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Canadian Herpetological Society Société d'Herpétologie du Canada |
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Wood Frog
Lithobates sylvaticus
Family: Ranidae
COSEWIC status:- Not Assessed
- No Status
- Least Concern
Description
The Wood Frog is a medium-sized frog that can grow to 8 cm in body length. Individuals are reddish-brown, tan or dark brown and have a dark mask around the eyes, a white stripe on the upper lip, prominent dorsolateral folds (folds of skin running down each side of the back), and a white belly that may be tinged with yellow or green. Wood Frogs may also have pronounced dark bars on the rear legs, a small amount of black mottling on the sides and a light-coloured stripe down the back. Larvae (tadpoles) have long tails with a large fin and lack front legs (newly hatched tadpoles are legless). Larvae are uniformly dark grey to dark brown with a faint, light-coloured stripe along the upper jaw and they have a light-coloured belly. The larvae may reach lengths of 6.5 cm before metamorphosis. The Wood Frog’s call is a series of clucks.
Similar Species
In eastern Canada there are no other frogs with both a mask and dorsolateral folds. In western Canada, Columbia Spotted Frogs, Oregon Spotted Frogs and Northern Red-legged Frogs can all have a dark mask but none of these species have a white underside.
Distribution
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The Wood Frog is the most widely distributed amphibian in North America and is found in every province and territory in Canada. It is absent from most of Nunavut except the southern tip of the territory and is also absent from the island of Newfoundland (but present in Labrador). Unlike most North American frogs, the majority of this species’ distribution is in Canada. The Wood Frog occurs throughout the northeastern United States and there are small, isolated populations in Colorado, Wyoming and northern Idaho.
Habitat
The Wood Frog is closely associated with deciduous and boreal forests, but this species is also found north of the tree line in the tundra. Wood Frogs breed in shallow (less than 1 m deep), fish-free ephemeral (temporary) wetlands within forested areas. They may also breed in flooded ditches, road ruts, ponds and shallow bays of lakes. Wood Frogs primarily forage in forest habitats and they hibernate under leaf litter on the forest floor.
Biology
Wood Frogs hibernate during the winter and are typically active from late March or early April until late September or October in southern Canada, but the active season is shorter further north. Wood Frogs are one of the first frogs to begin calling and breeding in spring, often arriving at breeding sites when there is still some snow on the ground. Breeding is explosive and occurs over a one or two week period in April or May, depending on latitude. During breeding, the male grasps the female (amplexus) and fertilization occurs externally in aquatic habitats as the female lays eggs. The female lays a globular mass of 300–2000 eggs that is 4–10 cm in diameter. The egg mass is usually deposited in very shallow water or is attached to vegetation within 15 cm of the water’s surface. The eggs are black on top and white underneath and are surrounded by two clear jelly envelopes, although the inner envelope is often indistinct. The eggs develop rapidly and hatch in one to three weeks, depending on water temperature. The tadpoles transform into frogs in 6–12 weeks. Males typically reach maturity in 1–2 years while females mature after 2–3 years, but individuals mature later than this in northern populations. Wood Frogs typically do not live more than 5 years. Wood Frogs are able to survive sub-freezing temperatures during hibernation by increasing the levels of glucose in their blood, which lowers the freezing point and also prevents the formation of ice particles within the cells. Wood Frogs are active during the day and night and they eat a wide variety of insects, spiders and other small invertebrates.
Threats
Wood Frogs are widespread and abundant in Canada. They occur throughout large expanses of northern Canada that are relatively undeveloped, and threats to this species are minimal throughout most of its range. The loss of forest habitat and associated breeding wetlands is likely the primary threat to this species. Pollution, such as herbicides, agricultural effluent and road salt, can be detrimental to frog populations by causing direct mortality as well as developmental deformities. Large numbers of Wood Frogs are killed on roads every year during migrations to and from breeding sites, and high levels of road mortality can cause long-term decline of frog populations. Pathogens such as chytrid fungus and Ranavirus can cause mass mortality of frog populations. Climate change may also pose future threats to Canada’s frog populations.