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 70mm (2.0 to 2.8in) 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 which was 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, the belly may be faintly mottled. Their body colour may change seasonally; exposure to sunlight causes darkening.[9]
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.
Habitat
Wood frogs are forest-dwelling organisms that breed primarily in ephemeral, freshwater wetlands: woodland vernal pools. They are nonarboreal and spend most of their time of the forest floor.[11] 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.
Spring mating calls
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.[12]
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.
Feeding
Picture of a wood frog on the shoreline of Kabekona Lake, Minnesota.
Wood frogs eat a variety of small, forest-floor invertebrates, with a diet primarily consisting of insects. The tadpoles are omnivorous, feeding on plant detritus and algae along with other tadpoles of their own and other species.[13]
The feeding pattern of the wood frog is 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.[14] 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.[14] The wood frog makes contact with the prey with just the tip of its tongue, much like a toad.[15] 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.[16][17]Urea is accumulated in tissues in preparation for overwintering,[citation needed] 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.[18][19] Frogs found in southern Canada and the American midwest can tolerate freezing temperatures of −3 to −6°C (27 to 21°F). However, wood frogs in Interior Alaska exhibit even greater tolerance, with some of their body water freezing while still surviving. Wood frogs in natural hibernation remain frozen for 193 +/- 11 consecutive days and reached an average (October-May) temperature of −6.3°C (20.7°F) and an average minimum temperature of −14.6±2.8°C (5.7±5.0°F). The wood frogs has evolved various physiological adaptations that allow it to tolerate the freezing of 65–70% of its 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.[20]
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).[21]
Studies on northern subpopulations found that Alaskan wood frogs had a larger liver glycogen reserve and greater urea production[22] 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.[23]
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.[24] Studies on different populations of wood frogs found that Alaskan wood frogs have larger liver glycogen reserves and higher enzymatic activity of glycogen phosphorylase, which facilitates freezing, compared to frogs in more temperate zones.
L. sylvaticus primarily breeds in ephemeral pools rather than permanent water bodies such as ponds or lakes.[25] 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 typically hibernate within 65 meters of breeding pools.[26] They emerge from hibernation in early spring and migrate to the nearby pools. There, males chorus, emitting duck-like quacking sounds.
Wood frogs are considered explosive breeders; many populations will conduct all mating in the span of a week.[27] 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.[28] 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][25] 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][25][29]
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][25] 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.[30][31] During the first stage, the larvae are adapted for rapid development, and their growth depends on the temperature of the water.[31][32] Variable larval survival is a major contributor to fluctuations in wood frog population size from year to year.[32] The second stage of development features rapid development and growth, and depends on environmental factors including food availability, temperature, and population density.[31]
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 and 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.[33]
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.[30] Most frogs breed only once in their lives, although some will breed two or three times, generally with differences according to age.[25][30][34] 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.[30]
Conservation status
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.[33]
Related Research Articles
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura. The oldest fossil "proto-frog" Triadobatrachus is known from the Early Triassic of Madagascar, but molecular clock dating suggests their split from other amphibians may extend further back to the Permian, 265 million years ago. Frogs are widely distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is in tropical rainforest. Frogs account for around 88% of extant amphibian species. They are also one of the five most diverse vertebrate orders. Warty frog species tend to be called toads, but the distinction between frogs and toads is informal, not from taxonomy or evolutionary history.
The American bullfrog, often simply known as the bullfrog in Canada and the United States, is a large true frog native to eastern North America. It typically inhabits large permanent water bodies such as swamps, ponds, and lakes. Bullfrogs can also be found in manmade habitats such as pools, koi ponds, canals, ditches and culverts. The bullfrog gets its name from the sound the male makes during the breeding season, which sounds similar to a bull bellowing. The bullfrog is large and is commonly eaten throughout its range, especially in the southern United States where they are plentiful.
The spotted salamander or yellow-spotted salamander is a mole salamander common in eastern United States and Canada. It is the state amphibian of Ohio and South Carolina. The species ranges from Nova Scotia, to Lake Superior, to southern Georgia and Texas. Its embryos have been found to have symbiotic algae living in and around them, the only known example of vertebrate cells hosting an endosymbiont microbe.
The spring peeper is a small chorus frog widespread throughout the eastern United States and Canada. They prefer permanent ponds due to their advantage in avoiding predation; however, they are very adaptable with respect to the habitat they can live in. In northern regions, the frog is able to endure below freezing temperatures due to the capacity of their livers to exude and flush the bloodstream with a glucose cryoprotectant which acts both as an anti-freeze in their blood, and allows organs like the heart to enter into a state of protected dormancy. They are so called because of their chirping call that marks the beginning of spring. Crucifer is derived from the Latin root meaning "cross-bearing". This could be a reference to the cross-like pattern on the spring peeper's dorsal side.
Lithobates pipiens formerly Rana pipiens, commonly known as the northern leopard frog, is a species of leopard frog from the true frog family, native to parts of Canada and the United States. It is the state amphibian of Minnesota and Vermont.
The common frog or grass frog, also known as the European common frog, European common brown frog, European grass frog, European Holarctic true frog, European pond frog or European brown frog, is a semi-aquatic amphibian of the family Ranidae, found throughout much of Europe as far north as Scandinavia and as far east as the Urals, except for most of the Iberian Peninsula, southern Italy, and the southern Balkans. The farthest west it can be found is Ireland. It is also found in Asia, and eastward to Japan. The nominative, and most common, subspecies Rana temporaria temporaria is a largely terrestrial frog native to Europe. It is distributed throughout northern Europe and can be found in Ireland, the Isle of Lewis and as far east as Japan.
The pickerel frog is a small North American frog, characterized by the appearance of seemingly "hand-drawn" squares on its dorsal surface.
The moor frog is a slim, reddish-brown, semiaquatic amphibian native to Europe and Asia. Moor frogs are known for their ability to freeze solid and survive thawing. The frog makes use of various cryoprotectants i.e. antifreeze that decrease its internal freezing temperature. The species is distributed over a large range, covering a significant portion of Eurasia. Male moor frogs are known to turn blue temporarily during the height of mating season. This coloration is assumed to signal a mate's fitness. Moor frogs typically mate through multimale amplexus a form of polyandry.
The Italian agile frog, also known as Lataste's frog, is a species of frog in the family Ranidae. The species is native to southern Europe, primarily found in the Po River Basin of Italy. It is one of the most endangered amphibian species in Europe, with its populations declining sharply in recent years, and has been the focus of multiple conservation plans.
Lithobates sphenocephalus or Rana sphenocephala, commonly known as the southern leopard frog, is a medium-sized anuran in the family Ranidae. It is native to eastern North America from Kansas to New York to Florida. It is also an introduced species in some areas. This species lives in cool, clear water in the north, whereas in the south it occurs in warmer turbid and murky waters of coastal and floodplain swamps, twilight zones of caves, and abandoned mines.
The pig frog is a species of aquatic frog found in the Southeastern United States, from South Carolina to Texas. Some sources also refer to it as the lagoon frog or the southern bullfrog.
The crawfish frog is a medium-sized species of frog native to the prairies and grasslands of the central United States. It gets its name because it inhabits the burrows of crayfish for most of the year. They have defined golden or black circles all over their body.
The northern red-legged frog is a species of amphibian whose range is the coastal region stretching from southwest British Columbia to southern Mendocino County in Northern California, and is protected in Oregon and California. As a member of the genus Rana, this species is considered a true frog, with characteristic smooth skin and a narrow waist. This frog requires still waters for breeding, and is rarely found at any great distance from its breeding ponds or marshes.
Forrer's grass frog or Forrer's leopard frog is a species of frog in the family Ranidae found in Mexico and Central America through Guatemala, El Salvador, Honduras, and Nicaragua to Costa Rica. It is a widespread and common frog found in lowland and seasonal tropical forests. It can also adapt to man-made habitats such as flooded agricultural lands and other water content systems. Reproduction requires permanent pools and lagoons.
The river frog is a species of aquatic frog in the family Ranidae. It is endemic to the southeastern United States. Its natural habitats are temperate rivers, swamps, freshwater lakes and freshwater marshes. It is threatened by habitat loss.
The big-footed leopard frog or bigfoot leopard frog, is a species of frog in the family Ranidae endemic to western central Mexico where it is found in the Nayarit, Jalisco, Michoacán, and Guanajuato states.
The Mississippi gopher frog, also known commonly as the dark gopher frog, the dusky gopher frog, and the St. Tammany gopher frog, is a critically endangered species of frog in the family Ranidae. The species is endemic to the southern United States. Its natural habitats are temperate coastal forests and intermittent freshwater marshes. This secretive frog is on average 3 in (8 cm) long, with a dark brown or black dorsal surface covered in warts. It is a federally listed endangered species of the United States.
Lithobates clamitans or Rana clamitans, commonly known as the green frog, is a species of frog native to eastern North America. The two subspecies are the bronze frog and the northern green frog.
Countergradient variation is a type of phenotypic plasticity that occurs when the phenotypic variation determined by a biological population's genetic components opposes the phenotypic variation caused by an environmental gradient. This can cause different populations of the same organism to display similar phenotypes regardless of their underlying genetics and differences in their environments.
↑ Conant R, Collins JT. (1998). A field guide to reptiles & amphibians: eastern and central North America. Third edition. New York (NY): Houghton Mifflin Company ISBN0395904528.
↑ Conant, Roger. (1958). A Field Guide to Reptiles and Amphibians. Houghton Mifflin Company, Boston.
↑ Berven, Keith A., and Thaddeus A. Grudzien. "Dispersal in the wood frog (Rana sylvatica): implications for genetic population structure." Evolution 44.8 (1990): 2047-2056.
↑ Redmer, Michael and Trauth, Stanley E. (2005). Amphibian Declines: The Conservation Status of United States Species M. Lannoo, ed. University of California Press ISBN0520235924.
1 2 Cardini, F. (1974). 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
↑ Cardini, F. (1973). Characteristics and Adaptedness of Feeding Behaviors of North American Anurans, Paper presented at June 1973 meetings of the Animal Behavior Society, Amherst, MA
↑ Storey KB; Storey JM (1984). "Biochemical adaption for freezing tolerance in the wood frog, Rana sylvatica". Journal of Comparative Physiology B. 155: 29–36. doi:10.1007/BF00688788. S2CID29760226.
↑ Bansal, Saumya (2016). "MicroRNA Regulation in Heart and Skeletal Muscle over the Freeze–thaw Cycle in the Freeze Tolerant Wood Frog". Journal of Comparative Physiology B. 186 (2, Springer Berlin Heidelberg, 2015): 229–41. doi:10.1007/s00360-015-0951-3. PMID26660652. S2CID16490101.
↑ Berman, D. I.; Meshcheryakova, E. N.; Bulakhova, N. A. (Jan 2016). "The Japanese tree frog (Hyla japonica), one of the most cold-resistant species of amphibians". Doklady Biological Sciences. 471 (1): 276–279. doi:10.1134/s0012496616060065. PMID28058600. S2CID254413388.
↑ Berven KA (1988). "Factors affecting variation in reproductive traits within a population of wood frogs (Rana sylvatica)". Copeia. 1988 (3): 605–615. doi:10.2307/1445378. JSTOR1445378.
Further reading
Baldwin R.F.; Calhoun A.J.K.; de Maynadier P.G. (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 (4): 443–454. doi:10.1670/0022-1511(2006)40[442:CPFASW]2.0.CO;2. S2CID85111873.
Heatwole H (1961). "Habitat selection and activity of the Wood Frog, Rana sylvatica Le Conte". American Midland Naturalist. 66 (2): 301–313. doi:10.2307/2423030. JSTOR2423030.
LeConte J (1824). "Remarks on the American species of the Genera Hyla and Rana". Annals of the Lyceum of Natural History of New York. 1: 278–282. (Rana sylvatica, new species, p.282).
Rittenhouse T.A.G.; Semlitsch R.D. (2007). "Postbreeding habitat use of wood frogs in a Missouri Oak-Hickory forest". Journal of Herpetology. 41 (4): 645–653. doi:10.1670/07-015.1. S2CID86284247.
Waldman B (1982). "Adaptive significance of communal oviposition in wood frogs (Rana sylvatica)". Behavioral Ecology and Sociobiology. 10 (3): 169–172. doi:10.1007/bf00299681. S2CID35527688.
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