The wood frog, Lithobates sylvaticus, has garnered attention from biologists because of its freeze tolerance. It is a frog species with a broad distribution over North America. These frogs are able to survive the freezing of their blood and other tissues.

Frogs Freeze

How Wood Frogs Survive Freezing Temperatures

  • Tolerance of Freezing: Wood frogs, like other northern frogs, have the ability to tolerate the freezing of their blood and tissues. They can endure temperatures as low as −3 to −6 °C (27 to 21 °F). Interior Alaskan wood frogs can even survive the freezing of a significant portion of their body water.
  • Cryoprotectants: The wood frog accumulates urea in its tissues and converts liver glycogen into glucose. Both urea and glucose act as cryoprotectants, limiting ice formation and reducing osmotic shrinkage of cells.
  • No Vital Signs: When frozen, wood frogs exhibit no detectable vital signs, including heartbeat, breathing, blood circulation, muscle movement, or brain activity. They can remain frozen for extended periods, with one study showing an average of 193 days.
  • Cellular Protection: The wood frog has evolved traits that prevent their cells from being damaged when frozen and thawed. They combat prolonged oxygen deprivation and extreme cellular dehydration. The accumulation of high amounts of glucose is one crucial mechanism, acting as a cryoprotectant.
  • Urea Recycling: Wood frogs recycle urea, the main waste in urine, into useful nitrogen, which helps them survive as they hibernate and freeze. Special microbes in their guts recycle the urea.

Regional Adaptations

Studies on northern subpopulations of wood frogs have revealed interesting adaptations. Alaskan wood frogs, for instance, have a larger liver glycogen reserve and greater urea production compared to those in more temperate zones. They also exhibit higher glycogen phosphorylase enzymatic activity, which aids in their adaptation to freezing.

Other Cold-Resistant Species

The phenomenon of cold resistance is also observed in other anuran species. The Japanese tree frog, for example, exhibits even greater cold tolerance than the wood frog, surviving in temperatures as low as −35 °C (−31 °F) for up to 120 days.

Habitat and Conservation

Wood frogs are forest-dwelling organisms that breed primarily in ephemeral, freshwater wetlands. They spend most of their time on the forest floor and require a landscape perspective for conservation, considering their wide-ranging movements among breeding pools and neighboring habitats.

While the wood frog is not currently endangered, urbanization is fragmenting populations in many areas. They are also primarily dependent on smaller wetlands for breeding, which are largely unprotected. Road salt contamination of freshwater ecosystems can also negatively affect tadpole development.

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