Amphibians of North Carolina
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Lithobates clamitans - Green Frog



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Taxonomy
Class: Amphibia Order: Anura Family: Ranidae Synonym: Rana clamitans
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.

Two subspecies were traditionally recognized by herpetologists, the Green Frog (Rana clamitans melanota) and the Bronze Frog (Rana c. clamitans). These are no longer recognized and are treated as one of several color phases of a single species, the Green Frog (Lithobates clamitans).
Species Comments:
Identification
Description: The adults are medium-sized frogs that exhibit substantial geographic variation in coloration and body size. Two color phases that were previously recognized as subspecies occur throughout the range. These are a green form that is more northern and commonly referred to as the Northern Green Frog, and a southern bronze form commonly referred to as the Bronze Frog. In addition to these regional groups, some northern populations in scattered populations from the Great Lakes region to the Northeast are extremely dark olive to black and are commonly referred to as the Black Frog (Dodd 2013, Mecham 1954). 'Green Frog' is the standardized common name that now applies to all populations of this species, regardless of the particular color form. North Carolina has both the green and bronze forms, as well as numerous intermediate types, with bronze individuals predominating in the southeastern part of the state (Beane et al. 2010). Many individuals in North Carolina with greenish heads have the green coloration restricted to the sides of the head.

Lithobates clamitans is best identified by a dorsolateral fold that extends on each side from behind the eye down the back before fragmenting about midway or so. A secondary fold branches off of the dorsolateral fold and extends around the tympanum. Populations in the northern part of the range can vary from green to olive green or brown above. Their skin tends to be somewhat rugose and the head is usually bright green. The dorsum, legs, and sides are often flecked with black spots, and the legs lack black bars above. The side of the body tends to have a dark, wavy, indistinct band that phases into darker mottling near the venter below. The center of the tympanum is boldly marked by a white or yellow spot, and the belly is grayish white to whitish and slightly mottled around its margins. The belly can be either white or more boldly marked with dusky reticulations (Dodd 2013).

Populations in the southern Coastal Plain from southeastern North Carolina to eastern Texas and up the Mississippi Embayment tend to be entirely or predominantly bronze colored above, except for the upper jaw that is often bright green and contrasts with the rest of the dorsal coloration. Specimens from this region also tend to be smaller and more slender than their northern counterparts, and there is no dorsal spotting between the dorsolateral folds. The dorsal skin surface is not as rugose and the venter is more likely to be silvery white and lack darker reticulations.

Sexually mature males of both forms differ from females in having throats that vary from light to bright yellow, a tympanum that is larger than the eyes (about the same size in females), and enlarged thumbs and hypertrophied forearms that are used to amplex the females. Sexual differences in size are not consistent across the range. Within a given local population or region, males may average smaller than females, be about the same size, or average larger (Dodd 2013).

The tadpoles can reach 80–100 mm TL and are olive green to drab brown above with small to large dark markings or fine mottling on the body. The throat and belly are dull whitish and the tail musculature is similar in color and patterning as the dorsum of the body. The tail fin is lighter than the musculature and mottled with brown specks or blotches that are equally represented on both the upper and lower fin. The dorsal fin terminates just past its juncture with the body and posterior to the spiracle -- which is located on the left side of the body (Dodd 2013). The eyes are dorsal when viewed from above.
Vocalizations: The males produce several types of calls, with the advertisement call the most well recognized one. It may consists of a single note that resembles a deep "c'tung" or a rapid sequence that has 2-7 notes (e.g., "c'tung - c'tung - c'tung - c'tung." The larger males have deeper voices than the younger ones, and the number of notes per call typically increases with overall chorus activity during a night (Wells 1978). Bee et al. (2008) analyzed individual advertisement calls of males and found that they were distinctive. The males are likely able to discriminate between strangers and adjacently territorial holders based on individual variation in advertisement calls.

In addition to the advertisement call, Wells (1978) described several other calls that mediate interactions between individuals. These include an encounter call that is made when a resident male approaches a territorial intruder, a growl that is issued when a male wins a physical contest with another, and a release call. Individuals also emit a shriek when jumping into water after being approached by a human or predator.
Technical Reference: Dodd (2013)
Online Photos:    Google
Observation Methods: Green frogs are most easily observed by walking the margins of wetlands or searching roads on rainy nights. Their distinctive advertisement call is readily heard around breeding sites during the summer months.

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AmphibiaWeb Account
Distribution in North Carolina
Distribution Comments: The Green Frog is widely distributed in eastern North America, including much of extreme southern Canada from southeastern Manitoba eastward to the Canadian Maritimes and St. Lawrence River Valley. The range includes much of the eastern US, with the southern limit in central Florida, the Gulf States, and eastern Texas. The western limit includes eastern Texas, eastern Oklahoma, central and southeastern Missouri, eastern Iowa, and eastern Minnesota. Populations have been introduced in western North America, including British Columbia, Washington, Arizona, Iowa, and Montana (Dodd 2013). This species occurs statewide in North Carolina where it has been found in almost every county in the state.
Distribution Reference: Dodd (2013)
County Map: Clicking on a county returns the records for the species in that county.
Key Habitat Requirements
Habitat: Lithobates clamitans is a semiaquatic species that generally stays close to bodies of water that are used for foraging, breeding, and overwintering. The juveniles and adults use all sorts of aquatic habitats, including both standing water, springs, and the sluggish portions of streams. The length of the larval period is variable depending on latitude and local site conditions. Many populations require a year or more to metamorphose and semipermanent or permanent habitats are requires to support local populations. The larvae are somewhat unpalatable to fish -- and have behavioral defenses also -- but in many areas they do best in either fish-free permanent sites or sites with well-developed, vegetated shallows that provide cover and protection for the larvae and adults (Richardson 2001, Werner and McPeek 1994).

This wide-ranging species uses a variety of habitats. Populations in the Coastal Plain have been found in wet and mesic hammocks with small streams, mixed hardwood forests with aquatic sites, flatwoods ponds, bayheads, cypress and titi swamps, cypress and gum swamps, cutoff pools of river swamps, steephead ravines, freshwater ponds, seepage bogs, and along rivers and small streams (Dodd 2013). Outside of the Coastal Plain, the Green Frog can be found in and around lakes and ponds, springs and seeps, woodland pools, peatlands, and the riparian zones of streams. Dispersing juveniles can commonly be found in large mud puddles, roadside ditches, ornamental ponds and other small artificial habitats. Both the juveniles and adults may use a variety of temporary upland habitats when moving to and from the breeding sites.

The Green Frog has been found in a large number of aquatic settings in North Carolina, including the backwaters and sluggish areas of streams and rivers, small woodland streams and creeks, springs, marshes, bogs, vernal pools in bottomlands, vegetated lakes and farm ponds, cypress swamps, Carolina bays, swamp forest pools, sinkhole ponds, borrow pits, quarry ponds and roadside ditches. The juveniles disperse widely from breeding sites and the adults occasionally move to new breeding sites. They are commonly seen on roads during rainy nights and may travel through a variety of disturbed upland habitats when on the move.
Environmental and Physiological Tolerances: The Green Frog appears to be moderately tolerant of acidic waters. It is generally restricted to breeding sites with a pH above 4.0 or higher, with local abundance being positively correlated with pH (Dodd 2013).
Biotic Relationships: The larvae typically live in semipermanent or permanent bodies of water that have numerous aquatic predators. Fish will readily eat the ovarian eggs, suggesting that they are palatable to predators (Licht 1969). However, the jelly envelope may offer a defense against fish and other predators (Smith et al. 2019). There is limited evidence that ostracods may also eat the eggs under natural conditions (Gray et al. 2010). Some of the documented predators on the larvae include predaceous diving beetles, water beetles (Hydrophylidae), whirly gig beetles, giant water bugs, water scorpions, backswimmers, dragonfly larvae, crayfishes, turtles, and fishes (Dodd 2013, Smith et al. 2019).

The larvae have chemical defenses against fishes, but they do not appear to be as strongly developed as those of some species such as the American Bullfrog (Fogarty and Hetrick 1973, Kats et al. 1988, Szuroczki and Richardson 2011, Werner and McPeek 1994). The tadpoles often respond to threats from fish and invertebrate predators by decreasing activity levels and moving into cover or away from the source of chemical cues, but this can compromise growth rates (Brown et al. 2019, Eklov and Werner 2000, Fraker 2010, Peacor and Werner 2000, Smith et al. 2010, Relyea and Werner 1999, Werner and McPeek 1994). The general consensus is that the tadpoles are vulnerable to fish predation, particularly when small, and rely more heavily on behavioral defenses to lower predation risk.

In addition to chemical and behavioral defenses, the presence of predators such as fishes and aquatic insects can trigger developmental changes in the young tadpoles. These may cause changes in body width, tail musculature, and fin morphology that may improve the likelihood of a tadpole escaping a lethal attack (Dodd 2013, Relyea 2001a, 2001b). The presence of extensive vegetated shallows in lake and ponds provides important cover that helps to reduces losses from fish predation. Werner and McPeek (1994) found that this species was uncommon or absent in permanent ponds with bluegill fish in Michigan and much more common in similar permanent or semipermanent ponds that lacked fish and had high densities of aquatic predators such as aquatic insects, newts, and Ambystoma tigrinum larvae.

The juveniles and adults are palatable to vertebrate predators (Formanowicz and Brodie 1979) and are undoubtedly taken by numerous species. Some of the known predators include fishes, large frogs (e.g., L. catesbeianus, L. clamitans), turtles (e.g., Chelydra serpentina), snakes (Agkistrodon; Nerodia; Thamnophis), crows, herons, hawks, and presumably mammals such as mink, otters, and raccoons (Dodd 2013). The juveniles and adults appear to minimize risk primarily via camouflage and remaining immobile, then leaping into the water if approached. A startle call is typically emitted when leaping that helps to disorient predators. Individuals may either crouch when approached by a snake -- which reduces their detectability -- or inflate their bodies to look more intimidating (Dodd 2013, Heinen and Hammond 1997, Marchisin and Anderson 1978).
See also Habitat Account for General Waters and Shorelines
Life History and Autecology
Breeding and Courtship: The adults breed during the warmer months of the year, but patterns are geographically variable. Dodd (2013) and Meshaka et al. (2009a) have comprehensive summaries of geographic patterns throughout the range. The calling and breeding season tends to be more prolonged in southern and coastal populations relative to those in northern or mountainous areas (e.g., Berven et al. 1979). The males begin calling with the arrival of warm weather in late winter, spring or early summer and may continue through August or September depending on the location and site conditions. Actual breeding and egg laying tends to begins a few weeks after the males begin calling and is more concentrated during the first few months of the breeding season.

Breeding in Florida occurs from March-September (Krysko et al. 2019), and in Georgia from March-August (Jensen et al. 2008). Populations in southern Louisiana breed from March-September, with a peak in April and May (Meshaka et al. 2009b). Populations farther north may not begin breeding until May or June. Some examples of reported breeding seasons include May and June in Indiana, May to July in Maryland, Ohio, and New Hampshire, and May to August in Michigan, Minnesota, New York, Ontario, and New Brunswick (Dodd 2013). Populations in Virginia breed from May to September, and Gaul and Mitchell (2007) reported breeding in coastal North Carolina from April to August. Beane et al. (2013) reported breeding for the Carolinas and Virginia from March-September, with a peak from April to June.

The adults sometimes migrate from surrounding habitats to the breeding habitats. Martof (1953b), for example, found that the adults in Michigan moved from nearby streams to ponds and lakes to breed. The males are territorial and defend small areas that are several meters in diameter during the breeding season. They typically issue their advertisement call either from shoreline vegetation, or while resting on vegetation mats or floating in the water (Martof 1953b, Wells 1977b). Intruders are aggressively challenged by using splashing displays and vocalizations (Wells 1978), and the encounters may escalate into attacks and wrestling bouts (Brode 1959, Dodd 2013, Schroeder 1968, Wells 1978). Submissive satellite males are sometimes present in territories. The dominant males often move to new locations in ponds or other breeding sites seasonally, which allows the satellite males to assume the abandoned territory (Wells 1977b). Martof (1953b) found that the males tend to maintain spatial relationship to one another, but these tend get disrupted due to shifting territories.

Wells (1977b) conducted detailed studies at a breeding site in New York and found that the larger males won most territorial disputes. They claimed high quality territories that had extensive amounts of vegetation in shallow water that were ideal oviposition sites. Females selected males as mates based on the quality of their territories and male size, and males with high-quality territories mated more often than those with lower-quality territories. Martof (1953b) noted that females that were immigrating from nearby streams spent only a short amount of time at the breeding sites compared to the males, and perhaps as little as a week or two. They moved back to nearby streams after breeding.

Gravid females that are ready to breed may swim in a submissive posture and visit several territories before selecting a mate. The cues by which females choose males are poorly understood, but may include both voice characteristics and physical features such as throat coloration and the size of the tympanum (Dodd 2013). A female that approaches a male will be quickly amplexed halfway between the axillary and pectoral region. The male hangs on with his enlarged thumbs and powerful forearms, and the eggs are laid in the male’s territory.

When laying eggs the female positions her cloaca just above the water surface. The male then adjust his position to bring his cloaca next to that of the female. A female will typically lay 30-50 eggs at a time. As they emerge the male uses his rear feet to direct the eggs to his cloaca where they are fertilized. The male then uses his toes and feet to push the eggs away from the amplexed pair (Dodd 2013). This process is repeated over a 10–25 minute time span until the entire clutch is laid and the pair departs. Females normally oviposit one clutch per season, but double-clutching has been observed (Martof 1956b, Wells 1976).
Reproductive Mode: When mating, each female produces a single, irregular, surface film (monolayer) of eggs that are deposited in shallow, vegetated areas of the breeding site. The surface film is typically < 30 cm in diameter and may either float freely or be anchored to vegetation (Dodd 2013). It is similar to that of the American Bullfrog, but smaller and contains far fewer eggs. The freshly laid eggs are black above and whitish below, and each is surrounded by two jelly envelopes that may fuse with those of neighboring eggs with time. The eggs vary from 1.2–2.1 mm in diameter, and the outer capsule is around 5–6 mm in diameter (Dodd 2013, Meshaka and Hughes 2014). Hatching normally occurs in three to five days.

Clutch sizes appear to vary geographically and with the size of females in local populations. Meshaka and Hughes (2014) reported a mean clutch size of 4,631 eggs (range = 2,334-6,467) in 11 dissected females from Pennsylvania, and Meshaka et al. (2009a) found 2,550 eggs (range = 1,600-4,200) for nine specimens from northern Louisiana. Clutch size was positively correlated with female length in both studies. The clutch sizes of 10 individuals from Nova Scotia ranged from 1,401 to 5,289 (Gilhen 1984). Other reports of clutch sizes based on very small sample sizes range from 1,451-5,730 eggs (Dodd 2013).
Aquatic Life History: The hatchlings and small larvae disperse from the ovipositing sites and do not form schools or other active social groups. They are active swimmers and feeders relative to most other ranids and often concentrate in warm, vegetated shallows during the warmer periods of each day to feed. Warkentin (1992) observed that the older tadpoles often moved from shallow water to deeper water at night, perhaps as a way to avoid shoreline predators.

The larvae are suspension feeders that feed on diatoms, filamentous algae, blue-green algae, organic debris and macrophyte plant material, as well as the associated organisms such as protozoans and crustaceans. They undoubtedly take small aquatic insects as they graze or feed in muddy sediments on pond bottoms. The overwintering larvae will also feed on the eggs of the Wood Frog. Petranka and Kennedy (1999) found that overwintering Green Frog tadpoles complete consumed all Wood Frog egg masses in two ponds in western North Carolina.

Growth rates and length of the larval period can vary markedly among populations depending on the latitude, elevation, site conditions, and other factors such as the seasonal time of hatching. Individuals in southern populations or those from early seasonal breeding bouts may complete the larval period before the end of the growing season, but those from northern latitudes or high elevation sites may overwinter for one or two years before transforming during the summer months (Babbitt 1937, Bleakney 1952, Dodd 2013, Martof 1956a, Richmond 1964, Berven et al. 1979). Pond drying may also trigger early metamorphosis (Dodd 2004, Martof 1956a). Berven et al. (1979) compared populations from a wide range of elevations in Virginia and found that variation in growth rates and developmental rates is both genetically based and subject to environmental modification. Larvae at relatively high elevation sites were larger at any developmental stage and were larger at metamorphosis.

Size at metamorphosis is highly variable depending on site conditions and the seasonal time of hatching. Dodd (2013) summarized some of the reported values that range from 19-47 mm SUL. Metamorphs are occasionally found at even smaller sizes, particularly when pond drying triggers early metamorphosis. Some examples of size variation include 28–36 mm SUL (mean = 33 mm) in Michigan (Martof 1956a), 28–39 mm (mean = 33 mm) in Nova Scotia (Gilhen 1984), 19–32 mm (mean = 27 mm) in northern Louisiana (Meshaka et al. 2009a), and 20–47 mm SUL (mean = 28 mm) in southern Louisiana (Meshaka et al. 2009b). Larval survivorship is poorly documented, but appears to be high for this species relative to other anurans. Shirose and Brooks (1997) estimated larval survivorship to be from 32.5-85% over a 6-year period in Ontario.
Terrestrial Life History: The tadpoles metamorphose during the warmer months of the year (April-October), and the young metamorphs spend only a short time at the breeding sites before dispersing into the surrounding country. Some may remain in the vicinity of a breeding site, but the great majority move away (Dodd 2013). Almost all of several hundred frogs that were tracked by Schroeder (1976) in Virginia left the breeding pond within a month. They moved away in all directions and used suitable habitat such as moist stream drainages and other wetlands along the way. Most frogs were recaptured from 183–448 m away, but eight were recaptured in an adjoining bog 2.5 km away, and two others 4.8 km away. Metamorphs at Martof’s (1953a) Michigan site only stayed for a week or so after transforming before moving to nearby streams.

Searcy et al. (2018) constructed a set of experimental ponds in Ontario that were different distances from two breeding sites (ca. 100-300 m) and found that the distance that the young moved away from the ponds was positively correlated with body size. Individuals at this site left the breeding ponds within a month after metamorphosing. Others have also reported the ability of the young to disperse relatively long distance that vary from 457-600 m (Dodd 2013, Martof 1953a, Oldham 1967). Many dispersing juveniles appear to finally settle in or near wetlands such as seasonal pools or woodland streams or creeks. In Louisiana they appear to forage in the surrounding woodlands (Liner (1954). Lamoureux and Madison (1999) and Lamoureux et al. (2002) tracked adults in New York and found that they left breeding sites to overwinter in nearby streams that did not freeze solid during the winter. They often made several foraging forays on land before moving to their final overwintering sites.

The young juveniles grow rapidly, particularly during the warmest summer months (Ryan 1953). In Michigan, Martof (1956) found that individuals grew an average of 34 mm and 18 mm during their first and second year after metamorphosing, then at very slow rates thereafter. Ryan (1953) found that marked juveniles in New York that were 31 mm or larger often reached 50-58 mm SUL before hibernation. They resumed growth the following May and became sexually mature soon thereafter, approximately one year after metamorphosing.

Studies elsewhere indicate that juveniles generally become sexually mature after a year. Males may breed after one year in northern populations, but females may require an additional year since the developing eggs require a full year to mature (Dodd 2013, Martof 1956, Wells 1977b). In their studies in Louisiana, Meshaka et al. (2009a, 2009b) found that L. clamitans reaches sexual maturity at a relative small size (e.g., 40 mm for males and 43 mm for females in southern Louisiana) and can become sexually mature in as little as 3-4 months after metamorphosing. Both sexes breed within a year or so after metamorphosing. Similar patterns were observed in specimens from northern Florida (Meshaka and Marshall 2012).

The juveniles and adults appear to be opportunistic, generalist, gape-limited predators that will attempt to eat any palatable prey that they encounter. They feed on both aquatic and terrestrial prey, with terrestrial prey being more important in the diet. Dodd (2013) provides a summary of the numerous dietary studies that have been done on this species. A diverse taxonomic array on insects form the bulk of the diet. Prey that are commonly taken include beetles, crane flies, katydids, grasshoppers, dragonflies and dragonfly nymphs, wasps, and ants. Other invertebrates such as amphipods, crayfishes, millipedes, spiders, and snails are also consumed. Larger individuals will eat their own tadpoles and occasionally cannibalize small juveniles. They also occasionally take other vertebrate prey, including small frogs such as the Southern Cricket Frog (Acris gryllus) and Spring Peeper (Pseudacris crucifer). There are three records of a Green Frog eating a hatchling Spotted Turtle (Clemmys guttata), a Meadow Jumping Mouse (Zapus hudsonius), and a Big Brown Bat (Eptesicus fuscus).
General Ecology
Population Ecology: Green Frog populations are probably not organized as metapopulations either locally or regionally given their ability to disperse long distances and their lack of strong habitat fidelity (Smith and Green 2005). Populations that use local breeding sites appear to be highly interconnected in many regions, and local ponds in some areas have turnover rates with recolonization occurring rapidly (Hecnar and M’Closkey 1996a, Skelly et al. 2003).
Community Ecology: Lithobates clamitans often shares breeding ponds with one or more large ranids and may interact via both competition and predation. The larvae often coexist with L. americanus, and studies by Werner (1994) suggests that bullfrog tadpoles may be superior competitors overall.

The adults often share areas around ponds and lakes with other ranids and may potentially compete for calling sites. Male L. clamitans, for example, can displace male L. virgatipes from calling sites (Given 1990). The adults may share sites with other ranids such as the Northern Leopard Frog, American Bullfrog, and Mink Frog and possibly compete for food due to their generalist diets (Stewart and Sandison 1972). These species often spatially segregate to varying degrees where they share habitats, which may lessen competitive interactions (McAlpine and Dilworth 1989). The American Bullfrog often preys on smaller ranids, including the Green Frog. Hecnar and M’Closkey (1997a) found that Green Frog numbers increased markedly at a site in Ontario after bullfrogs went extinct, presumably due to release from predation and/or competition.
Adverse Environmental Impacts
Status in North Carolina
NHP State Rank: S5
Global Rank: G5
Status Comments: This is one of the most common frogs in North Carolina and it tolerates habitat disturbance and alteration better than most species. Populations often occur on timbered landscapes so long as breeding sites are present, and they readily use farm ponds and lakes as breeding sites which helps compensate for the loss of natural habitats. Populations in North Carolina appear to be stable and show no evidence of significant declines.

Photo Gallery for Lithobates clamitans - Green Frog

10 photos are shown.

Recorded by: Jim Petranka
Madison Co.
Comment: A mature tadpole; the upper and lower tail fin of this species has similar amounts of blotching that resembles the blotching on the body.
Recorded by: Steve Hall and Bo Sullivan
Richmond Co.
Recorded by: Steve Hall and Bo Sullivan
Scotland Co.
Recorded by: L. Osteen
Orange Co.
Recorded by: tom ward
Buncombe Co.
Recorded by: tom ward
Buncombe Co.
Recorded by: tom ward
Buncombe Co.
Recorded by: K. Long
Rockingham Co.
Recorded by: Steve Hall
Durham Co.
Recorded by: Steve Hall
Orange Co.