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Lithobates sylvaticusWood Frog

By Katie Kiehl

Geographic Range

Wood frogs, Lithobates sylvaticus, 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. (Conant and Collins, 1998)

Habitat

Wood frogs inhabit a wide variety of habitats including tundra, thickets, wet meadows, bogs, coniferous and deciduous forests. Wood frogs are aquatic breeders and require fish-free seasonal or semi-permanent bodies of water to reproduce, but will migrate from their primary habitat to breed. These frogs seek out terrestrial locations with ample cover to hibernate which may or may not be near a breeding pond. (Redmer and Trauth, 2005)

  • Aquatic Biomes
  • temporary pools

Physical Description

Wood frogs range from 3.5 to 7.6 cm. Females are much larger than males. This species exhibits a number of color morphs, usually browns, tans and rust, but can also be found in shades of green and gray. In all cases however, they can be distinguished by a black patch that extends over the tympanum to the base of the front limb. It is this characteristic that causes them to be referred to as the frog with the "robber's mask". They are also known to have a white outline across the upper lip. Most specimens have a light yellowish brown middorsal lateral fold. The underparts of the frogs are white becoming pale orange-yellow towards the rear, with male frogs having more brilliant colors on the ventral aspect of the legs. (Dickerson, 1931; Mansker, 1998)

  • Sexual Dimorphism
  • female larger
  • male more colorful
  • Average mass
    7.88 g
    0.28 oz
    AnAge
  • Range length
    3.5 to 7.6 cm
    1.38 to 2.99 in
  • Average basal metabolic rate
    0.00416 W
    AnAge

Development

The time it takes for fertilized eggs to hatch is largely 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 undergo metamorphosis when they reach 50 to 60 mm in length between 65 and 130 days post-hatch. Juveniles measure 16 to 18 mm in length after metamorphosis. Juvenile males reach reproductive maturity from 1 to 2 years post-metamorphosis, whereas females may not reach reproductive maturity for 2 to 3 years post-metamorphosis. Like all frogs, wood frogs exhibit indeterminate growth. (Redmer and Trauth, 2005)

Reproduction

Wood frogs exhibit "explosive" breeding in late winter or early spring when the first warm rains occur. Frogs wake from hibernation and migrate to breeding ponds. Individuals show some site fidelity year to year. Not much information is known regarding how males attract a mate. In an explosive breeding situation the success of the male in finding an available and willing female is strictly density-dependent. (Redmer and Trauth, 2005)

Wood frogs are seasonal breeders that breed from early March to May. They are the first frogs to begin calling, often before the ice is completely off the breeding ponds. While the calls of these male frogs are very abundant in season, once the breeding season is over they become silent. During the breeding season however, they create a chorus of duck-like quacking sounds, described by some as a "lot of chuckling". Once mate choice is accomplished and amplexus occurs, the female will 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 grass, or they can be free standing. After about a week or so the egg mass begins to flatten out, allowing it to rest on the surface of the water. The jelly around the eggs becomes green in color creating a great camouflage. The green color of the jelly is due to the presence of numerous 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 reach reproductive maturity when they are 1 to 2 years old, while females take a bit longer and cannot reproduce until they are 2 to 3 years old. (Conant and Collins, 1998; Redmer and Trauth, 2005)

  • Breeding interval
    Wood frogs breed once yearly.
  • Breeding season
    Wood frogs breed from March to May.
  • Range number of offspring
    1000 to 3000
  • Range time to hatching
    9 to 30 days
  • Range age at sexual or reproductive maturity (female)
    2 to 3 years
  • Range age at sexual or reproductive maturity (male)
    1 to 2 years

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)

  • Parental Investment
  • no parental involvement
  • pre-fertilization
    • provisioning
    • protecting
      • female

Lifespan/Longevity

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)

  • Typical lifespan
    Status: wild
    3 to 5 years
  • Range lifespan
    Status: wild
    3 (high) years
    AnAge

Behavior

Wood frogs are a diurnal species. These frogs perform seasonal migrations to breeding ponds in late winter or early spring. Individual wood frogs show significant breeding site fidelity and will often migrate within the same area throughout their lives. Though they amass at breeding ponds, these frogs are mostly solitary.

Wood frog tadpoles have been shown to have the strongest powers of kin recognition yet discovered in amphibian larvae. These tadpoles can recognize kin using maternal and paternal factors. They have been documented (by marking them with dye and releasing them into natural habitats) to aggregate back together. This may be a survival mechanism allowing them the potential benefit of food, thermoregulation, and defense against predators. (Blaustein and Walls, 1995; Redmer and Trauth, 2005; Savage, 1961)

Home Range

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)

Communication and Perception

Males actively search for females during the breeding season; however, they are unable to tell males from females by sight. Sex recognition is accomplished by the males embracing other frogs (regardless of sex) and releasing those that are not fat enough to be females full of eggs. If a male is embraced he lets out a loud croak. A female will also be let go if spawning has already occurred, because of her thin body size. (Harding, 1997)

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)

Food Habits

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)

  • Primary Diet
  • carnivore
    • eats non-insect arthropods
  • Animal Foods
  • eggs
  • carrion
  • insects
  • terrestrial non-insect arthropods
  • mollusks
  • terrestrial worms
  • Plant Foods
  • algae

Predation

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)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

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.

Economic Importance for Humans: Positive

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.

  • Positive Impacts
  • research and education
  • controls pest population

Economic Importance for Humans: Negative

There are no known negative effects of wood frogs on humans.

Conservation Status

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. 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. (Harding, 1997)

Other Comments

Wood frogs utilize a cryogenic freezing process during hibernation. In the winter, as much as 35 to 45% of a frog's 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. (Redmer and Trauth, 2005)

Contributors

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.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

carrion

flesh of dead animals.

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

migratory

makes seasonal movements between breeding and wintering grounds

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

tundra

A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.

visual

uses sight to communicate

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..

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..

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