Amphibians of North Carolina
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NC Records

Lithobates heckscheri - River Frog

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Taxonomy
Class: Amphibia Order: Anura Family: Ranidae Synonym: Rana heckscheri
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.
Species Comments: Despite its large size, the River Frog is one of the least studied species of ranid frogs in eastern North America.
Identification
Description: The adults are large frogs that vary from 8.3–15.5 cm SUL, although most rarely exceed 12 cm SUL. The dorsal ground color varies from light to dark brown or grayish olive and is overlain with varying levels of black mottling or blotching. A series of narrow dark bars is present on the upper sides of the legs, and the dorsal surface of the skin has numerous raised tubercles. A series of white spots is often present on the lower jaw that contrast with a dark ground color. These are sometimes present on the upper jaw, but are usually less conspicuous.

The chest, belly, and undersides of the legs are patterned with contrasting white and dark gray to blackish markings. These form a reticulate pattern on the back legs and a more mottled pattern with dark gray or blackish pigmentation predominating elsewhere. The darker coloration is often more prominent on the throat and chest region, and a pale crescent-shaped line may girdle the groin area (Beane et al. 2010, Dodd 2013, Jensen et al. 2008). This species lacks a dorsolateral fold that extends down the back, but does have a supra-tympanic fold that extends from behind the eye and around the upper margin of the tympanum before ending near the insertion of the front limb. The tip of the longest hind toe is free of webbing. The males in local populations average slightly smaller than females and have larger tympanums that are noticeably larger than the eyes.

The young tadpoles are black with gold to white transverse bands on the snout and body. These disappear when the tadpoles are about half grown (Dodd 2013). The older tadpoles vary from dark greenish olive to blackish above and have fine pale greenish-yellow spots or flecks over the dorsum. The lateral line system is conspicuous and shows as a series of light, linear dots. The tail has a prominent black band that extends along the upper tail musculature to about two-thirds of its length. The tail fin is very distinctive in being clear except for a narrow but conspicuous band of black coloration along the entire margin. The tadpoles grow to very large sizes and can reach 160 mm TL. The older tadpoles and young metamorphs are distinctive in having brick-red eyes (Dodd 2013).
Vocalizations: The advertisement call has been described as being a loud, distinctive snore or deep growl that last a second or two. It is often repeated several times in sequence with a brief gap in time between growls. Other vocalizations include a long squeal that may be issued if an individual is grabbed, and a brief startle call that is made before jumping in the water (Wright 1932).
Technical Reference: Dodd (2013)
Online Photos:    Google   iNaturalist
Observation Methods: The adults can be found by searching the margins of wetlands and looking for individuals that are near shorelines. Searching for the conspicuous schooling tadpoles may be the best way to document local populations.
AmphibiaWeb Account
Distribution in North Carolina
Distribution Comments: The River Frog is only found in the southeastern Coastal Plain, with the historic range extending from the Lumber and Cape Fear river systems in the southern Coastal Plain of North Carolina southward through South Carolina, Georgia, northern Florida, and extreme southern Alabama before ending in extreme southern Mississippi. Populations in North Carolina appear to have been extirpated (Beane 1998).
Distribution Reference: Beane (1998), 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 River Frog is a semiaquatic species that is associated with Coastal Plain streams and rivers and standing water habitats. It can be found along the margins of sluggish sections of rivers and streams, but more commonly is found in associated wetlands such as swampy backwaters, fluvial swamps, cut-off overflow pools, oxbow ponds, and large beaver ponds. Individuals also occur around the margins of ponds and lakes, sinkhole ponds, and permanently flooded borrow pits, particularly where riparian hardwood forests or cypress swamps adjoin the sites (Allen 1938, Dodd 2013, Jensen et al. 2008, Mount 1975, Wright 1924). Shaded banks with shrub zones or hardwoods such as cypress or gum are preferred over open, sunny habitats. Populations that were historically present in North Carolina presumably used similar habitats. The few habitats that were reported include several borrow pits, a cypress lake, and areas along the banks of the Lumbar River (Beane 1998).
Biotic Relationships: The defenses of this species against predators are poorly documented. It is uncertain if the larvae have chemical defenses against aquatic predators such as fishes and odonates. Altig and Christensen (1981) fed tadpole skin extracts to a mouse and did not observe any adverse effects. However, many species that coexist with fishes have chemical defenses and more studies are needed.

Altig and Christensen (1981) found that the larvae respond strongly to chemical cues from either humans or a mutilated conspecific larva by swimming wildly and erratically. In the field, larvae responded in a similar way when exposed to human cues (washing hands in water) or cues from a gartersnake (Thamnophis sp.). A group of 200 or so tightly packed tadpoles appeared to explode as they frenetically fled from the source of the chemicals. The response to humans remains a mystery. The large size of the older tadpoles undoubtedly allows them to avoid attacks from small gape-limited predators.

The documented predators of tadpoles and recent metamorphs include the Southern Watersnake (Nerodia fasciata), the Loggerhead Musk Turtle (Sternotherus minor), and the Boat-tailed Grackle (Allen 1938, Wright 1932). Dodd (2013) noted that the adults are easily approached by human observers and may go limp when handled. It is uncertain if they produce toxic or noxious skin secretions.
See also Habitat Account for General Blackwater Shorelines
Life History and Autecology
Breeding and Courtship: Very little is known about the breeding biology. This is mostly a warm-weather species and the advertisement calls have been heard from April to August in Georgia (Jensen et al. 2008, Wright 1932) and elsewhere. The males may call from land along shorelines or from shallow water (Dodd 2013).
Reproductive Mode: Very little information is available on the reproductive biology. Allen (1938) transplanted three amplexed pairs to his private lake and reported that the females laid around 5,000 eggs per mass in vegetated areas near the shore. Wright (1932) reported an estimated clutch of 14,000 eggs from a single female. Hatching at Allen's site occurred in 10–15 days. The egg mass is probably oviposited in a sheet-like surface film as seen in other summer-breeding ranids in eastern North America, but has not been described in detail. The freshly laid eggs that were measured by Wright (1932) were 1.5–2 mm in diameter, but he was not completely certain that these were from L. heckscheri. More detailed descriptions of the eggs and egg masses are needed.
Aquatic Life History: The tadpoles swim about and feed in large schools that may consist of many hundreds or thousands of individuals (e.g., Dodd and Barichivich 2017, Wright 1932). Altig and Christensen (1981) observed schools that moved along a bank and remained near the upper water column while moving. Two schools contained larvae of mixed sizes that differed by 65 mm TL, which indicates that different age cohorts intermingle. Wright (1932) made similar observations of different age classes schooling together. The larvae often can be heard making a smacking sound as dozens of tadpoles simultaneously gulp air at the surface as a school moves through shallow water (Dodd 2013). Allen (1938) noted that schools remained within about a 46 m stretch of lakeshore during the tadpoles' entire development. They often fed in shallow water during the day, then moved to deeper water at night.

The adaptive value of schooling in this species is not fully understood, but it may facilitate the location of rich food patches or perhaps reduce predation risk from fish or other predators. Altig and Christensen (1981) found that chemical cues from food (rabbit chow) that was placed near the nostrils of a tadpole stimulated the individual to begin feeding, and that this in turn triggered nearby tadpoles to do the same. Punzo (1991b, 1992b) showed that tadpoles in small groups learned to avoid mild electric shocks and reached experimental food patches more quickly relative to isolated individuals. Collectively, these studies suggest that social facilitation occurs and that individuals often respond in adaptive ways to other school members.

This species is mostly restricted to permanent habitats, which implies a larval period of around a year or so. The larvae grow rapidly and are probably able to escape many gape-limited predators such as small sunfishes with a few months after hatching. Allen (1938) reported that hatchlings that emerged in late June had grown to 10-13 cm TL by September. Growth was slower during the cooler winter months, then accelerated with the spring warm-up. The larvae reached 15-17 cm TL shortly before metamorphosis began in early April after about a 10-month larval period. Wright (1932) observed transforming tadpoles at a Florida site that were estimated to vary from 80-100 mm TL. At a second site there were transforming tadpoles in June that were mixed with three size classes of young tadpoles from eggs laid earlier in the spring, suggesting a larval period of slightly longer than a year. Young metamorphs that were measured by Wright (1932) were mostly 30-39 mm SUL (maximum = 49 mm SUL).

Limited observations indicate that local populations often have synchronized metamorphosis with large numbers emerging over a short period. Jensen and Wright (2014) observe an estimated 4,000 or so metamorphs emerging from a small area along the margin of a swamp in Georgia on 31 May. This large aggregation was present again on 6 June, but was gone when the site was revisited on 15 June.
Terrestrial Life History: Very little is known about the natural history of the young metamorphs. Jensen and Wright (2014) noted that thousands of metamorphs that were part of a mass emergence and that were present on 6 June were gone when the site was revisited on 15 June. It is uncertain if they dispersed widely into the surrounding area since direct observations were not possible. Allen (1938) found one juvenile 183 m from its natal pond a month after it transformed and noted that none that emerged from a lake shore a few weeks earlier remained there. These observations suggest significant movements away from the breeding sites soon after individuals metamorphose.

Hansen (1957) studied a population of juveniles and adults at a large sinkhole pond in northern Florida over a two-year period. The frogs were not active until seasonal air temperatures reached 18°C. Temperatures around 25°C appeared to be optimal based on counts of frogs. The juveniles and adults normally sat at night on moist substrates such as sphagnum moss while facing the water. Most were found within 0.25 m of the water's edge, but on cool nights some individuals remained in water that was warmer than the surrounding air. The adults appeared to establish home ranges along the shore and moved an average distance of 9-10 meters between captures, with no difference between the sexes. The juveniles moved slightly farther between captures (mean = 11.4 m). On average, the adults used about a 17 m length of shoreline. Hansen (1957) noted that the juveniles grew rapidly, but did not have sufficient data to generate a growth curve or estimate the time required to reach sexual maturity. Wright (1932) observed several individuals on land in vegetation such as pickerel weed beds that were close to water.

The juveniles and adults appear to be generalist, gape-limited predators, but the diet is poorly documented. The known prey include crayfishes, millipedes, centipedes, spiders, beetles, true bugs, tabanid flies, cockroaches, and caddisflies (Dodd 2013, Lamb 1980).
General Ecology
Population Ecology: Very little is known about the sizes of local populations or genetic population structure. Given that this species may breed in rivers and creeks and connected wetlands, it is possible that the species is capable of long-distance dispersal through downstream larval drift.
Community Ecology: The River Frog shares breeding sites with numerous amphibians, including other large ranids, but ecological interactions with other amphibians have not been studied.
Adverse Environmental Impacts
Habitat Loss: Dodd (2013) noted that there is very little information on the status of this species in much of its range and that populations have undoubtedly been lost as habitats were destroyed and river floodplains altered.
Interactions with Humans: Lithobates heckscheri has declined in some areas of the range, but the reasons for declines are poorly documented. The adults will allow human to approach closely and it is possible that many have suffered from humans that harvest these and other large ranids for food.
Status in North Carolina
NHP State Rank: SX
Global Rank: G5
Status in North Carolina: E
Status Comments: The River Frog appears to still be common in Florida and Georgia (Krysko et al., 2019, Jensen et al. 2008), but has declined near the periphery of the range. The last museum-vouchered specimen for Mississippi was collected in 1959 and this species may have been extirpated from the state due to coastal development. Aresco (2004) noted only six breeding localities in Alabama and that there have been no records for 25 years. However, one recent record from Alabama (iNaturalist) appears to be valid. The species was last collected in North Carolina in 1975. Beane (1998) was unable to locate any populations despite an exhaustive search for this species in 1992-1993 with much help from volunteers. Searches since then have failed to find any specimens. This species appears to have been extirpated from the state.