The wood frog is a medium-sized, tan frog with a distinct, dark brown “mask” stripe on each side of the head extending from the snout to behind the tympanum (rounded external ear). The general color is pink tan, light brown, red brown, or dark brown. Scattered small, dark markings may be present on the back, and the hind legs usually have brown or dark brown bars. There is a prominent white line along the upper lip. The dorsolateral fold (ridge of skin running along each side of the back) is distinct and extends to the groin. The belly is white with scattered dusky markings.
The call of a male wood frog is a rapid, hoarse “waaaduck” sound that lasts about one second and may be quickly repeated three or four times; they sound like quacking ducks. The sound of a small chorus of this species carries only a short distance.
Similar species: Young green frogs (including their bronze frog color morph) can have a similar color, but they lack the distinctive dark brown mask through each eye.
Missouri has eight members of the true frog family. These are typically medium- to large-sized, have long legs, smooth skin, and well-developed webbing between the hind toes. Another common characteristic is a glandular fold or ridge of skin along each side of the back (these are called dorsolateral folds).
Adult length (snout to vent): 1½–2¾ inches; occasionally to 3½ inches. Males are smaller than females.
Scattered in a few north-central and eastern counties, and in the southeastern and southwestern Missouri Ozarks.
Habitat and Conservation
The wood frog is a secretive and solitary species that can be difficult to observe after its short breeding season.
During the summer, wood frogs live along shady ravines, among dead leaves, or along north-facing rock outcroppings and bluffs. A study in Warren County showed the wood frog extensively uses forested, rocky ravines that are moist and cool. Wood frogs travel up to 1,300 feet from the nearest breeding sites, and movement within these habitats is mainly at night during or following rainfall.
Wood frogs are also generally associated with cool, north-facing wooded hillsides, where there is ample shade and some moisture. Oak-hickory forest, often with sugar maple, is the preferred habitat. A wood frog among dead oak and maple leaves is nearly invisible. During the spring and summer, most adult wood frogs completely bury themselves under the deciduous leaf litter, with only an occasional eye or other part of the body visible. During summer drought conditions, they are often found within cave entrances in the Ozark Highlands.
Wood frogs overwinter on land beneath a deep layer of leaves or in damp soil under brush piles or logs. As with several other frogs and toads that overwinter close to the surface under cover objects, wood frogs protect themselves from freezing by producing glucose in their blood that acts like antifreeze.
This generally northern species is called a “glacial relict” because past glaciations pushed it to the southern part of its range, where it endures in cool locations.
Wood frogs eat a variety of insects and other invertebrates. The diet of young wood frogs may include small grasshoppers, leafhoppers, and small crickets.
Wood frog tadpoles prefer to eat algae and other plant material when available, but if such food sources become scarce, they will shift their diet to eggs and larvae of amphibians, including wood frog eggs and larvae.
A species of conservation concern in Missouri. For a time in the 1970s it was classified in our state as being in danger of extinction, but new populations were discovered. Still, the frog is considered rare and vulnerable in Missouri.
Wood frogs were successfully reintroduced into a fishless pond in St. Louis County in 1980 and have spread throughout the general area. Today, the wood frog appears to be naturally expanding its range in Missouri but remains a species of conservation concern due to loss of temporary, fishless wetlands, conversion of forested habitat, and isolated populations.
Taxonomy: The true frog family (Ranidae) is the largest and most widespread family of frogs. It contains 365 species in 14 genera and probably originated in Africa. Representatives of this cosmopolitan family occur on every major land mass except New Zealand, Antarctica, most oceanic islands, the West Indies, and southern South America. The largest genus in the family in the New World (North and South America) is Lithobates (formerly Rana), with about 50 species. Missouri’s species, formerly in genus Rana, are all in genus Lithobates. As of taxonomic understandings in 2016, the Rana genus is considered restricted to the eastern hemisphere and western North America. In Missouri, the genus Lithobates is represented by eight species.
Active between February and October. The wood frog is an explosive breeder during late winter and early spring. Small, fishless, woodland ponds and ephemeral (temporary seasonal) pools are used for breeding. In Missouri, breeding takes place between early February and late March, depending on local weather conditions.
Wood frogs require heavy, warm rains and an air temperature of at least 50°F to stimulate breeding. Males move to a breeding pool right after sunset and normally vocalize until about midnight. During ideal, warm and moist weather conditions, wood frogs will call during the day. Spring peepers chorus and breed along with wood frogs; the peepers' loud, high-pitched calls can easily overpower the muted calls of the wood frogs. Gravid females begin arriving at the breeding site on the first night the males begin to call. By the second or third night, all the eggs are laid and few wood frogs remain at the pond or pool. The males normally call while floating in open water.
A wood frog egg mass contains 500–1,000 eggs, and in a breeding pond, there can be 50–100 masses within a few square yards. The eggs hatch within 10–14 days. Development is rapid, and metamorphosis takes place from May to mid-June.
Lifespan is relative short; few females live beyond four years, and few males beyond three.
When a perfectly camouflaged wood frog is sitting quietly among dead oak and maple leaves, it is nearly invisible. When you happen to see one of these rare frogs on a woodsy outing, you have received a special gift.
Management of the wood frog should focus on maintaining and creating the temporary wetlands it needs for breeding and larval growth within large areas of intact forests. Preserving a 984-foot buffer of mature forest around breeding wetlands within the broader forested landscape is important for maintaining healthy populations of wood frogs. Preserving the forests' connections to breeding wetlands via forested ravines is critical for wood frog movement and survival.
This species helps to check the huge growth potential of various insect populations, while also serving as food for snakes, raccoons, mink, and skunks. The eggs and tadpoles are food for various aquatic predators.
As wood frog egg masses age, they become greenish due to infiltration by symbiotic green algae (Oophilia amblystomatis). These algae can take up carbon dioxide and wastes produced by the developing embryo and provide the embryo with oxygen and sugar. It may also help camouflage the eggs. This is the same alga that is associated with the eggs of spotted salamanders and some other amphibian species in North America.