Lithobates sylvaticusWood Frog

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. They are the only frogs found north of the Arctic Circle. (Conant and Collins, 1998)

Habitat

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.

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 spot on the upper lip. Most specimens have a light yellowish brown middorsal lateral fold. The underparts of the frogs are yellowish and sometimes greenish-white, 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 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.

Reproduction

Even though males do call, they generally have a non-calling behavioral mating tactic. The males move around the breeding area actively searching for a female. Occasionally this results in a male to male fight for a female already in amplexus. Both of these reproductive strategies are typical of explosive breeders. In an explosive breeding situation the success of the male in finding an available and willing female is strictly density-dependent.

Wood frogs are seasonal breeders that begin very early in the spring. They are the first frogs to begin calling, often before the ice is completely off the breeding ponds. These frogs mate as early as March and the breeding season will last until the beginning of May at the very latest. While the calls of these male frogs are very abundant in season, once the breeding season is over you will no longer hear their calls. During the time of the calls however, they create a duck-like quacking sound, 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 grasses, 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 mass then looks like a floating mass of green pond scum. The green color of the jelly is due to the presence of numerous small green algae. Tadpoles undergo complete metamorphosis in 2 months and reach sexual maturity in approximately 2 years.

Wood frogs have a great deal of selective pressures on both sexes. A larger female is often correlated with a stronger fecundity, for larger females are known to produce larger clutches. This may lead to a higher survival rate in offspring. On the other hand, male mating success is also positively size-dependent, allowing larger females the ability to "win" the male. (Conant and Collins, 1998)

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

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

  • Parental Investment
  • pre-fertilization
    • provisioning
    • protecting
      • female

Lifespan/Longevity

No information is available on the lifespan of wood frogs.

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

Behavior

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; Savage, 1961)

  • Average territory size
    100 m^2

Home Range

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

Communication and Perception

As stated in the reproduction section, 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 nature.

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.

Food Habits

Wood frogs eat a variety of insects and other small invertebrates, especially spiders, beetles, bugs, moth larvae, slugs and snails. Larvae consume algae. (Chenard, 1998)

  • Animal Foods
  • insects
  • terrestrial non-insect arthropods
  • mollusks
  • terrestrial worms
  • Plant Foods
  • algae

Predation

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

  • Known Predators

Ecosystem Roles

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

Economic Importance for Humans: Positive

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.

  • Positive Impacts
  • research and education
  • controls pest population

Conservation Status

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.

Other Comments

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

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

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.

herbivore

An animal that eats mainly plants or parts of plants.

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.

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.

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

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

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..

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..