Description
Adult Wood Frogs are 37 to 83 mm in length, and females are larger. Prominent dorsolateral folds extend from the head to near the vent. The lateral edge of each fold is darker than the medial edge. The smooth to moderately rough back often has short folds between the dorsolateral folds. Toes are webbed, with two or three phalanges of the fourth toe web-free. Dorsal coloring may be gray to tan to vivid reddish brown; some individuals are coppery or golden. Females are usually more reddish. Black or dark brown markings may be present on the back and sides, and many northern and western specimens have a middorsal white line. A conspicuous dark brown or blackish mask extends from the snout to just behind the tympanum. The white venter is sometimes darkly mottled on the throat and breast, and is smooth except for a granular region under the thighs. There also is a prominent dark marking in the pectoral region. The tympanum is smaller than the eye. Males have paired vocal sacs, stout forelegs, and a "thumb" and enlarged webbing between the toes during the breeding season (Martof 1963).

Distribution and Habitat

Country distribution from AmphibiaWeb's database: Canada, United States

U.S. state distribution from AmphibiaWeb's database: Alaska, Alabama, Arkansas, Colorado, Connecticut, District of Columbia, Delaware, Georgia, Idaho, Illinois, Indiana, Kentucky, Massachusetts, Maryland, Maine, Michigan, Minnesota, Missouri, Montana, North Carolina, North Dakota, New Hampshire, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Virginia, Vermont, Wisconsin, West Virginia, Wyoming

Canadian province distribution from AmphibiaWeb's database: Alberta, British Columbia, Manitoba, New Brunswick, Newfoundland and Labrador, Nova Scotia, Northwest Territories, Nunavut, Ontario, Prince Edward Island, Quebec, Saskatchewan, Yukon

View distribution map in BerkeleyMapper.

The Wood Frog is the only cold-blooded tetrapod known to occur north of the Arctic Circle in the Western Hemisphere. It is found over most of Alaska and Canada and throughout the northeastern part of the United States. Its northern limit lies along the treeline from Alaska to Labrador. Its range extends southward along the coast to Maryland and in the Appalachian Mountains to northern Georgia and northeastern Tennessee. The southern edge of the range passes northward through southern Illinois and the northeastern corner of South Dakota, the northeastern half of North Dakota, northern Idaho and westward in Canada to near the Pacific coast. Isolated populations are found in southeastern Wyoming and northern Colorado, in eastern Kansas, in the Ozark Mountains of Arkansas and Missouri, and perhaps in areas north of the Arctic tree line.

It is a terrestrial species, often found in or near moist wooded areas, sometimes considerable distances from open water.

Life History, Abundance, Activity, and Special Behaviors
The Wood Frog is well known for hibernating and freezing during winters. Nitrogen balancing has been known to play a role in osmoregulation, cryoprotection, and metabolic inhibition in amphibians. In some vertebrates, gut bacteria aid this balance by producing urease, an enzyme that breaks down urea. However, until 2018 it was unknown if gut bacteria played a role in amphibian nitrogen-recycling. Wiebler et al. (2018) investigated the role of hindgut microbiomes in Rana sylvatica, which is known for storing urea during its hibernation, by comparing the urease activity and microbial community in hibernating males with active males and by artificially increasing the concentration of urea in the blood stream. They found that while active frogs have a greater concentration of bacteria in their hindgut, hibernating frogs have a greater diversity of bacteria that produce urease, and have more urease activity. Additionally, increasing the concentration of urea in the blood stream increase bacterial urease activity. They, thus, provided the first report of nitrogen-recycling by hindgut bacteria in amphibians (Wiebler et al. 2018).

Comments

This species was featured as News of the Week on 14 January 2019:

Human activities ranging from vehicle traffic to industry are making the world an increasingly noisy place to live in; two recent studies show frogs have found ways to cope with the human soundscape. Tennessen et al. (2018) studied wood frogs (Rana sylvatica) in northeastern United States where noise from vehicle traffic is physiologically stressful to recently metamorphosed tadpoles, negatively impacting frog health. However, these researchers found that wood frogs from populations living near human noise have rapidly evolved to no longer be stressed by noisy human environments. In Panama, predators like bats and midges avoid noisy urban areas because they rely on sounds to hunt. Halfwerk and colleagues (2018) found that male túngara frogs (Engystomops pustulosus) from urban habitats can flexibly adjust their calls. Urban dwelling male túngara create more conspicuous calls, which are more attractive to females. When these urban males are placed in the forest, they adjust their calls to be less conspicuous and therefore less obvious to predators. Male túngara frogs from forests are unable to flexibly adjust their calls if they are placed in the city. Together, these studies show that some frogs species can rapidly evolve to deal with noisy human environments whereas others can adjust their behaviors accordingly (Written by Max Lambert)

This species was featured as News of the Week on 25 October 2021:

Amphibian larval development is highly temperature sensitive; larvae develop faster at warmer temperatures and slower at cooler ones. Countergradient variation is a form of local adaptation where – for example, in the Wood frog (Rana sylvatica)- tadpoles living in colder ponds (due to more enclosed, shaded forest canopies) are adapted to develop at faster rates than tadpoles from populations living in warmer ponds (with more open, sunnier forest canopies). This allows tadpoles in cold ponds to developmentally catch up to tadpoles in warmer habitats. The original study (Skelly 2004) found that this countergradient variation occurs at very small 'microgeographic' spatial scales, i.e., Wood frog populations are adapting to local environmental conditions well within the distance that the frogs can disperse and interbreed within a metapopulation. Arietta and Skelly (2021) performed the identical experiment with the same Wood frog populations 17 years later (~ 6-9 frog generations) and found persistent microgeographic countergradient evolution as before with overall Wood frog embryonic developmental rates accelerated by 14-19%. During this period, the forest canopy over these ponds generally became more shaded, but pond temperatures still increased across the metapopulation. With warmer ponds, it would be expected that the Wood frog populations would evolve slower tadpole development rates, not faster. Critically, ponds experiencing more change (i.e., canopy cover and temperature change) also had declining populations and several disappeared altogether. Overall, their experiment illustrates how amphibians might be able to adapt to environmental change relatively quickly and at relatively small spatial scales, but, even so, there are limits on rescuing populations from rapid environmental change through evolutionary adaptation. (Written by Max Lambert)

This species was featured as News of the Week on 14 March 2022:

One of the silverlinings of the COVID-19 pandemic has been reduced human activities on the landscape (sometimes called the "anthropause"), notably a decrease in motor vehicle traffic during lockdown. This provided a unique opportunity while monitoring amphibians in northeastern United States. Using a community citizen science project, The Maine Big Night: Amphibian Migration Monitoring, Leclair et al. (2021) collected data on migrating amphibians crossing roads at sites throughout Maine during amphibian mating seasons, from March to May in 2018 through 2021. Almost 8,000 amphibians representing 16 species were recorded at 199 sites surveyed during these four years. They found a 50% decrease in frog mortality in 2020 compared to the other survey years, mainly due to decreased frog deaths in March and April. Wildlife collision data for other species in Maine (e.g., deer, turkeys, moose) were consistent with this trend of lower wildlife mortality in spring 2020. Thus, there was a significant reduction in frog deaths in Maine because of the traffic reductions during the COVID-19 lockdown. In the same time period, increasing precipitation correlated with increasing frog deaths, but not in salamanders, suggesting that environmental factors influence frog and salamander movements differently. Roads can be significant barriers to amphibian migrations; thus, even small changes can have large effects for these populations, especially for frogs. (Written by Carol Spencer)

References

Martof, B. S. (1963). ''Rana sylvatica (Le Conte). Wood Frog.'' Catalogue of American Amphibians and Reptiles. American Society of Ichthyologists and Herpetologists, 86.1-86.4.

Wiebler JM, Kohl KD, Lee Jr RE, Costanzo JP. (2018). ''Urea hydrolysis by gut bacteria in a hibernating frog: evidence for urea-nitrogen recycling in Amphibia.'' Proceedings of the Royal Society B, 285, 20180241.

Species Account Citation: AmphibiaWeb 2022 Rana sylvatica: Wood Frog <https://amphibiaweb.org/species/5162> University of California, Berkeley, CA, USA. Accessed Dec 9, 2022.

Citation: AmphibiaWeb. 2022. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 9 Dec 2022.

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