5 to -6°C for frogs indigenous to the Midwestern United States and southern Canada, but apparently is much lower for frogs of northern populations. With support from the National Science Foundation, our current research project aims to elucidate the molecular and physiological basis for extreme freeze tolerance in Alaskan wood frogs.

Figure 3:Biophysical and physiological responses to freezing and thawing of the wood frog. A: exothermic rise in body temperature after freezing begins (at time zero), followed by gradual cooling; B: initial increase in heart rate followed by cardiac arrest; C: accumulation of ice in tissues to a survivable equilibrium level; D: rapid accrual of the cryoprotectant, glucose, in tissues; E: protective dehydration of tissues during freezing and rapid rehydration following thawing. Adapted from our review article.

Natural freeze tolerance, which was first reported for amphibians by William D. Schmid some 30 years ago, is a remarkably complex adaptation. During freezing, ice forms only in the blood and spaces outside cells; intracellular freezing is lethal to nearly all organisms. Freezing/thawing stress is ameliorated by "cryoprotectants," certain organic solutes that limit cellular shrinkage and protect membranes and macromolecules. Cryoprotectants used by the wood frog include glucose, which is quickly synthesized in the liver after freezing begins, and urea, which gradually accumulates in tissues during autumn and early winter. As a bonus, high levels of urea also contribute to a metabolic depression that reduces energy use during dormancy. Tissue damage is further minimized because much of the water within the organs is translocated to the coelom before it freezes. Post-thaw resumption of normal behavioral and physiological functions can occur within 12-24 hours, but may be delayed several days if the freezing exposure was severe. A time-lapse video of the usual thawing/recovery process can be viewed here . In the video, time is compressed 120:1.

Research in our Laboratory for Ecophysiological Cryobiology addresses the mechanisms permitting various species of insects, amphibians, and reptiles to thrive in cold environments. Some of these animals spend over half of their lives in hibernation, yet relatively little is known of their winter biology. Understanding this facet of their life history may help predict consequences of climate change for their survival, as cold tolerance governs the distributional patterns of many ectotherms. Furthermore, investigation of natural cold-hardiness mechanisms may contribute to knowledge in various applied disciplines, such as integrated pest management, tissue cryopreservation, and organ banking.