The Bullfrog

Lithobates catesbeianus (Shaw, 1802)

Introduced

 

Adults reach at least 200 mm in body length (usually 90-150 mm), and can reach weights of 800 g.  The mean snout to vent length for males is 152 mm (range 111-178) and for females the mean SUL is 162 mm (range 120-183).  It can be recognized by the absence of dorsolateral folds, and an exceptionally large tympanum (larger than the eye in males). The tips of the digits are blunt, webbing between toes is well developed. The dorsal skin is rough with scattered minute tubercles. There is a prominent supratympanic fold. The males have pigmented nuptial pads. The vocal sacs open on each side of the mouth the rictus.  Coloration varies widely depending on the locality and the temperature of the frog, brown (when cold) to shades of green (when warm) (Conant and Collins 1975). The dorsum is green, and it may have a netlike pattern of gray or brown. The venter is off white and sometimes mottled with gray or yellow. Dorsal coloration can vary from brown (when cold) to shades of green (when warm); some may have spots or blotches of a darker color on the dorsum.  Male adult bullfrogs have an exceptionally large tympanum.  The tympanum is slightly larger than the eye in females.  In Arizona, it is most easily confused with the Tarahumara Frog, which has a tympanum smaller than the eye.

 

Larvae. The following description is from Storer (1925). Total length 113 to 135 mm head-and-body 41-50 mm; greatest width of body 22-30.3 mm ; internarial width 5.3-6 mm ; interorbital width 8-12.5 mm; spiracle sinistral, aperture directed backward and slightly upward, center of aperture about midway of body length; greatest height over tail fins 25 mm ; height of muscular part of tail 12-17 mm ; greatest width across mouth region 7.8 mm. Coloration in life: Upper surface of body greenish olive to dark olive, with many minute specks of black; sides of tail with spots (up to 2 mm. in diameter) of yellow; under surface of body yellow to white, sometimes mottled with dusky, never iridescent; iris golden yellow. Labial teeth in 3/3(or 2/3) rows, above mouth, uppermost complete, next divided, on either side of mouth, third row if present including only a few teeth on either side; fourth row, below month, divided in midline, fifth complete across entire mouth region, sixth undivided but shorter than fifth; papillae at sides of mouth area in three rows, extending in part along lower border; one row of papillae bordering mouth region midventral.

 

Distribution and Habitat. The natural distribution of the American Bullfrog is eastern North America, ranging from Nova Scotia southward to central Florida and westward to eastern Wyoming, Colorado and New Mexico. It occurs throughout most of Texas and into northwestern Mexico. Thus, it was found just east of the Rocky Mountains.  However, it has been widely introduced into western North America and many other countries, including those in Europe, Asia and South America (Lever 2003). Adult American Bullfrogs prefer warm, stagnant, or slow-moving water with dense vegetation along the shoreline.  They can inhabit ponds and streams with predatory fishes. The American Bullfrogis mostly aquatic and can be found at the edges of lakes, marshes, or cypress bays (Conant and Collins 1975). Yet, they disperse over long distances – in one case 6.8 miles across open grassland (Kahrs 2006). The American Bullfrog was introduced to Arizona as a game species at an unknown date, presumably in the late 1800’s. The species now occupies almost all substantial permanent body of water in Arizona and can reach exceptional population densities. At some locations bullfrog removal has been very successful but is probably a temporary measure until they re-colonize the removal zones.

 

Some native Bullfrog populations may be declining, due to habitat loss, habitat degradation, water pollution, and pesticide contamination (Baker 1942; Walker 1963; Bury and Whelan 1984; Harding 1997; Mossman et al. 1998). Wetland drainage, shoreline development, and damage to wetlands from housing developments and recreation have decreased bullfrog habitat quality and availability in many areas (Bury and Whelan 1984; Hunter et al. 1992; Casper 1998). At the same time historically used sites are degraded and eliminated, the creation of new farm and golf course ponds, roadside ditches, suburban retention and detention ponds have increased habitat. At these locations high densities may be produced and allow the species to expand its range.

 

Bullfrogs prefer warmer, lentic habitats (George 1940) with heavily vegetated shoals, sluggish backwaters and oxbows, farm ponds, reservoirs, marshes, and still waters with dead woody debris and dense and often emergent vegetation (Storer 1922, Bury and Whelan, 1984). Shorelines of lakes and streams also support adult individuals. Skelly et al. (1999) found bullfrogs restricted to open canopy ponds with permanent water. Bullfrogs prefer relatively high body temperatures (26–33⁰C; Lillywhite 1970) and thermoregulate, basking and altering their posturing to control body temperatures (Lillywhite 1970, 1971, 1974b). Adults remain inactive until water temperatures approach 15⁰C (Harding 1997). Unlike many other frogs, bullfrogs can coexist with predatory fishes, this is like due to their skin sections that make them distasteful or toxic to fishes (Hecnar 1997).

 

Diet. Bullfrog feeding behavior has been described using the adjectives voracious, opportunistic, and cannibalistic. They are usually ambush predators and will attack any live animal smaller than themselves (Bury and Whelan 1984). Bullfrogs may locate and eat smaller frogs by following the sounds of chorusing frogs (Green and Pauley 1987; Harding 1997) or distress calls from other frogs (Collins and Collins 1991). Bullfrogs will also eat carrion (Guarisco 1985). Frost (1935) found that small bullfrogs eat mostly insects, while larger individuals eat frogs, crayfish, and mice. In Arizona, most stomach contents of adults consisted of invertebrates such as snails and insects (Schwalbe and Rosen 1988). McCoy (1967) examined 52 stomachs of frogs at a farm pond in Oklahoma, of these 45 contained invertebrates of just two species: a water beetle and a crayfish. Thirty-three of the 45 frogs (73%) had eaten beetles, which contributed 48% of the total volume of food in these stomachs. One-third of the 45 frogs contained remains of crayfish, which constituted 52% of the total invertebrate food volume. Other invertebrate animals were absent. The other four frog stomachs contained vertebrate prey. One frog had eaten a 350 mm water snake (Nerodia rhombifer) and a second frogs contained a box turtle (Terrapene carolina). Two stomachs contained anuran amphibians a Woodhouse’s Toad and the second contained remains of three adult Gray Treefrogs.

 

Rosen and Schwalbe (1995) found bullfrogs preyed on garter snakes, including the threatened Mexican Garter Snakes, numerous frogs. Additionally, these frogs ate many other vertebrates and many invertebrates. At the time of writing they did not know of any examples of overlap between populations of the native leopard frogs (L. chiricahuensis and L. yavapaiensis) and bullfrogs in southern Arizona. Leopard frogs were abundant at both San Bernardino National Wildlife Refuge and Buenos Aires National Wildlife Refuge before bullfrog proliferation, and in 1981, bullfrogs and leopard frogs were both still widespread at Buenos Aires National Wildlife Refuge. Leopard frogs apparently were extirpated from the San Bernardino National Wildlife Refuge study area by 1989. In 1993-94 relict populations of Chiricahua Leopard Frogs (2-20 adults each) were found five to eleven miles east of San Bernardino National Wildlife Refuge. These populations were in areas not occupied by bullfrogs in areas that may have been too dry for non-native predators. The semi-aquatic Checkered Garter Snakes (Thamnophis marcianus) coexist in abundance with Bullfrogs while the highly aquatic Mexican Garter Snake, had small, declining populations where it overlapped with bullfrogs. The aquatic habits of the Bullfrog allowed it to decimate the Mexican Garter Snake while the more terrestrial Checkered Garter Snake could survive. Neonate Mexican Garter Snakes do not survive where they occur with bullfrogs. Larger garter snakes can co-exist with the Bullfrog once the young snakes outgrow vulnerability to Bullfrog predation they survive.

 

Reproduction. Sexual maturity varies with latitude and altitude. It takes 1–2 years for males and 2–3 years for females.  They may live up to 10 years in the wild; captives have been reported to live for 16 years (Oliver 1955a, Goin and Goin 1962, Schroeder and Baskett 1968).  Eggs are laid in thin rafts on the water surface.  Females may produce clutches of 48,000 eggs or more that hatch in 3–5 days.  Tadpoles favor warm-water environments and development time is temperature-dependent.  Development can be as short as a few months in the south to three years at higher latitudes and elevations.

 

Breeding occurs in late spring and early summer at southern latitudes later in the summer at northern latitudes (Bury and Whelan 1984). Bishop et al. (1997) reports 17 May to 8 June as peak calling dates in southern Ontario. In Québec, breeding occurs from late May to mid-July (Bruneau and Magnin 1980). In the Great Lakes in June–July (Pentecost and Vogt 1976, Harding 1997). In Michigan (and probably elsewhere, especially in southern latitudes), double clutching occurs, with some females active early and then three weeks later (Emlen 1977). In Kansas, calling begins when air temperatures are above 21⁰C (Fitch 1956c). This is also true for other locations and the time of reproduction may change from year to year depending on the temperature, a cold or warm spring-summer may postpone or advance breeding dates (personal observation JCM). In Texas, bullfrogs breed from March–October (Blair 1961a). Older females may produce two clutches/year, with second clutches containing substantially smaller eggs (Howard 1978b).

 

Reproduction occurs in vegetation choked shallows water of permanent bodies of water (Pope 1964a). Male bullfrogs aggressively defend oviposition sites (Ryan 1980) by pushing, shoving, and biting competitors.

 

Females select a mate by entering his territory (Ryan 1980) and female mate choice becomes more discriminating with age, with older females consistently selecting the oldest, largest males as mates (Howard 1978a). Older females sometimes vocalize within male choruses, which may elicit increased male–male competition and assist females in selecting high-quality males (Judge et al., 2000).

 

Embryo mortality depends on female choice of the oviposition site, the best of which are presumably controlled by the largest males and where water temperatures do not exceed 32⁰C. Eggs developing in water at temperatures of more than 32⁰C show developmental abnormalities (Howard 1978b). Howard (1979) estimated male reproductive success in a Michigan population. The most successful males fertilized three females, the average was 0.71 matings/male. Males defending territories in the middle of ponds are susceptible to predation by the Snapping Turtle (Emlen 1976; Howard 1978a).

 

Eggs are laid in thin rafts on the water surface, a clutch covering up to a square meter, and hatching in three to five days (Bury and Whelan 1984), producing up to 20,000 eggs/clutch (Schwalbe and Rosen 1999).

 

Howard (1978b) found the number of zygotes sired by successfully males ranged from 4,928–59,035 (mean 11,149), with the number of hatchlings ranging from 299–29,377 (mean 5,582). Females may lose up to 27% of their body mass during oviposition (Judge et al. 2000).

 

Tadpoles prefer relatively warm water, 24–30˚C (Brattstrom 1962b) and have a preferred body temperature ranging from 15–26.7˚C, which changes with acclimation, developmental stage, and season (Ultsch et al. 1999). Oxygen consumption increases about exponentially as a function of temperature (Burggren et al. 1983; Feder 1985). The time to metamorphosis is temperature dependent and varies from a few months at southern latitudes to three years in on the northern edge of the range (Collins 1979; Bury and Whelan 1984).

 

Larvae feed mostly on algae, aquatic plant material, and some invertebrates (Treanor and Nichola 1972; Bury and Whelan 1984). Growth rates and digestive abilities of tadpoles fed differing algal diets was studied by Pryor (2003). Coprophagy was demonstrated by Steinwascher (1978a), who suggested that this increased the amount of time food was resident in the digestive tract and may also allow some microbial digestion.

 

Tadpoles are solitary (Punzo 1992b) and occur in deeper water until just prior to metamorphosis, at which time they spend more time near the surface (Goodyear and Altig 1971; Smith 1999). Tadpoles are bimodal breathers, drawing oxygen from the water and from the air (Crowder et al. 1998; Ultsch et al. 1999). Their distribution within ponds is likely determined by temperature, and oxygen concentrations, and predator avoidance. The larvae avoid predators by seeking cover (Pearl et al. 2003).

 

Metamorphosis is asynchronous. Newly metamorphosed individuals generally frequent shorelines where vegetative cover is thick enough to afford protection from predators (Casper and Hendricks 2005, personal observation JCM).

 

Population Sizes. The high population densities are supported by the ability of adult frogs to sustain themselves on cannibalized juvenile frogs (Jennings and Hayes 1994). Thus, transforming larvae can sustain a dense adult Bullfrog population when other prey has been depleted. This may increase the probability that native species may be extirpated by bullfrog predation.  Most fish find Bullfrog tadpoles distasteful (Kruse and Francis 1977, Smith et al. 1999), and the larger aquatic predators such as snakes and wading birds that feed on Bullfrogs in their native range are absent or present in low numbers in the arid Southwest (Rosen and Schwalbe 1995).

 

The home range size is plastic and dependent on the population size. Currie and Bellis (1969) found a mean activity radius of 2.6 meters in an Ontario pond and is reduced at higher densities and increases with individual body size, males have a larger home range than females. Raney (1940) found no evidence of site fidelity in a New York population, and frogs dispersed up to 3.2 km from the home pond and dispersal distances of 7–8 km likely (Schwalbe and Rosen 1999). Maximum movement distances of up to 1,600 m (with a mean of 402 m), as well as homing movements were reported by Ingram and Raney (1943). McAtee (1921) noted a bullfrog that returned to its lakeshore territory within two days after being moved about 0.4 km away. In Arizona, Schwalbe and Rosen (1999) reported an adult bullfrog every 1.8 m along some shorelines. In Kentucky, Cecil and Just (1979) calculated densities of 0.9–13.2 tadpoles/m2 of pond, biomass of tadpoles as 11–103 g/m2 of pond, and survival rates of 11.8 to 17.6%. Treanor (1975) reports densities in California canals from 6.6–119 frogs/km, with average numbers reduced by 20–25% in areas where harvest occurred. Bullfrog density in an Ontario pond was 0.9 and 1.3/m2 (Currie and Bellis 1969) and 8.8–45.8 frogs/hectare in a 7-hectare Illinois lake (Durham and Bennett 1963).

 

Bullfrogs are highly territorial and polygynous. The largest males control the highest quality oviposition sites preferred by females (Howard 1978a, b). Males defend circular territories of two to five meters in diameter (Harding 1997). Bullfrogs recognize the calls of neighbors and respond more vigorously to the calls of strangers (Davis 1987). Most territorial disputes that involve physical encounters are won by largest males by engaging in wrestling, shoving, and pouncing (Blair 1963, Durham and Bennet 1963). Younger males unable to hold territories employ male parasitism (by intercepting females attracted to large males) and opportunism (temporarily holding a vacated territory until threatened by a larger male (Howard 1978a). Mauger (1988) observed medium-sized males engage in opportunism, where they vocalized from the fringes of a large male territory until forced to flee or they inhabited recently vacated territories. The smallest males practiced parasitism, maintaining a low posture without vocalizing to intercept females in route to a large-male territory. Small males near a larger male’s territory invade the territory when the large male was in amplexus, without challenging or displacing the resident male.

 

Bullfrogs are an important food source for other wildlife and humans, and the species is considered a game species and hunted for their legs. Cecil and Just (1979) suggested predation was the major factor controlling bullfrog population size in permanent ponds. Bullfrog tadpoles are relatively immune to fish predation because of unpalatability (Walters 1975, Kruse and Francis 1977, Nelson, 1980, Kats et al. 1988, Werner and McPeek 1994) and are one of only a few species likely to persist after fish invasion (Seale, 1980). Bullfrog eggs are preyed upon by leaches, fish, and salamanders. Tadpoles and juveniles are eaten by insects, fish, salamanders, frogs, turtles, alligators, snakes, birds, and mammals. Adult frogs are preyed upon by alligators, snakes, birds and mammals (Casper and Hendricks 2005).

 

Artificial stagnant water habitats are created by dams and tanks that favored Bullfrogs, invasive crayfish, and predatory fish.  Bullfrogs are known to have contributed to declines and extirpations of native leopard frogs throughout its range in the western United States due to predation and competition (Hayes and Jennings 1986; Hammerson 1999; Lannoo 1996; McAllister et al. 1999; Rorabaugh 2005).

 

Defense. Eggs and larvae avoid predation by fish and some salamanders by being unpalatable or larvae being less active activity (Woodward, 1983). Hobson et al. (1967) reported that adults have a state of rest characterized by alertness without a loss of reactivity, aiding in predator avoidance. Upon disturbance, adults retreat to deeper water with a series of long leaps with a great deal of splashing (Smith, 1961). Bullfrogs usually squawk when fleeing, this often triggers a many other bullfrogs jumping from a shoreline into deeper waters (Schwalbe and Rosen, 1999). Bullfrogs may emit a piercing scream when seized, which may startle a predator enough to allow escape (Harding, 1997).

 

Partial resistance to the venoms of cottonmouths (Agkistrodon piscivorus) and copperheads Agkistrodon contortrix) has also been reported (Heatwole et al., 1999). Bullfrog tadpoles avoid novel predators by seeking shelter but ignore predators from their native range (Pearl et al. 2003).

 

Diseases. Bullfrogs are known reservoirs for many microorganisms, only a few of these pathogens appear to be of major importance in nature (Desser 1992; Faeh et al. 1998). An intraerythrocytic virus outbreak was reported in Canada (Crawshaw,1997). Saucedo et al. (2019) describe an outbreak of ranavirus in a captive American Bullfrog in Sinaloa, Mexico. The farm experienced high mortality in an undetermined number of juveniles and sub-adult bullfrogs. Affected animals displayed clinical signs and gross lesions such as lethargy, edema, skin ulcers, and hemorrhages consistent with ranavirus infection. They suggest the probability of the virus becoming endemic within native amphibian populations was high because of the high number native amphibian species present in the area of the frog farm that reproduce in the water. Ranaviruses are considered the second deadliest pathogens for amphibian populations throughout the world and they are emerging. They can infect fishes, amphibians, squamates, and chelonians. (Brunner et al. 2015; Saucedo et al. 2019).

 

Fungi are reported as well (Goodchild 1953; Hill and Parnell 1996), including a recent outbreak of chytrid fungus (Bd) in Arizona (Sredl et al. 2000), which has been implicated as the cause of amphibian declines in Australia and Panama (Berger et al. 1998). Lefcort and Eiger (1993) showed how infections in tadpoles (i.e., fever, malaise) can lead to increased predation. In Kentucky, Cecil and Just (1979) considered microbial infections as a secondary factor controlling bullfrog population size in permanent ponds. Disease can be an important factor in tadpole survival (Gibbs et al. 1971).

 

Parasites. Bullfrogs host many parasites including helminths (Bursey and DeWolf, 1998), platyhelminths such as trematodes (Crawshaw 1997) and nematodes (Modzelewski and Culley 1974), protozoans and mesozoans (Smith et al., 1996, 2000), and leeches (Siddall and Desser 1992). Trematode metacercariae have been implicated in bullfrog limb abnormalities (Christiansen and Feltman 2000).

 

Taxonomy. Shaw (1802:106) described Rana catesbeiana based on an illustration (Shaw 1802:106, pl. 33) not known to exist. The type locality was given as “many parts of North America . . . Carolina . . .  Virginia” and restricted to South Carolina by Kellogg, 1932: 197: restricted to Charleston, Charleston County, South Carolina by Smith and Taylor (1950:360). Restricted to the vicinity of Charleston, South Carolina by Schmidt (1953:79). The restrictions are invalid for reason of not being based on disclosed evidence following Fouquette and Dubois (2014:407). Daudin (1802:58) described  Rana pipiens based upon syntypes: the frog figured on pl. 18 of the original publication and five individuals noted in the original publication as being in the MNHNP with the type locality given as l’Amerique Septentrionale, et sur-tout dans la Caroline . . . la Virginie. Primary homonym of Rana pipiens Schreber, with which Daudin was aware. Cuvier (1816,2:39) proposed the replacement name Rana taurina for Rana pipiens Daudin, 1802. Merrem (1820:175) described Rana mugiens based several different species including in part on Catesby’s 1754, pl 72, that may be Rana grylio (following Frost), Rana catesbeiana of Shaw, 1802, Rana pipiens of Latreille, and others. Harlan (1826:59) described Rana scapularis without designating any type material but gave the type locality as Pennsylvania, that was restricted to “vicinity of Philadelphia”, USA, by Schmidt (1953:79). LeConte (1855:425) described Rana conspersa  based on syntypes including ANSP 2918, according to Malnate, 1971:350 with the type locality Type locality of Pennsylvania which was later restricted to “Riceborough, Liberty County, Georgia” by Schmidt 1953:79. Cope (1889:424) used the combination with the spelling Rana catesbyana. Werner (1909:86) used the combination and spelling Rana catesbyana. Boulenger (1920:10) used the combination Rana (Rana) catesbeiana. Hsü (193:19) described Rana nantaiwuensis based on a holotype presumed lost in World War II from the type locality: “Nantaiwu, Amoy [= Xiamen Shi, Fujian Province], China. Considered a junior synonym or incertae sedis within Hoplobatrachus by Dubois, 1987 1986:60 without discussion. Placed in the synonymy of Rana catesbeiana by Zhao and Adler (1993: 140). Previously considered a synonym of Rana spinosa by Liu and Hu (1961:156). Angel (1947:253) used the combination Rana mugicus which appears to be an incorrect subsequent spelling of Rana mugiens but based on specimens of Lithobates pipiens; see discussion by Smith (1948:517–518). Dubois (1987:41) used Rana (Rana) catesbeiana —by implication. Dubois (1992 61:331) used the combination Rana (Aquarana) catesbeiana. Hillis and Wilcox (2005:305) used the combination Rana (Novirana, Aquarana) catesbeiana. Invalid name formulation under the International Code of Zoological Nomenclature (1999) as discussed by Dubois (2007:395). Frost et al. (2007:369) used Lithobates catesbeianus by implication. Dubois (2006:829) Lithobates (Aquarana) catesbeianus. Hillis (2007: 335–336) used the combination Rana (Aquarana) catesbeiana by implication. Fouquette and Dubois, 2014:407 used the combination Rana (Lithobates) catesbeiana.

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