Wood frog adaptations to overwintering in Alaska: new limits to freezing tolerance

Followed wood frogs through natural Alaskan winters in their own overwintering sites, measuring body temperature, the repeated freeze–thaw cycles they experienced, and the cryoprotectant concentrations they accumulated.

Studied in

Lithobates sylvaticus

Sample size

frogs overwintering in natural Alaskan hibernacula

Alaskan wood frogs survived far lower temperatures and longer freezing than laboratory work had established, accumulating much higher glucose concentrations, and the repeated partial thaws of autumn appeared to drive that accumulation.

• Field measurement means less control over exposure, so the relationship between the freeze–thaw cycling and the glucose accumulation is inferred rather than manipulated.
• One population at the northern edge of the range; other populations are less cold-hardy.

• Letting the winter do the experiment

  Laboratory work established how much freezing a wood frog survives. Do frogs in an actual Alaskan winter match it?

• Wood frog

  A frozen wood frog has ice through roughly two-thirds of its body water, no heartbeat and no breathing, for weeks — and thaws into a working frog. The ice is outside its cells, and it is flooded with glucose.