Amphibians
Wood Frog
Lithobates sylvaticus · also called Rana sylvatica
The wood frog is a small brown frog with a black “robber’s mask” and an astonishing trick: every winter it freezes into a solid block, heart stopped, and every spring it thaws out and carries on. That ability has let it colonise ground no other frog can survive.
Overview
Wood frogs live in moist woodland across the northern half of North America, from the Appalachians and upper Midwest across the whole of Canada and Alaska to the Arctic tundra. Outside the brief breeding season they are terrestrial, foraging on the forest floor among leaf litter and logs, well away from open water. What ties them to a place is the presence of fishless temporary ponds — vernal pools — where they breed.
Freezing solid
Most frogs in cold climates overwinter underwater, below the ice, where it never quite freezes. The wood frog does the opposite: it hibernates on land, under leaves and snow, and lets itself freeze.
As ice begins forming on its skin, the frog’s body launches a chemical response — the liver dumps massive amounts of glucose into the blood and cells, and urea builds up as the kidneys shut down. These act as natural antifreeze, protecting the interior of every cell. Ice is allowed to form only in the gaps between cells and around the organs. At the peak, the frog’s heartbeat and breathing cease entirely and up to 70% of its body water is frozen, and it can survive in that state for weeks, through repeated freeze–thaw cycles.
When spring warmth arrives, the frog thaws from its core outward, the heart flickers back to life within a few hours, and the animal is soon active. No other North American amphibian tolerates freezing to this degree, and it is why the wood frog is the only one found within the Arctic Circle.
Explosive breeding
Wood frogs are among the first amphibians to breed each year, moving to vernal pools while snow still lies on the ground. Breeding is explosive: the entire local population converges on a pool, males chorus with a low chuckling call, mating and egg-laying happen within days, and the adults leave. The eggs and fast-developing tadpoles must complete metamorphosis before the pool dries in summer.
Status
The wood frog is Least Concern and abundant. Its vulnerability is local and specific — the loss of vernal pools to filling, drainage, and development, and the fragmentation of the surrounding forest that the adults need to reach those pools. It is widely used as a bioindicator for the health of temporary-wetland systems, and its freeze physiology is actively studied for insights into preserving human organs and tissues.
Wood Frog: common questions
How does the wood frog survive being frozen?
When ice starts to form on its skin in autumn, the frog's liver rapidly converts stored glycogen into huge amounts of glucose, and its kidneys stop clearing urea, so both flood the bloodstream and cells. These act as cryoprotectants, keeping the fluid inside the frog's cells from freezing and drawing water out of the cells before it can turn to damaging ice crystals. Ice forms only in the spaces between cells and organs. The frog's heart and breathing stop, up to about 70% of its body water becomes ice, and it can stay like that for weeks. In spring it thaws from the inside out, the heart restarts within hours, and the frog hops away.
How far north do wood frogs live?
Farther than any other frog or toad in North America. Wood frogs range across essentially all of Alaska and Canada to the edge of the Arctic Ocean, well north of the Arctic Circle, in country where the ground stays frozen for more than half the year. Their freeze tolerance is what makes this possible — they overwinter under leaf litter and snow, not in unfrozen water like most frogs.
What is "explosive breeding"?
Wood frogs spend most of the year dispersed through the forest, but in early spring, often while snow still lies around, they migrate en masse to temporary woodland ponds ("vernal pools"). The males call — a soft, duck-like clucking chorus — and the whole population mates and lays eggs within a few days to two weeks, then leaves. Timing it to fishless temporary pools means the tadpoles develop before the water dries and with no fish to eat them, but it is a race against the pool drying out.
Are wood frogs endangered?
No — they are Least Concern and abundant across a vast range. Local populations are lost when vernal pools are filled or drained and when the forest around them is cleared, cutting off the pools the adults need, and they are studied as an indicator of wetland health. Their remarkable physiology also makes them important in research on organ preservation and cold tolerance.