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.
Habitat Regions: terrestrial
Terrestrial Biomes: forest
Wood frogs are food for a wide variety of birds, such as herons, and snakes.
Known Predators:
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.
Range length: 3.5 to 7.6 cm.
Other Physical Features: ectothermic ; bilateral symmetry
Sexual Dimorphism: female larger; male more colorful
Average mass: 7.88 g.
Average basal metabolic rate: 0.00416 W.
No information is available on the lifespan of wood frogs.
Range lifespan
Status: wild: 3 (high) years.
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.
Biogeographic Regions: nearctic (Native )
Wood frogs eat a variety of insects and other small invertebrates, especially spiders, beetles, bugs, moth larvae, slugs and snails. Larvae consume algae.
Animal Foods: insects; terrestrial non-insect arthropods; mollusks; terrestrial worms
Plant Foods: algae
Primary Diet: carnivore (Eats non-insect arthropods); herbivore (Algivore)
Wood frogs provide important food for many animals as well as helping to control insect populations.
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
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.
Development - Life Cycle: metamorphosis
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.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
IUCN Red List of Threatened Species: least concern
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.
Communication Channels: acoustic
Perception Channels: visual ; tactile ; acoustic
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.
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.
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 (male): 2 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); oviparous
Average age at sexual or reproductive maturity (male)
Sex: male: 547 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 912 days.
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)
Lithobates sylvaticus[1][2] or Rana sylvatica,[3] commonly known as the wood frog, is a frog species that has a broad distribution over North America, extending from the boreal forest of the north to the southern Appalachians, with several notable disjunct populations including lowland eastern North Carolina. The wood frog has garnered attention from biologists because of its freeze tolerance, relatively great degree of terrestrialism (for a ranid), interesting habitat associations (peat bogs, vernal pools, uplands), and relatively long-range movements.
The ecology and conservation of the wood frog has attracted research attention in recent years because they are often considered "obligate" breeders in ephemeral wetlands (sometimes called "vernal pools"), which are themselves more imperiled than the species that breed in them. The wood frog has been proposed to be the official state amphibian of New York.[4]
Wood frogs range from 51 to 70 mm (2.0 to 2.8 in) in length. Females are larger than males.[5][6] Adult wood frogs are usually brown, tan, or rust-colored, and usually have a dark eye mask.[7] Individual frogs are capable of varying their color; Conant (1958) depicts one individual when light brown and dark brown at different times. The underparts of wood frogs are pale with a yellow or green cast;[8] in northern populations, belly may be faintly mottled. Body colour may change seasonally; exposure to sunlight causes darkening.[9]
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The contiguous wood frog range is from northern Georgia and northeastern Canada in the east to Alaska and southern British Columbia in the west.[10] It is the most widely distributed frog in Alaska. It is also found in the Medicine Bow National Forest.
Wood frogs are forest-dwelling organisms that breed primarily in ephemeral, freshwater wetlands: woodland vernal pools. Long-distance migration plays an important role in their life history. Individual wood frogs range widely (hundreds of metres) among their breeding pools and neighboring freshwater swamps, cool-moist ravines, and/or upland habitats. Genetic neighborhoods of individual pool breeding populations extend more than a kilometre away from the breeding site. Thus, conservation of this species requires a landscape (multiple habitats at appropriate spatial scales) perspective. They also can be camouflaged with their surroundings.
A study was done on wood frogs dispersal patterns in 5 ponds at the Appalachian Mountains where they reported adult wood frogs were 100% faithful to the pond of their first breeding but 18% of juveniles dispersed to breed in other ponds.[11]
Adult wood frogs spend summer months in moist woodlands, forested swamps, ravines, or bogs. During the fall, they leave summer habitats and migrate to neighboring uplands to overwinter. Some may remain in moist areas to overwinter. Hibernacula tend to be in the upper organic layers of the soil, under leaf litter. By overwintering in uplands adjacent to breeding pools, adults ensure a short migration to thawed pools in early spring. Wood frogs are mostly diurnal and are rarely seen at night, except maybe in breeding choruses. They are one of the first amphibians to emerge for breeding right when the snow melts, along with spring peepers.
Wood frogs eat a variety of small, forest-floor invertebrates. Omnivorous, the tadpoles feed on plant detritus and algae, and also attack and eat eggs and larvae of amphibians, including those of wood frogs.[12]
The feeding pattern of the wood frog is basically similar to that of other ranids. It is triggered by prey movement and consists of a bodily lunge that terminates with the mouth opening and an extension of the tongue onto the prey.[13] The ranid tongue is attached to the floor of the mouth near the tip of the jaw, and when the mouth is closed, the tongue lies flat, extended posteriorly from its point of attachment.
In the feeding strike, the tongue is swung forward as though on a hinge, so some portion of the normally dorsal and posterior tongue surface makes contact with the prey. At this point in the feeding strike, the wood frog differs markedly from more aquatic Lithobates species, such as the green frog, leopard frog, and bullfrog.[13] The wood frog makes contact with the prey with just the tip of its tongue, much like a toad.[14] A more extensive amount of tongue surface is applied in the feeding strikes of these other frog species, with the result that usually the prey is engulfed by the fleshy tongue and considerable tongue surface contacts the surrounding substrate.
Similar to other northern frogs that enter dormancy close to the surface in soil and/or leaf litter, wood frogs can tolerate the freezing of their blood and other tissues.[15][16] Urea is accumulated in tissues in preparation for overwintering, and liver glycogen is converted in large quantities to glucose in response to internal ice formation. Both urea and glucose act as cryoprotectants to limit the amount of ice that forms and to reduce osmotic shrinkage of cells.[17][18] Frogs found in southern Canada and the American midwest can tolerate freezing temperatures of −3 to −6 °C (27 to 21 °F). The wood frogs have evolved various physiological adaptations that allow them to tolerate the freezing of 65–70% of total body water. When water freezes, ice crystals form in cells and break up the structure, so that when the ice thaws the cells are damaged. Frozen frogs also need to endure the interruption of oxygen delivery to their tissues as well as strong dehydration and shrinkage of their cells when water is drawn out of cells to freeze. The wood frog has evolved traits that prevent their cells from being damaged when frozen and thawed out. The wood frog has developed various adaptations that allow it to effectively combat prolonged ischemia/anoxia and extreme cellular dehydration. One crucial mechanism utilized by the wood frog is the accumulation of high amounts of glucose that act as a cryoprotectant.[19]
Frogs can survive many freeze/thaw events during winter if no more than about 65% of the total body water freezes. Wood frogs have a series of seven amino acid substitutions in the sarco/endoplasmic reticulum Ca2+-ATPase 1 (SERCA 1) enzyme ATP binding site that allows this pump to function at lower temperatures relative to less cold-tolerant species (e.g. Lithobates clamitans).[20]
Studies on northern subpopulations found that Alaskan wood frogs had a larger liver glycogen reserve compared to those in more temperate zones of its range. These conspecifics also showed higher enzymatic activity of the glycogen phosphorylase which facilitates the freezing.[21]
The phenomenon of cold resistance is observed in other anuran species. The Japanese tree frog shows even greater cold tolerance than the wood frog, surviving in temperatures as low as −35 °C (−31 °F) for up to 120 days. [22]
L. sylvaticus primarily breeds in ephemeral pools rather than permanent water bodies such as ponds or lakes.[23] This is believed to provide some protection for the adult frogs and their offspring (eggs and tadpoles) from predation by fish and other predators of permanent water bodies. Adult wood frogs emerge from hibernation in early spring and migrate to nearby pools. There, males chorus, emitting duck-like quacking sounds.
Wood frogs are considered explosive breeders, many populations will conduct all mating all in the span of a week.[24] Males actively search for mates by swimming around the pool and calling. Females, on the other hand, will stay under the water and rarely surface, most likely to avoid sexual harassment.[25] A male approaches a female and clasps her from behind her forearms before hooking his thumbs together in a hold called "amplexus", which is continued until the female deposits the eggs.[6][23] Females deposit eggs attached to submerged substrate, typically vegetation or downed branches. Most commonly, females deposit eggs adjacent to other egg masses, creating large aggregations of masses.[6][23][26]
Some advantage is conferred to pairs first to breed, as clutches closer to the center of the raft absorb heat and develop faster than those on the periphery, and have more protection from predators.[6][23] If pools dry before tadpoles metamorphose into froglets, they die.[6] This constitutes the risk counterbalancing the antipredator protection of ephemeral pools. By breeding in early spring, however, wood frogs increase their offspring's chances of metamorphosing before pools dry.
The larvae undergo two stages of development: fertilization to free-living tadpoles, and free-living tadpoles to juvenile frogs.[27][28] During the first stage, the larvae are adapted for rapid development, and their growth depends on the temperature of the water.[28][29] Variable larval survival is a major contributor to fluctuations in wood frog population size from year to year.[29] The second stage of development features rapid development and growth, and depends on environmental factors including food availability, temperature, and population density.[28]
Some studies suggest that road-salts, as used in road de-icing, may have toxic effects on wood frog larvae. A study exposed wood frog tadpoles to NaCl and found that tadpoles experienced reduced activity, weight, and even displayed physical abnormalities. There was also significantly lower survivorship and decreased time to metamorphosis with increasing salt concentration. De-icing agents may pose a serious conservation concern to wood frog larvae.[30]
Following metamorphosis, a small percentage (less than 20%) of juveniles will disperse, permanently leaving the vicinity of their natal pools. The majority of offspring are philopatric, returning to their natal pool to breed.[27] Most frogs breed only once in their lives, although some will breed two or three times, generally with differences according to age.[23][27][31] The success of the larvae and tadpoles is important in populations of wood frogs because they affect the gene flow and genetic variation of the following generations.[27]
Although the wood frog is not endangered or threatened, in many parts of its range urbanization is fragmenting populations. Several studies have shown, under certain thresholds of forest cover loss or over certain thresholds of road density, wood frogs and other common amphibians begin to "drop out" of formerly occupied habitats. Another conservation concern is that wood frogs are primarily dependent on smaller, "geographically isolated" wetlands for breeding. At least in the United States, these wetlands are largely unprotected by federal law, leaving it up to states to tackle the problem of conserving pool-breeding amphibians.[1]
The wood frog has a complex lifecycle that depends on multiple habitats, damp lowlands, and adjacent woodlands. Their habitat conservation is, therefore, complex, requiring integrated, landscape-scale preservation.[1]
Wood frog development in the tadpole stage is known to be negatively affected by road salt contaminating freshwater ecosystems.[30]
Lithobates sylvaticus or Rana sylvatica, commonly known as the wood frog, is a frog species that has a broad distribution over North America, extending from the boreal forest of the north to the southern Appalachians, with several notable disjunct populations including lowland eastern North Carolina. The wood frog has garnered attention from biologists because of its freeze tolerance, relatively great degree of terrestrialism (for a ranid), interesting habitat associations (peat bogs, vernal pools, uplands), and relatively long-range movements.
The ecology and conservation of the wood frog has attracted research attention in recent years because they are often considered "obligate" breeders in ephemeral wetlands (sometimes called "vernal pools"), which are themselves more imperiled than the species that breed in them. The wood frog has been proposed to be the official state amphibian of New York.
