Where Do Frogs Go in Winter? Hibernation in Ponds and Mud

Why frogs disappear in winter

Frogs are ectotherms, sometimes called cold-blooded animals: their body temperature follows that of their surroundings. As autumn turns cold, they become too slow to hunt or escape predators, and their insect prey disappears. So they find a sheltered place and enter a long dormancy, usually called hibernation.

During this time a frog’s heart rate and breathing slow right down, and it uses very little energy. It does not eat at all, living instead on fat and glycogen it stored during summer and autumn. Where a frog spends the winter depends on the species and the climate, and there are three main strategies.

Strategy 1: Under logs, leaves and compost heaps

Many frogs and nearly all toads overwinter on land. They choose places that stay damp but frost-free: under logs, rocks and roots, deep in leaf litter, inside rotting tree stumps, in old mammal burrows, in cracks in walls and in garden compost heaps. Toads can dig themselves into loose soil.

This is the main strategy for the European common frog, especially females and young frogs. Because it cannot survive being frozen, the frog must find a spot below the frost line, and it often shares good shelters with other frogs, toads and newts. In Britain, common frogs usually hibernate from about late October or November to January or February.

Strategy 2: At the bottom of ponds

Other frogs spend the winter underwater. Many male common frogs overwinter in ponds, as do North American species such as the American bullfrog, green frog and leopard frogs. They do not dig deep into the mud. Instead, they rest on the bottom, partly settle into soft sediment or tuck themselves among dead leaves, and some move around slowly even under the ice.

These frogs survive because cold water holds plenty of oxygen and their slowed bodies need very little. All the oxygen they use passes in through their skin, so they do not need to come up to breathe. The whole process is explained in Can frogs breathe underwater? The risk is winterkill: if a pond stays frozen over for a long time and rotting plants use up the oxygen in the water, frogs at the bottom can suffocate.

Strategy 3: Freezing solid on land

The most remarkable winter survivors are a handful of North American frogs that can freeze. The best known is the wood frog, which ranges farther north than any other North American frog, beyond the Arctic Circle in Alaska and Canada. It spends the winter just under the leaf litter on the forest floor, where it may freeze completely.

When ice starts to form on its skin, the wood frog’s liver rapidly releases large amounts of glucose into the blood. Together with urea, this sugar acts like antifreeze inside the cells, keeping them from freezing and drying out, while ice forms in the spaces between cells and in the body cavity. A frozen wood frog has no heartbeat, no breathing and no detectable brain activity. Frogs in southern Canada and the US Midwest survive about −3 to −6 °C (27–21 °F), and wood frogs in interior Alaska have survived months frozen at much lower temperatures, around −16 °C (3 °F) or colder.

Other freeze-tolerant frogs include the spring peeper, the gray treefrog, Cope’s gray treefrog and several chorus frogs. For the full story see How do wood frogs survive freezing?

When do frogs wake up?

Frogs become active again when the air and water warm up, usually after a few mild, wet nights. In southwest England and other mild parts of western Europe, the first common frogs can appear in December or January. Across most of lowland Britain and western Europe, frogs come out between January and March and head straight to their breeding ponds.

In colder places hibernation lasts much longer. Common frogs in the Alps and the far north may not emerge until May or June, after six to eight months of dormancy. Wood frogs thaw in early spring and are among the first frogs to breed, sometimes calling from woodland pools while ice still rims the edges.

What about frogs in warm climates?

Tropical frogs do not hibernate, because their habitats stay warm all year. However, frogs in hot, dry climates face the opposite problem: summer drought. Many desert and savanna frogs spend the dry season buried underground in a dormant state called estivation, sometimes inside a protective cocoon of shed skin, and emerge only after heavy rain.

Helping frogs through winter

Frogs and other amphibians benefit when winter shelters are left alone. Log piles, rock piles, compost heaps, leaf litter and patches of long grass all provide places to hide. Wildlife groups recommend leaving these undisturbed from autumn to spring and avoiding forking over compost heaps in the coldest months, when frogs, toads and newts may be sleeping inside.