Amphibians of North Carolina

Scientific Name:	Common Name:		Family (Alpha):

NC Records
Ranidae Members:
Lithobates sylvaticus - Wood Frog

caption

Taxonomy

Class: Amphibia

Order: Anura

Family: Ranidae

Synonym: Rana sylvatica


Taxonomic Comments: Frost et al. (2006) placed species in North American that were formerly in the very large genus Rana into a separate genus, Lithobates, to distinguish them from a large and predominantly Eurasian genus Rana (sensu stricto). There have been numerous arguments put forth for well over a decade about whether these species should be placed back into Rana or retained as Lithobates, with some supporting placing Lithobates as a subgroup within Rana and others supporting the recognition of both genera. There has been no clear resolution of the issue, and both Lithobates and Rana continue to be widely used in recent published literature on North American species. Here, we follow the recommendations of the Society for the Study of Amphibians and Reptiles' Standard English Names Committee and use Lithobates for North American representatives of this group. The Wood Frog exhibits substantial geographic variation in body size, body proportions, and color patterning. Over the years, some geographic groups have been described as separate subspecies or even species, such as Rana maslini for geographic isolates in north-central Colorado and south-central Wyoming. Martof and Humphries (1959) recognized five morphotypes from different portions of the range that do not correspond to historic nomenclature, but did not formally recognize these phenotypes taxonomically. Currently, all of these forms -- along with past described species and subspecies -- are treated as a single, geographically variable species with no recognized subspecies (Dodd 2013).
Species Comments: This very wide-ranging and common species has been the subject of numerous life history studies. Here we focus mostly on the published literature for eastern North America that is most relevant to North Carolina populations. Dodd (2013) has a very comprehensive review that covers studies from throughout the entire range of the species.

Identification

Description: The adults are medium-sized ranids with a tan, reddish-brown, or dark brown ground color above and a conspicuous dark brown patch or mask on each side of the head. The patch begins as a narrow band near the snout, then widens markedly in front of the eye before terminating obliquely just above the front leg. The patch typically mask most of the eye and the entire tympanum, and is margined below by a white stripe that extends along the upper lip rearward to near the insertion of the front leg. A conspicuous dorsolateral fold is present on each side that extends from just behind the eye before terminating near the back leg. The back is uniformly colored and lacks blotching, but the sides often have a series of scattered dark brown blotches. Dark bars or blotches are usually present on the upper surfaces of the back legs. The venter is whitish or grayish white and essentially unmarked. The general description above is applicable to North Carolina specimens, but not to all populations of this wide-ranging species. Specimens in northern and northwestern areas of the range often have dark blotches or spotting dorsally, a light line along the posterior face of the thigh and leg, and a median light stripe bordered by a dark streak that extends from the snout down the middle of the back to the end of the urostyle (Dodd 2013). Average body size also varies geographically with a general tendency for average size to decrease with latitude (Davenport and Hossack 2016, Martof and Humphries 1959). The largest individuals are found in the southern Appalachians, including western North Carolina. Regional size differences can also be pronounced. Berven (1982a), for example, found that coastal populations In Maryland and Virginia were about 25% smaller based on SUL than mountain populations. Sexually active males have swollen thumbs for clasping females and a pair of lateral vocal sacs. They also tend to be darker than the females and are smaller on average. In North Carolina the males are typically dark brown versus light reddish brown or tan for the females. Dodd (2013) has detailed summaries of size variation from throughout the range, with males typically averaging around 10-15% smaller than females based on SUL. The older tadpoles are medium-sized and rather non-descript. The body and tail musculature vary from dark gray to brown, and the dorsal tail fin terminates just anterior to the junction of the tail and body (Dodd 2013). Most tadpoles have a faint white, cream, or goldish stripe along the upper jaw that may be broken. The venter is light and may be slightly pigmented along the sides, but the gut is not usually visible. In North Carolina this is often the only ranid tadpole of its size in the western mountain region during the spring and early summer months.
Vocalizations: The advertisement call can be a simple, deep 'cluck' or a more drawn out and raspy sounding ‘craw-awauk' or 'craw-aw-auk.' These are usually issued individually with a pause of 3-6 seconds or so before another is produced. They are sometimes produce in sequence with two or three issued with little pause between them. Frogs are rarely heard calling alone and are most often in choruses where individual voices may be difficult to distinguish. Small choruses have often been likened to a groups of barnyard ducks quacking, but larger choruses like the example below are more difficult to characterize. In addition to the advertisement call, individuals also have a soft one- to three-syllable call that they make very sporadically from terrestrial overwintering sites, particularly during the fall months (Dodd 3013). Zweifel (1989) heard the calls in New Jersey throughout the autumn and winter from isolated frogs buried beneath leaf litter in his yard. They were heard mostly at night and on only 49 occasions during a 14-year period. Their adaptive function, if any, remains a mystery.
Technical Reference: Dodd (2013)
Online Photos: Google iNaturalist
Observation Methods: The juveniles and adults are secretive and are rarely encountered outside of the breeding season. The adults are most easily observed when calling and mating at the breeding sites during late winter. The conspicuous egg masses that are laid communally offer a good way to document local populations.

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AmphibiaWeb Account

Distribution in North Carolina

Distribution Comments: Lithobates sylvaticus has the largest range of any North American frog. The range encompasses an expansive area that includes most of Alaska, along with much of southern and central Canada from Yukon and British Columbia eastward to Quebec and Labrador. In the eastern US the range extends from Maine and other northeastern states southwestward to western North Carolina, northern Georgia, and as a disjunct in north-central Alabama. It extends westward to central Tennessee, western Kentucky, eastern Illinois, Wisconsin, Minnesota, and northeastern North Dakota. Major disjuncts occur in Arkansas, Missouri, northwestern North Dakota, and in the Rocky Mountains of Colorado, Wyoming, and Idaho (Dodd 2013). Populations occur along eastern coastal regions to as far south as the western Chesapeake Bay region of Virginia, and as a geographic isolate in eastern North Carolina. Populations in North Carolina are widespread and common in the Blue Ridge Mountains where they are mostly found at lower elevation sites. Scattered populations occur in the western Piedmont and foothills, and disjunct populations occur in Hyde and Tyrrell counties in the Coastal Plain (Beane et al. 2010). The Coastal Plain isolate may reflect a remnant group from the last glacial retreat and needs more careful study.
Distribution Reference: Beane et al. (2010), Dodd (2013)
County Map: Clicking on a county returns the records for the species in that county.


GBIF Global Distribution

Key Habitat Requirements

Habitat: The Wood Frog uses a variety of habitats across its range, including Alaskan tundra, subalpine woodlands, willow and alder thickets, wet meadows, bogs, and temperate forests (AmphibiaWeb 2022, Dodd 2013). In the eastern North America this species is strongly affiliated with closed-canopy, deciduous and boreal forests, with a general preference for deciduous forests where they are available locally. In much of the eastern US they prefer mesic hardwood forests or mixed hardwood-conifer forests, while farther north they can be found in northern conifer or mixed conifer-hardwood associations (Eigenbrod et al. 2008, Gibbs 1998b, Guerry and Hunter 2002). Forested sites with well-developed canopies, thick leaf-litter, and suitable seasonal or semipermanent breeding sites provide optimal habitat (Babbitt et al. 2003, Baber et al. 2004, Egan and Paton 2004, Herrmann et al. 2005).

In eastern North America the adults characteristically breed in fish-free seasonal or semipermanent ponds and rarely in permanent habitats with fish. Natural habitats that are commonly used include vernal ponds, bogs, marshes, fens, oxbow ponds, abandoned beaver ponds, and scour pools along rivers and streams. Numerous artificial habitats are also used so long as they are in close proximity to forests. Examples include flooded ditches, road ruts and puddles, seasonal depressions associated with roadway construction, borrow pits, catchment ponds, and ornamental and urban ponds. This species will also quickly colonize constructed wetlands so long as fish are not present.

In North Carolina, populations are strongly affiliated with mesic deciduous forests or mixed deciduous forests such as hardwood-White Pine or hardwood-Eastern Hemlock associations. The coastal disjunct populations are generally associated with bottomland hardwoods, although most records are based on individuals crossing roads on rainy nights. The adults breed in a variety of seasonal and semipermanent habitats, particularly vernal ponds, floodplain ponds and scour pools along rivers, fens and bogs, ridge top ponds, and beaver ponds. They also frequently use flooded ditches along railroad tracks and roadways, flooded tire ruts and large mud puddles, ornamental and urban ponds, and on rare occasion farm ponds that lack fish. The breeding sites are almost always either within or in close proximity to forests.

Environmental and Physiological Tolerances: Lithobates sylvaticus is an acid-tolerant species and frequently reproduces successfully in acidic wetlands such as northern bogs and fens. Dodd (2013) provides a comprehensive summary of studies on acid tolerance of this species, including information on geographic variation in acid tolerance across the range.

Biotic Relationships: The Wood Frog usually breeds in seasonal wetlands that lack fish, but other aquatic predators such as Ambystoma larvae, Eastern Newts, and aquatic invertebrates are often well represented. The eggs and larvae lack chemical defenses and are palatable to a host of predators. The eggs and hatchlings are eaten by Eastern Newts, Ambystoma larvae, caddisfly larvae, and leeches (e.g., Davis and Folkerts 1986, Walters 1975, Cory and Manion 1953, Trauth and Neal 2004). Raccoons will also eat the eggs (Redmer 2002).

Eastern Newts are important predators that can sometime consume most of the eggs at a breeding site, particularly in permanent ponds that lack fish and support robust populations of newts (e.g., Kross and Richter 2016). Overwintering Green Frog tadpoles can also be important egg predators in permanent ponds that lack fish. Petranka and Kennedy (1999) found that Green Frog tadpoles completely consumed all Wood Frog eggs in two ponds in western North Carolina. Similar observations of almost complete egg losses from Green Frog tadpoles have been made elsewhere (Jennette 2010, Vasconcelos and Calhoun 2006). Wood Frog tadpoles will also cannibalize their eggs and hatchlings. Petranka and Thomas (1995) found that the eggs and hatchlings of relatively late-breeding females in ponds in western North Carolina were heavily cannibalized by larger tadpoles that hatched after the main explosive breeding bout earlier in the year. Cannibalism selects for females that engage in synchronized, explosive breeding and against those that breed relatively late.

The larvae are also palatable, although they appear to develop chemical defenses immediately before metamorphosing as granular glands develop in the skin (Formanowicz and Brodie 1982). The known predators include fishes, dragonfly naiads, belostomatid and dyticid beetles, notonectids, waterstriders, crayfishes, several species of Ambystoma larvae, Eastern Newts, grackles, and Eastern Garter Snakes (Dodd 2013, Formanowicz and Brodie 1982, Kats et al. 1988, Redmer 2002, Szuroczki and Richardson 2011). The larvae often form large aggregates along pond margins when feeding during the day, and many individuals reduce predation risk simply through safety in numbers.

The tadpoles monitor many predators in their environment by using chemical cues. They respond to chemical cues by reducing their movements and moving into cover, which reduces encounter rates with predators (Skelly 1994). The larvae will respond to fish chemicals even though they rarely encounter fish in the wild, as well as those of other predators such as Eastern Newts and odonate larvae (Anholt et al. 2000, Chivers and Mirza 2001, Richardson 2001, Relyea 2000, 2001b, Fraker 2010). The responses to predators is often stronger when predators are fed a Wood Frog tadpole prior to testing.

In addition to behavioral defenses, the tadpoles have phenotypically plastic developmental responses. When confined with predators such as odonates, the young tadpoles develop shorter bodies and deeper tail fins relative to controls. These morphological changes make them less likely to be killed by odonates (Relyea 2001a, 2002b). A trade-off is that individuals with predator-induced phenotypes grow slower and have longer larval periods relative to controls (Van Buskirk and Relyea 1998, 2002b). By having developmentally plastic responses, the tadpoles can adapt to local conditions such as predator densities or predation risk that often vary markedly from year to year in seasonal ponds.

The breeding adults can enhance the survival of their future offspring by selecting optimal habitats for egg-laying. Hopey and Petranka (1994) examined the response of adults to a set of experimental ponds in western North Carolina that either had sunfish (Lepomis auritus) added or no fish added just before breeding began. Wood Frogs strongly avoided all ponds with fish and laid all of their egg masses in fish-free ponds. In other ponds in western North Carolina that were tracked for several years, breeding adults quickly abandon their traditional breeding sites following fish invasions and moved to nearby fish-free habitats (Petranka and Holbrook 2006).

The juveniles and adults are taken by numerous predators. Dodd (2013) summarized the known predators that include crayfishes, a giant water bug (Lethocerus sp.), bass, American Bullfrogs, Common Gartersnakes (Thamnophis sirtalis), Northern Watersnakes (Nerodia s. sipedon), numerous species of birds, and mammals such as raccoons and skunks.

See also Habitat Account for General Cool Mesic Forests and Shrublands

Life History and Autecology

Breeding and Courtship: The Wood Frog is an early season breeder, with breeding progressing seasonally from southern to northern populations. In some areas such as Virginia there is a strong seasonal difference in the initiation of the breeding season in coastal versus mountain populations. This is usually one of the first anurans to arrive at local breeding ponds following the arrival of warmer weather with the new year. The adults live in surrounding forests and migrate to the breeding sites during relatively warm, rainy weather. This species is an explosive or synchronized breeder, with most populations having one or two major breeding bouts, followed by a few stragglers that may appear later. In most populations the great majority of eggs -- including those in North Carolina (Petranka et al. 2004a) -- are usually laid within a week or so (Dodd 2013).

Calling and breeding in eastern North America occurs from late December through June depending on the latitude and elevation. Some representative examples that are provided by Dodd (2013) are January to early May in the southern Appalachians depending on elevation (Huheey and Stupka 1967, Dodd 2004), late February to early April in eastern Maryland, late March to mid-April in New York, mid-April to late May in Quebec, and late May to early June along the south shore of the Hudson Bay.

Populations in Alabama at the southern limit of the range breed from mid-January through early March (Davis and Folkerts 1986), while those in Georgia typically begin breeding in January or early to mid-February depending on the year and local site conditions (Jensen et al. 2008). Tennessee populations mostly breed from January through March (Niemiller and Reynolds 2011). Egg laying in North Carolina can occur as early as the last week of December (Jim Petranka, personal observation), but more commonly occurs from mid-January through mid-March depending on the elevation and local site conditions. Breeding generally occurs during bouts of warm, rainy weather when air temperatures reach 5–10°C or higher (Dodd 2013, Howard 1980).

The males arrive a day or two before the females. Small vernal pools often have high densities of males that can be seen calling while floating on the water surface. They swim about for short distances as they interact with neighboring males, perhaps to check and see if they are females. There is no evidence of territoriality, but males tend to space apart at somewhat regular intervals on the pond surface (Dodd 2013, Howard 1980, Seale 1982). They are wary and will quickly dive beneath the water and hide on the bottom if approached.

Calling can occur both during the day and at night, and local and regional populations vary depending on site conditions. Davis and Folkerts (1986), for example, found that Wood Frogs in Alabama mostly called at night, with a peak in activity during the first few hours after dark. Populations in the northern portion of the range are more likely to call during the day, which reflects cool temperatures at night. Adults in North Carolina and other areas of the southern Appalachians call during the day and night.

Males almost always outnumber females in the ponds at any given moment and compete intensely for females (Berven 1990, Crouch and Paton 2000, Dodd 2013, Howard and Kluge 1985). They will attempt to amplex any female that enters a breeding site, and it is not uncommon to see several males attempting to amplex a female (Howard 1980, Wells 1977a). Mating is nonrandom and males aggressively compete for larger females that produce larger clutches (Howard and Kluge 1985). Large males are generally more successful at holding on to large females and fending off smaller males that try to dislodge them. They also enter the breeding ponds ahead of smaller males, which enhances their mating success (Berven 1981, Dodd 2013, Howard 1980, Howard and Kluge 1985). Howard and Kluge (1985) found that 61% of the males at a breeding site in Michigan did not mate due to a combination of skewed sex ratios and competition for mates.

Amplexus normally occurs in water, with the male grasping the female just below her front legs. The amplexed pair may move substantial distances within a breeding site before egg laying and fertilization occurs, typically at a communal egg-laying site within the pond. Egg laying often begins within a few hours to a day after amplexus, and females lay their entire clutch in less than 15-30 minutes (Dodd 2013).

Reproductive Mode: Each female normally lays a single, globular mass that eventually swells to about the size of a large orange or small grapefruit. Most egg masses are laid communally, with dozens or hundreds of females often laying in a very confined area. Adjoining egg masses are usually placed close enough to touch one another, and with time tend to fuse as the jelly envelops swell to produce what is sometimes referred to as a raft of egg masses. These are often placed in shallow water and in relatively warm areas of a breeding site (Dodd 2013, Seale 1982). Northern populations often lay in somewhat deeper water, while those in southern populations often lay with the tops of the masses at the water surface. A small percentage of females (usually no more than 20%) may lay eggs singly and not as part of a communal group. Females in later breeding bouts may lay around the margins of an older raft, create a new raft elsewhere, or lay eggs singly in other areas of the pond.

The freshly laid eggs are blackish to grayish black above and whitish below, and are surrounded by a well-developed jelly layer. The jelly envelopes and membranes are usually colonized by a unicellular green alga (Oophilia amblystomatis), which may give them a greenish coloration. The temperature within the center of an individual egg mass is often 1–3°C warmer than the surrounding water. The dark, developing embryos and hatchlings act as miniature solar collectors on sunny days, while the jelly serves as insulation. The temperature at the center of communal groups or rafts on sunny days can be 4–7°C warmer than the surrounding water, which accelerates embryonic development (Dodd 2013, Herreid and Kinney 1967, Howard 1980, Seale 1982, Waldman 1982a, Waldman and Ryan 1983).

Development rates are temperature dependent and can vary markedly depending on external factors such as annual weather patterns and water temperatures at oviposition sites within ponds. This species generally has a prolonged developmental period due to early breeding in relatively cold water. The embryos in most populations normally require from 14-24 days to reach the hatching stages (Dodd 2013), but Davis and Folkerts (1986) reported hatching in 7-9 days in Alabama populations.

Davis and Folkerts (1986) noted that each female has two ovisacs and can potentially produce two egg masses. The eggs from the two ovisacs are normally laid simultaneously and fuse to form a single mass. In some instances a female may conceivably lay two separate masses in different areas of a pond. The egg mass counts in their ponds in Alabama ranged from 350-708 eggs per mass versus 618-966 for ovarian egg counts, which suggest that this may happen occasionally. However, Seale (1982) found no difference between egg counts for egg masses (mean = 895) versus ovaries (mean = 840) in Pennsylvania females, which suggests that females typically lay a single mass.

The clutch size of females tends to be positively correlated with female SUL, and average clutch sizes can vary substantially among populations (Dodd 2013). Davenport and Hossack (2016) analyzed much of the published data on clutch sizes and found that the average clutch size of females did not change with latitude, while the average ovum size decreased. Because female body size also decrease with latitude, northern females are able to produce similar sized clutches as larger southern females by producing smaller eggs. Average clutch size varied from 465-956 eggs among 17 populations that were analyzed from throughout the range, with a grand mean of 704 eggs per mass.

Aquatic Life History: The hatchlings often aggregate in large groups within communal egg masses where they form blackish masses. These undoubtedly act as heat collectors that accelerate development until the tadpoles have functional mouthparts. The hatchlings often feed on the remains of the egg masses and associated algae before finally dispersing more widely within the breeding site.

The tadpoles primarily function as microphagous suspension-feeders that feed on algae, detritus suspensions, and other living or dead plant material, along with their associated microorganisms and microinvertebrates. They use their keratinized mouthparts to break up algae and other plant material and consume items such as diatoms, filamentous algae, blue-green algae, and protozoans (Dodd 2013, Munz 1920). The tadpoles will also opportunistically prey on invertebrates and feed on the eggs of other anurans. The larvae do not form organized social schools, but often concentrate in warm water around pond margins on sunny days where they can reach very high local densities. Biesterfeldt et al. (1993), for example, reported larval densities of over 1,800 tadpoles per square meter of pond bottom in western North Carolina. In North Carolina the tadpoles can greatly reduce the densities of benthic invertebrates (Petranka and Kennedy 1999), and often feed heavily of the eggs of other amphibians such as the Spotted Salamander and American Toad (Petranka et al. 1994a, 1998).

The tadpoles are capable of rapid growth, but growth rates are mediated by numerous factors, including ambient water temperatures, food quality and quantity, the density of conspecifics and heterospecific competitors, and the kinds and densities of predators (Berven 1990, Castano et al. 2010, Dodd 2013, Wilbur 1977a). Berven (1990) found evidence of density-dependent regulation during the larval stage in a multi-year study of two ponds in Maryland. In years when hatchling densities were relatively high, growth rates were suppressed, the length of the larval period increased, and individuals were smaller at metamorphosis.

Large tadpoles of this and many other anurans can suppress the growth of smaller tadpoles in laboratory tanks even when fed ad libitum. The guts of stunted tadpoles become packed with round, colorless cells (Anurofeca richardsi; formerly Prototheca richardsi) that are believed to be parasitic and that either compete for intestinal nutrients or inhibit nutrient absorption across the intestinal wall. This stunting phenomenon has been viewed as a form of interference competition since larger tadpoles are less likely to have stunted growth. Petranka (1989b) and Biesterfeldt et al. (1993) examined the importance of interference competition in natural ponds with high densities of Wood Frog tadpoles and found little evidence that it occurs outside of laboratory tanks.

The length of the larval period varies depending on site conditions. Reported values for the eastern US that were summarized by Dodd (2013) include 73–113 days in Maryland (Berven 1982b), 115–130 days in Georgia (Camp et al. 1990), 70–87 days in Illinois (Redmer 2002), and 68–94 days in Michigan (Berven 2009). Sizes at metamorphosis based on SUL include 15-21 mm (mean = 18 mm) in Georgia (Camp et al. 1990), 14–18 mm in Maryland (Berven 1990), 17–21 mm in Minnesota (Bellis 1961), and 14-29 mm (mean = 18 mm) for five ponds in Missouri (Semlitsch and Drake 2015).

Survivorship to metamorphosis is also variable and can be zero when ponds dry prematurely, predators such as newts reach high densities, or diseases such as Ranavirus infections occur in ponds (e.g., Berven 1995, Kross and Richter 2016, Petranka et al. 2003, Timm et al. 2007b). Berven (1990) reported premetamorphic survival at 4.5% in one pond and 0.95% in another in Maryland. Seigel (1983) reported 66% survivorship from egg deposition to metamorphosis in New Jersey in a pond with low densities of hatchlings and few predators, while Semlitsch and Drake (2015) estimated larval survival in Missouri to vary from 1.3-6.1% depending on the pond and year.

Terrestrial Life History: The young metamorphs tend to leave their natal ponds within a week or two after metamorphosing. Timm et al. (2007b) found that most juveniles left their study ponds in Massachusetts within a 1-2 week period, with 80% leaving during a five-day period for a given pond in a given year. Paton and Crouch (2002) reported a combined emigration period of around one-month for metamorphs at seven ponds in Rhode Island. Movements of both the adults and juveniles occur during rain events. Timm et al. (2007c) found that juvenile dispersal tended to be nonrandom at individual breeding ponds in any one year, but more random when data was pooled across years. Vasconcelos and Calhoun (2004) also found non-random movement patterns from ponds in Maine, with most metamorphs and adults moving towards forested habitats.

Berven and Grudzien (1990) examined the movements of juveniles from a series of ponds in western Virginia that were monitored for several years. Although 100% of the adults faithfully returned to their home ponds to breed in subsequent years, only about 80% of the metamorphs returned to their home ponds to breed as adults. An estimated 21% of marked males and 13% of marked females moved to non-natal ponds. The mean dispersal distances for males and females that moved between ponds was 1,140 m and 1,276 m, respectively, while the longest estimated distance moved was 2,530 m by two individuals. Vasconcelos and Calhoun (2004) documented adult movements between three ponds in Maine that were in close proximity to one another (maximum distance 200 m). Males were 98% and females 88% faithful overall to their home ponds across years. Both juveniles and adults were collected 300 m from the ponds, which was the maximum distance that was monitored with drift fences.

After moving from the breeding ponds both the juveniles and adults seek out moist microhabitats in forests that they use for foraging and hiding. Heatwole (1961) found them using swamplands in Michigan where they frequently remained near the margins of drying ponds. However, they are commonly found far away from the nearest bodies of water in mesic habitats with well-developed leaf litter and moist substrates (e.g., deMaynadier and Hunter 1998, Wyman 1988). Baldwin at al. (2006) found that dispersing Wood Frogs in Maine initially selected damp leaf litter near wetland margins, but eventually moved to closed-canopy habitats that were 102-340 m from the breeding pond. They were strongly dependent on wet to moist microhabitats such as sphagnum-bordered forested streams for surviving dry periods. Frogs that were studied by Rittenhouse and Semlitsch (2007) in Missouri mostly used stream drainages with deciduous leaf litter for summer retreats. Adults monitored by Groff et al. (2017) in Maine moved from breeding sites to higher and drier habitats, with one individual moving as far as 1210 m. They tended to select summer foraging areas that had relatively deep leaf litter, dense canopy cover, and cool ground temperatures.

After settling into their summer habitats, the adults tend to have small home ranges. They usually move only short distances of a few to several meters between observations, but occasionally make long-distance movements of 50 m or more (Baldwin et al. 2006, Bellis 1965). Bellis (1965) estimated an average home range of 65 m2 in a Minnesota bog. The juveniles and adults are gape-limited, generalist predators that take a wide variety of prey. They tend to be more active during rainy weather and largely function as sit-and-wait predators that may move short distances to attack prey in their immediate vicinity (Dodd 2013). Some of the known prey as summarized by Dodd (2013) include springtails, ants, flies, beetles, larval lepidoptera, crane flies, spiders and gastropods. A great variety of other invertebrate taxa are also taken. The young grow rapidly, with males typically becoming sexually mature 1-2 years after metamorphosing. The females usually require an additional year to lay their first clutch of eggs (Bastien and Leclair 1992, Berven 1982a, 1990, Dodd 2013, Howard 1980).

The juveniles and adults overwinter on land where they shelter in the soil or under logs, leaf litter, and other surface cover (Dodd 2013, Groff et al. 2016). Regosin et al. (2003) found that males in Missouri tended to move closer to the breeding ponds than did females as winter approached. This allowed males who compete for mates to be close to breeding ponds when breeding begins several months later. The Wood Frog is well known for its ability to tolerate subfreezing temperatures by using blood glucose as a cryoprotectant. Individuals can survive whole body freezing for 8 days at –2.5°C and for 2 weeks at –5°C in the fall when blood glucose levels reach a seasonal peak (Dodd 2013, Layne 1995, Storey and Storey 1987).

General Ecology

Population Ecology: The sizes of local breeding populations often fluctuate markedly across years, which reflects fluctuations in juvenile recruitment from year to year. It is not uncommon for local pond populations to consist of hundreds to several thousand breeding adults. Howard and Kluge (1985) estimated a Michigan population to fluctuate between 5,202 to 8,955 individuals over a four-year period, and Berven (2009) reported a population fluctuation from 594–6,196 adults over the course of 21 years of study. His long-term data suggests that the upper population size of this population is set by density-dependent factors operating in the terrestrial stages of the life cycle. In particular, higher juvenile population size resulted in lower juvenile survival, delayed maturation, smaller adult body size, and reduced fecundity. Local populations that use small, isolated breeding sites such as vernal pools are often much smaller than the ones described above.

Studies of population genetics and dispersal indicate that populations in close proximity (less than one to several kilometers) are highly connected and lack strong genetic structure (e.g., Gabrielsen et al. 2013, Skibbe et al. 2021, Squire and Newman 2002). This likely reflects the ability of the juveniles to disperse long distances and their lack of strong fidelity to the breeding ponds (Berven and Grudzien 1990). The adults may also shift from one breeding site to another in response to fish invasions or other disturbances (Petranka et al. 1994b, Petranka and Holbrook 2006).

Community Ecology: Wood Frog tadpoles often share breeding sites with other amphibians. Their ecological roles are complex since they can function both as microphagous suspension-feeders that can compete with other anurans, as well as predators that can impact other community members. In western North Carolina and other areas of the southern Appalachians, Wood Frog tadpoles can reach very high densities in seasonal ponds and often act as keystone species. They can greatly reduce the densities of benthic invertebrates in ponds that serve as important food items for the Spotted Salamander and other ambystomatid salamanders (Petranka and Kennedy 1999). They often feed heavily on the eggs of other amphibians such as those of the Spotted Salamander, American Toad, Cope's Gray Treefrog, and Upland Chorus Frog. Petranka et al. (1998) found that the tadpoles can be major predators on Spotted Salamander embryos, and in some ponds can consume most of the egg masses prior to hatching. American Toad eggs are also highly vulnerable to predation from Wood Frog tadpoles, and choice experiments show that female toads will actively avoid laying in ponds with Wood Frog tadpoles (Petranka et al. 1994a). Egg predation on anuran eggs such as those of the Upland Chorus Frog (Sours and Petranka 2007) may be a way to eliminate potential competitors.

Because this species breed very early in the year, it often has a significant size advantage that makes it a superior competitor with later arrivers such as the Spring Peeper (Morin and Johnson 1988). Relyea (2000) found that predators can often mediate competitive interactions between the Wood Frog and the Northern Leopard Frog. In the absence of predators the Wood Frog was the superior competitor. In the presence of predatory odonates and fish, the tadpoles altered their behavior and morphology in a way that made them inferior competitors with leopard frogs.

Adverse Environmental Impacts

Habitat Loss: The Wood Frog is strongly dependent of forests for its survival. Several studies have shown that this species is absent or underrepresented in landscapes that have been deforested to varying degrees (Gibbs et al. 2005). In Maine, Connecticut, and Ontario, for example, the amount of forest cover is positively correlated with the probability of finding Wood Frogs breeding in associated wetlands (Eigenbrod et al. 2008, Gibbs 1998b, Guerry and Hunter 2002). Homola et al. (2019) found that urbanization and features such as interstate roads can significantly restrict interpopulation connectivity for wood frogs in Maine.

Habitat Fragmentation: Habitat fragmentation can impact Wood Frog populations both by eliminating forests from the landscape and adding barriers to movement such as roadways (e.g., Findlay et al. 2001). The sizes of local pond populations are negatively correlated with nearby road density (Dodd 2013, Egan and Paton 2004, Karraker and Gibbs 2011, Veysey et al. 2011), perhaps due to road-related mortality or the use of road deicing compounds in some regions.

Effects of Introduced Species/Induced Increases of Native Species: Emerging infectious diseases are considered to be one of the most important factors contributing to global amphibian declines and have been implicated in the local extinction of several species. Ranavirus is a highly virulent pathogen that affects many amphibian species, including the larvae of the Wood Frog. Infected larvae develop characteristic bloody, hemorrhagic patches on the body and die shortly thereafter. Ranavirus is highly infectious and lethal. Populations in western North Carolina frequently have complete die-offs of larvae during outbreaks, with no larvae surviving to metamorphosis (Petranka et al. 2003, 2007). This pathogen also infects Spotted Salamander larvae that share breeding ponds with Wood Frog larvae, and both presumably serve as sources of transmission between species.

Status in North Carolina

NHP State Rank: S5

Global Rank: S5

Status Comments: Populations have likely declined historically in North Carolina due to the widespread loss of seasonal wetlands such as vernal ponds, floodplain pools, fens, and marshes. These were at one time common along the floodplains and alluvial valleys of major streams and rivers in the mountains, but most are now gone. The losses have been compensated to some extend by Wood Frogs using small, fish-free constructed ponds and flooded ditches. Unfortunately, many local populations are now isolated from nearest neighbors and are at greater risk of being extirpated in the future. Numerous populations still remain in the mountains, and the remaining populations appear to be relatively stable. Deciduous forest habitat is abundant in the mountains and presumably less limiting than suitable breeding sites.

Stewardship: Populations are best maintained by having a cluster of fish-free, seasonal ponds that are surrounded by an extensive forest buffer zone of around 200 meters or more. Many populations in the Blue Ridge are isolated from nearest neighbors, and wetland restoration projects need to be designed that allow local populations with little connectivity to nearest neighbors to survive. Petranka et al. (2007) examined the long-term population persistence of the Wood Frog at a wetland restoration site in Graham County. The design strategy was to create a large number of wetlands on site that varied in size, depth, and hydroperiod, and that provided multiple breeding sites for all resident species of amphibians. The Wood Frog population on site persisted over a 13-year monitoring period and remained robust. This occurred despite the fact that the output of metamorphs in many ponds was near zero due to outbreaks of Ranavirus, fish invasions, and drought. They provide specific recommendations for wetland design that are relevant for restoring wetlands for seasonal pond breeders in North Carolina.

Photo Gallery for Lithobates sylvaticus - Wood Frog		20 photos are shown.
	Recorded by: Andrew W. Jones Polk Co.	Recorded by: J. Cameron Buncombe Co.
	Recorded by: Paul Bartels Buncombe Co.	Recorded by: K. Bischof Transylvania Co.
	Recorded by: K. Bischof Transylvania Co.	Recorded by: K. Bischof Transylvania Co.
	Recorded by: David George Swain Co.	Recorded by: Travis McLain Watauga Co.
	Recorded by: R. Spainhour Surry Co.	Recorded by: R. Spainhour Surry Co.
	Recorded by: Jim Petranka Madison Co. Comment: The larvae are rather non-descript like this one; note the lack of conspicuous blotching on the tail fin and the poorly developed light stripe on the upper jaw.	Recorded by: K. Bischof Transylvania Co.
	Recorded by: Jim Petranka and Becky Elkin Madison Co.	Recorded by: tom ward Buncombe Co.
	Recorded by: tom ward Buncombe Co. Comment: Note the Eastern Newts that are feeding on the eggs.	Recorded by: K. Bischof, A. Sicard Avery Co.
	Recorded by: K. Bischof, A. Sicard Avery Co. Comment: A large feeding aggregate of tadpoles. These can be major predators on the eggs of other amphibians.	Recorded by: K. Bischof Avery Co. Comment: A raft of egg masses.
	Recorded by: Becky Elkin Madison Co.	Recorded by: Jim Petranka Buncombe Co.

Lithobates sylvaticus - Wood Frog

Photo Gallery for Lithobates sylvaticus - Wood Frog

20 photos are shown.