Wood Frog (Lithobates sylvaticus): The Frog That Freezes

Appearance: the frog with a robber’s mask

The easiest way to identify a wood frog is its mask: a dark brown or black patch that runs from the snout through the eye and over the round eardrum to the base of the front leg. Below it, a pale line runs along the upper lip. This look has earned it the nickname “the frog with the robber’s mask”.

The rest of the body is usually brown, tan or rusty, but individuals can be pinkish, grayish or even greenish, and a single frog can turn lighter or darker over time. Two raised ridges of skin, called dorsolateral folds, run down the sides of the back. The underside is pale, often with a yellowish or greenish tint, and in northern populations it can be faintly mottled.

Females are larger than males. In the breeding season males develop swollen thumb pads that help them grip females.

How big are wood frogs?

Wood frogs are small to medium-sized frogs. Adults measure 3.5–7.6 cm (1.4–3 in) from snout to vent, the standard frog measurement that leaves out the legs, and most are 5–7 cm (2–2.8 in). An average adult weighs only about 8 g (0.3 oz).

Habitat and range

Wood frogs live across the northern half of North America. Their range stretches from northern Georgia and Alabama through the Appalachians and the northeastern United States, around the Great Lakes, and right across Canada to Labrador, British Columbia and Alaska. Isolated populations live in the Medicine Bow area of Wyoming and Colorado and in lowland eastern North Carolina. This is the only North American frog found north of the Arctic Circle, and the most widespread frog in Alaska.

As the name suggests, the wood frog is a forest animal. It lives in moist deciduous, mixed and coniferous woodland, including the boreal forest, as well as forested swamps, bogs, wet meadows and the edge of the tundra in the far north. Unlike most of its relatives, it spends most of the year on land, hidden among dead leaves on the forest floor. Read more in Where do wood frogs live?

What do wood frogs eat?

Adult wood frogs eat small invertebrates of the forest floor: beetles, moth caterpillars, spiders, slugs, snails and worms, along with many other insects. They hunt mainly by day, sitting still until something moves, then lunging forward. Unlike bigger pond frogs such as the bullfrog, which wrap their fleshy tongue around their prey, the wood frog touches its target with just the tip of its tongue, much as a toad does.

Tadpoles are omnivores. They graze on algae and decaying plant matter, and they also eat dead animals and the eggs and larvae of other amphibians, sometimes including other tadpoles.

The frog that freezes

The wood frog’s most remarkable talent is surviving winter frozen. In autumn, adults move a short distance from their breeding pools, often onto higher ground, and settle just under the leaf litter. Unlike pond frogs, they do not dig deep or hide below the frost line, so in cold weather they freeze.

Up to about 65–70% of the frog’s body water can turn to ice. As freezing begins, the liver pours large amounts of glucose into the blood, and together with urea stored in autumn this sugar keeps the inside of cells from freezing. A frozen wood frog has no heartbeat, no breathing and no detectable brain activity, yet it revives when it thaws. Frogs from southern Canada and the Midwest survive about −3 to −6 °C (27–21 °F), while Alaskan wood frogs tracked in the wild stayed frozen for about 193 days in a row and survived temperatures down to about −18 °C (0 °F).

How this works is explained in How do wood frogs survive freezing?, and other frogs’ winter strategies are in Where do frogs go in winter?

Breeding in vernal pools

Wood frogs are among the first frogs to breed each year, often right after the snow melts and sometimes while ice still covers part of the pond. Along with the spring peeper, they are usually the first amphibians heard calling in spring. Breeding takes place in late winter or early spring, from about February in the south to May in the north. Wood frogs are “explosive breeders”: in many populations all mating takes place within a week or two.

They breed mostly in vernal pools, temporary ponds that fill with snowmelt and rain in spring and dry up in summer. Because these pools dry out, they contain no fish, which would eat eggs and tadpoles. The price is a race against time, since tadpoles die if a pool dries before they change into frogs.

Males call from the water with a short, rough clucking or quacking, and a chorus can sound like a group of ducks. Males grab any frog they meet and let go if it is another male or a female that has already laid her eggs. For more on frog calls, see Why do frogs croak?

Each female lays one round mass of about 1,000–3,000 eggs, 10–13 cm (4–5 in) across, attached to twigs or plants or floating free. Females often lay next to one another, forming large communal rafts of egg masses. Eggs in the middle of a raft stay warmer and develop faster than those at the edge. Over time the jelly turns green with tiny algae, which helps camouflage the eggs.

From egg to frog

Depending on the water temperature, the eggs hatch in about 9 to 30 days. The olive-brown to black tadpoles grow quickly and change into froglets about 2 to 4 months after hatching, usually by early summer. Wood frog tadpoles are known for strong kin recognition: they can tell siblings from unrelated tadpoles and tend to gather with them. The stages of this change are explained in How do tadpoles turn into frogs? Only a small share of tadpoles survive to leave the pool, and changes in tadpole survival from year to year are a major reason why wood frog numbers rise and fall.

Young males can breed one to two years after metamorphosis, and females two to three years after. Most wood frogs return to the pool where they hatched, and adults are highly faithful to the pool where they first bred, although a minority of young frogs, fewer than one in five, disperse to new pools.

Lifespan and predators

Wood frogs have short lives. Most live about 3 to 5 years in the wild, and males tend to die younger than females. Most adults breed only once, and few more than two or three times.

Adults are eaten by garter snakes, ribbon snakes, northern water snakes, herons, raccoons, skunks, mink and larger frogs. Tadpoles fall prey to diving beetles, water bugs and salamander larvae, and leeches, eastern newts and aquatic insects eat the eggs. The wood frog’s main defense is camouflage among dead leaves. When seized, it may give a loud, piercing cry that can startle a predator into letting go.

Conservation status

The IUCN Red List classifies the wood frog as Least Concern (assessed in 2015) because it is common and widespread over a huge range. Locally, however, populations are declining. Woodland clearance, roads and suburban growth split forests from the pools where the frogs breed, and many frogs are killed crossing roads on their spring migration. In the United States many small, isolated wetlands such as vernal pools have little federal protection, so their conservation is left largely to states.

Other threats include road salt washing into breeding pools, which harms tadpoles, as well as polluted runoff and disease outbreaks such as ranavirus, which can kill large numbers of tadpoles. Because they need both healthy forest and clean pools, wood frogs are considered good indicators of the health of their environment.

Myths and facts

Myth: Any frog can be frozen and brought back to life. Only a handful of frogs survive freezing, among them the wood frog, spring peeper, gray treefrogs and chorus frogs of North America. Most frogs, such as the European common frog, die if they freeze solid.

Myth: The wood frog lives in ponds. It visits ponds and pools for only a few weeks in spring. The rest of the year it lives on land in the woods, which is how it got its name.

Fact: Wood frogs live farther north than any other North American amphibian or reptile. They reach beyond the Arctic Circle in Alaska and northern Canada, where winters can last more than half the year.

Fact: Its scientific name means “of the woods”. The American naturalist John Eatton Le Conte described the species in 1825 as Rana sylvatica, from the Latin silva, a wood or forest. Many scientists later moved North American true frogs into the genus Lithobates, which is why both names appear in field guides today.