The wood frog (Lithobates sylvaticus or Rana sylvatica ) 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.
Wood frogs range from 51 to 70 mm (2.0 to 2.8 in) 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; in northern populations, belly may be faintly mottled. Body colour may change seasonally; exposure to sunlight causes darkening.
The contiguous wood frog range is 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. 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.
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.
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. 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. The wood frog makes contact with the prey with just the tip of its tongue, much like a toad. 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.
L. sylvaticus primarily breeds in ephemeral pools rather than permanent water bodies such as ponds or lakes. 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. 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. 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. 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.
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. If pools dry before tadpoles metamorphose into froglets, they die. 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. During the first stage, the larvae are adapted for rapid development, and their growth depends on the temperature of the water and has a higher mortality rate. The second stage of development features rapid development and growth, and depends on environmental factors including food availability, temperature, and population density.
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.
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. Most frogs breed only once in their lives, although some will breed two or three times, generally with differences according to age. 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.
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.
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.
Wood frog development in the tadpole stage is known to be negatively affected by road salt contaminating freshwater ecosystems.
