Wood frogs range from 3.5 to 7.6 cm. They exhibit a number of different skin colors, usually browns, tans, and rust, but they can also be found in shades of green and gray. They can be distinguished by a black "robber's mask" that extends over the tympanum (outer ear) to the base of the front leg. They also feature a white line outlining their upper lips. Most wood frogs have a light yellowish-brown fold around their sides and mid-back. They have two back ridges on either side that extend from behind the eye, down the side of the back, and to the legs. The underparts are white becoming pale orange-yellow towards the rear, with male frogs having more bright colors on their thighs. Males have two vocal sacs for calling. Females are much larger than males. (Dickerson, 1931; Mansker, 1998)
Wood frogs are native to North America and live only in the United States and Canada. They are the most widespread North American amphibian and are the only frogs found north of the Arctic Circle. Specifically, wood frogs are found from Alaska in the west to Labrador, Canada in the east. They also inhabit the northern Mid-west and northeast regions of the United States and range from North Dakota in the west to Maine in the east, and south to northern Georgia and Tennessee. (Conant and Collins, 1998)
Wood frogs are common in woodlands across their range. They can be found in a variety of habitats including tundra, thickets, wet meadows, bogs, and coniferous or deciduous forests. Wood frogs are aquatic breeders and require fish-free seasonal bodies of water to reproduce such as ponds, woodland pools, or water-filled ditches. When not breeding, wood frogs may migrate away from water and live under logs, fallen branches, or leaves. These frogs hibernate in winter and will hide under logs or leaves to survive the cold months. (Redmer and Trauth, 2005)
The time it takes for fertilized eggs to hatch is dependent on water temperature. Eggs that are laid in colder waters in early March may take a month to hatch, whereas eggs laid later when water temperatures are warmer may take only 10 to 14 days. Tadpoles are olive-brown to black in color and measure 49.8 mm in length. Tadpoles metamorphosize into juvenile frogs when they reach 50 to 60 mm in length, usually when they are 65 to 130 days old. Juveniles measure 16 to 18 mm in length after metamorphosis. Juvenile males reach adulthood and are able to breed when they are 1 to 2 years old whereas females take longer and are not adults until they are 2 to 3 years old. Like all frogs, wood frogs exhibit indeterminate growth and will continue growing throughout their lives. (Redmer and Trauth, 2005)
Wood frogs exhibit "explosive" breeding in late winter or early spring when the first warm rains occur. Explosive breeding consists of many frogs gathering at ponds where they all actively scramble to find a mate. The more frogs there are at a pond, the better the chances are for an individual frog to successfully find a mate. Frogs wake from hibernation and migrate to breeding ponds. Many wood frogs return to the same ponds to reproduce year after year. (Redmer and Trauth, 2005)
Wood frogs breed every year during the spring from early March to May. During this time males begin to call to attract females. They create a duck-like quacking sound, described by some as a "lot of chuckling". Once mates are chosen and breeding occurs, females lay a globular egg mass in the water, most often in the deepest part of a pond. Each egg mass measures about 10 to 13 cm in diameter, and can contain from 1000 to 3000 eggs. The masses can either be attached to a twig or grasses, or they can be left free-floating. After about a week the egg mass begins to flatten out, allowing it to rest on the surface of the water. The jelly around the eggs becomes green, creating a kind of camouflage. The mass then looks like a floating mass of green pond scum. The green color of the jelly is due to the presence of many small green algae. The eggs hatch after 9 to 30 days and the tadpoles will undergo metamorphosis when they are 2 months old. Male juvenile frogs grow into adults and are able to breed when they are 1 to 2 years old, while females take a bit longer and are not adults until they are 2 to 3 years old. (Conant and Collins, 1998; Redmer and Trauth, 2005)
Like many frogs, wood frogs do not provide any further parental care after fertilizing and laying the eggs. Eggs are supplied with a nutritious yolk sac to sustain the tadpoles during the early stages of life. The parents select breeding sites without fish to increase the likelihood that their young will survive. (Redmer and Trauth, 2005)
Wood frogs frogs are expected to live to 4 or 5 years old for males and females, respectively, living in Quebec and southern Illinois. Studies done in several other states showed wood frogs live to be 3 to 4 years old for males and females, respectively. It is unknown why males consistently have shorter lifespans than females. (Redmer and Trauth, 2005)
Wood frogs are a diurnal species and therefore are active during the day. These frogs perform seasonal migrations to breeding ponds in late winter or early spring. Individual wood frogs generally return to the same ponds to breed year after year and will often migrate within the same area throughout their lives. Though they gather in great numbers at breeding ponds, these frogs are mostly solitary.
Wood frog tadpoles have been shown to have the ability to recognize their own brothers and sisters. They have been documented (by marking them with dye and releasing them into natural habitats) to group together according to family. This may be a survival mechanism allowing them the potential benefit of food, temperature control, and defense against predators. (Blaustein and Walls, 1995; Redmer and Trauth, 2005; Savage, 1961)
Wood frog territory size is currently unknown. Home range for these frogs is estimated to be an average of 83.6 square meters. (Redmer and Trauth, 2005)
Wood frog males will actively search for females during the breeding season; however, they are unable to tell males from females by sight. Gender recognition is accomplished by the males embracing other frogs (regardless of gender) and releasing those that are not fat enough to be females full of eggs. If a male is embraced he will let out a loud croak. A female will also be let go if spawning has already occurred, because she is thin.
The call of a wood frog is often compared with the sound of a quacking duck or a squawking chicken. They tend to repeat the call several times in a row when trying to attract females. Wood frogs use auditory forms of communication nearly exclusively during the breeding season. (Harding, 1997)
Adult wood frogs eat a variety of terrestrial insects and other small invertebrates, especially spiders (Order Araneae), beetles (Order Coleoptera), moth larvae (Order lepidoptera), slugs (Order Stylommatophora) and snails (Order Stylommatophora). Wood frog larvae consume algae, decaying plant and animal matter, and eggs or larvae of other amphibians. (Chenard, 1998; Harding, 1997)
Adult wood frogs have many predators including larger frogs, garter snakes, ribbon snakes, water snakes, herons, raccoons, skunks, and mink. Tadpoles are preyed upon by diving beetles, water bugs, and Ambystoma salamander larvae. Leeches, eastern newts, and aquatic insects may eat wood frog eggs.
Wood frogs have developed several anti-predator mechanisms. Older tadpoles develop poison glands that repel many predators. Adult wood frogs have noxious skin secretions but they are only effective in deterring shrews. These frogs rely on their cryptic coloration to camouflage into the forest floor and escape predators. If captured, wood frogs may emit a piercing cry that may startle the attacker enough to release the frog. (Harding, 1997)
Wood frogs have many predators and thus provide food for many animals in an ecosystem. They also feed on many terrestrial and aquatic invertebrates and therefore control insect populations.
There are no known negative effects of wood frogs on humans.
Wood frogs, along with other amphibians, are great indicators of environmental health. Population declines in species of amphibians should be of great concern. Wood frogs may also help to control invertebrate pests.
Though wood frogs are fairly common in most areas of appropriate habitat, loss of habitat to agriculture and suburban development has put them on the list of "species of special concern" in some areas. Populations may decline if breeding ponds are drained or forest habitats are logged. Many migrating frogs are killed while crossing busy roads to access breeding ponds. Studies have shown that eggs and larvae may be harmed by acid rain or toxic runoff that enter breeding pools.
The International Union for Conservation of Nature and Natural Resources (IUCN) consider wood frogs to be of "Least Concern" as they are an abundant and widespread species. Wood frogs are common throughout Michigan. (Harding, 1997)
Tanya Dewey (editor), University of Michigan-Ann Arbor, George Hammond (editor), Animal Diversity Web Staff, Rachelle Sterling (editor), Special Projects.
Allison Poor (editor), University of Michigan-Ann Arbor.
Katie Kiehl (author), Michigan State University, James Harding (editor), Michigan State University.
Blaustein, A., S. Walls. 1995. Aggregation and Kin Recognition. American Naturalist, 121: 449-454.
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Harding, J. 1997. Amphibians and Reptiles of the Great Lakes Region. Ann Arbor, MI: The University of Michigan Press.
Mansker, A. 1998. "Critter of the Week: Rana sylvatica" (On-line). Accessed November 17, 1999 at http://think.ucdavis.edu/~yamara/ucdlife/traditions/critters/rana.html.
Redmer, M., S. Trauth. 2005. Amphibian Declines. London, England: University of California Press, Ltd.
Savage, R. 1961. The Ecology and Life History of the Common Frog. London: Sir Isaac Pitman and sons, LTD..