Definitions

Wood Frog

Wood Frog

Wood frog is the common name given to Rana sylvatica. The wood frog has a broad North American distribution, extending from the southern Appalachians to the boreal forest.

As for other northern frogs hibernating close to the surface in soil and/or leaf litter, wood frogs tolerate the freezing of their blood and other tissues. 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. Frogs can survive multiple freeze/thaw events during winter if not more than about 65% of the total body water freezes.

Physical description

Wood frogs range from to in length. Females are larger than males. Adult wood frogs are usually brown, tan, or rust colored, and usually have a dark eye mask. 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.

There are no similar species. If you see a small brown frog with a dark eye mask in the woods, it's a wood frog. The first evasive leap is fast and long. Close observation will often glimpse a second short dive under the leaf litter, making the frog appear to disappear.

Feeding

Wood frog adults eat a variety of small, forest-floor invertebrates. Omnivorous, wood frog tadpoles feed on plant detritus, algae and also attack and eat eggs and larvae of amphibians, including those of wood frogs .

The feeding motor pattern of the wood frog, basically similar to that of other ranids, 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 . Note that 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 that some portion of the normally dorsal and posterior tongue surface makes contact with the prey. And it is at this point in the feeding strike that the wood frog differs markedly from more aquatic Rana species, such as the green frog, leopard frog, and bullfrog . The wood frog makes contact with the prey with just the very tip of its tongue, much like a toad . A more extensive amount of tongue surface is applied in the feeding strikes of the other 3 frog species, with the result that usually the prey is engulfed by the fleshy tongue and considerable tongue surface contacts the surrounding substrate.

Geographic Range

The wood frog ranges from northern Georgia and northeastern Canada in the east to Alaska and southern British Columbia in the west. It is the most widely distributed frog in Alaska.

Habitat

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 meters) 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 kilometer away from the breeding site. Thus, conservation of this species requires a landscape (multiple habitats at appropriate spatial scales) perspective

Rana sylvatica primarily breeds in ephemeral pools rather than permanent water bodies such as ponds or lakes. This is believed to provide some protection of 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. Females deposit eggs in floating masses, often aggregated with those of other females in rafts. 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. If pools dry before tadpoles metamorphose into froglets, they die. This constitutes the risk counterbalancing the anti-predator protection of ephemeral pools. By breeding in early spring, however, wood frogs increase their offspring's chances of metamorphosing before pools dry. Following metamorphosis, a small percentage (less than 20%) of juveniles will disperse, permanently leaving the vicinity of their natal pool. The majority of offspring are philopatric, returning to their natal pool to breed.

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, adult Rana sylvatica ensure a short migration to thawed pools in early spring.

Conservation status

The wood frog is not endangered or threatened. Its habitat, however, is steadily disappearing due to housing and other construction, road building, wetland drainage, and other forms of so-called development.

The wood frog has a complex life cycle that depends on multiple habitats, damp lowlands, adjacent woodlands. Wood frog habitat conservation is, therefore, complex, requiring integrated, landscape-scale preservation.

Notes

References

  • (2005): Phylogeny of the New World true frogs (Rana). Mol. Phylogenet. Evol. 34(2): 299–314. PDF fulltext
  • (2007) Constraints in naming parts of the Tree of Life. Mol. Phylogenet. Evol. 42: 331–338.
  • Database entry includes a range map and a brief justification of why this species is of least concern.
  • 2006. Conservation planning for amphibian species with complex habitat requirements: a case study using movements and habitat selection of the wood frog Rana sylvatica. Journal of Herpetology 40:443-454.
  • Habitat selection and activity of the Wood Frog, Rana sylvatica Le Conte. American Midland Naturalist 66:301-313.
  • 2003. Terrestrial habitat use and winter densities of the wood frog (Rana sylvatica). Journal of Herpetology 37:390-394.
  • 2007. Postbreeding habitat use of wood frogs in a Missouri Oak-Hickory forest. Journal of Herpetology 41:645-653.
  • Adaptive significance of communal oviposition in wood frogs (Rana sylvatica). Behavioral Ecology and Sociobiology 10:169-172.
  • Specializations of the Feeding Response of the Bullfrog, Rana catesbeiana, for the Capture of Prey Submerged in Water. M.S. Thesis, U. of Massachusetts, Amherst, MA
  • Characteristics and Adaptedness of Feeding Behaviors of North American Anurans, Paper presented at June 1973 meetings of the Animal Behavior Society, Amherst, MA
  • A Field Guide to Reptiles and Amphibians. Houghton Mifflin Company, Boston.

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