What do they look like?

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. Females tend to be more brightly colored than males. In all cases, however, they can be distinguished by a black patch that extends over the tympanum (outer ear) to the base of the front leg. It is this characteristic that causes them to be referred to as frogs with the "robber's masks." They also have white spots on their upper lips. Most wood frogs have a light yellowish brown fold around their sides and mid-back. The underparts are yellowish and sometimes greenish-white, with male frogs having more bright colors on their thighs. They have two back ridges on either side that extend from behind the eye, down the side of the back, and to the legs. Males have two vocal sacs for calling. Females are much larger than males.

Where in the world do they live?

Wood frogs, Rana sylvatica, are only native to the Nearctic region. They are found from northern Georgia and in isolated colonies in the central highlands in the eastern to central parts of Alabama, up through the northeastern United States, and all the way across Canada into Alaska. They are found farther north than any other North American reptile or amphibian. They are the only frogs found north of the Arctic Circle.

What kind of habitat do they need?

Wood frogs are common in woodlands across their range. They are most commonly found in woodlands in the summer, under stones, stumps and leaf litter in the winter, and wood ponds in the breeding season.

How do they grow?

The eggs have a very good tolerance of temperature and those that are laid in water that afterwards freezes are not killed. They develop once temperature rises again. The length of incubation for these eggs varies depending on temperature. If laid in cold waters, then development is slow, and lasts at least a month; if, however, the eggs are laid in waters with a higher temperature, the development is much quicker, lasting only 9 to 10 days. After about a week to a month the eggs hatch and tiny, almost black, tadpoles emerge. The tadpoles are about 38 to 48 mm in length. It can take them a further 61 to 115 days to undergo metamorphosis and become froglets. The froglets are usually very small. They develop into full grown, sexually mature, adults generally within the next 2 years.

How do they reproduce?

Wood frogs breed very early in the spring. 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, 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 afloat. 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. Tadpoles will undergo metamorphosis in 2 months and will reach sexual maturity in approximately 2 years.

Female wood frogs provide their eggs with yolk before laying them. Once the eggs are laid and fertilized, the parents abandon them.

How long do they live?

No information is available on the lifespan of wood frogs.

How do they behave?

Wood frog tadpoles have been shown to have the strongest powers of kin recognition yet discovered in amphibian young. These tadpoles can recognize their brothers and sisters using maternal and paternal factors. They have been documented (by marking them with dye and releasing them into natural habitats) to group together according to relatedness. This may be a survival mechanism allowing them the potential benefit of food, temperature control, and defense against predators.

Home Range

Wood frogs tend to be very territorial. They generally occupy an area of about 100 square meters.

How do they communicate with each other?

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.

What do they eat?

Wood frogs eat a variety of insects and other small invertebrates, especially spiders, beetles, bugs, moth larvae, slugs and snails. Larvae consume algae.

What eats them and how do they avoid being eaten?

Wood frogs are food for a wide variety of birds, such as herons, and snakes.

What roles do they have in the ecosystem?

Wood frogs provide important food for many animals as well as helping to control insect populations.

How do they interact with us?

Wood frogs, along with other amphibians, are great indicators of environmental health. Recent population declines in species of amphibians should be of great concern. Wood frogs may also help to control pests.

Are they endangered?

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.

Some more information...

Wood frogs have perfected the cryogenic freezing process. In the winter, as much as 35-45% of the frogs body may freeze, and turn to ice. Ice crystals form beneath the skin and become interspersed among the body's skeletal muscles. During the freeze the frog's breathing, blood flow, and heart beat cease. Freezing is made possible by specialized proteins and glucose, which prevent intracellular freezing and dehydration.

Contributors

Katie Kiehl (author), Michigan State University: February, 2000. James Harding (editor), Michigan State University: February, 2000.

Allison Poor (editor), University of Michigan: January, 2005.

References

Blaustein, A., S. Walls. 1995. Aggregation and Kin Recognition. American Naturalist, 121: 449-454.

Chenard, P. 1998. "Wood Frogs" (On-line). Accessed November 17th, 1999 at http://www.ednet.ns.ca/cgi-bin/redirmu/educ/museum/mnh/nature/frogs/wood.htm.

Conant, R., J. Collins. 1998. Reptiles and Amphibians. New York: Houghton Mifflin Company.

Dickerson, M. 1931. The Frog Book. Garden City, New York: Doubleday, Doran, and Company, Inc..

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.

Savage, R. 1961. The Ecology and Life History of the Common Frog. London: Sir Isaac Pitman and sons, LTD..