Taxonomic name: Lithobates catesbeianus (Shaw, 1802)
Synonyms: Rana catesbiana Shaw, 1802
Common names: bullfrog (English), grenouille taureau (French), North American bullfrog, Ochsenfrosch (German), rana toro (Spanish), Stierkikker (Dutch)
Organism type: amphibian
The American bullfrog (Lithobates catesbeianus (=Rana catesbeiana)) is native to North America. It has been introduced all over the world to over 40 countries and four continents. Many introductions have been intentional with the purpose of establishing new food sources for human consumption. Other populations have been established from unintentional escapes from bullfrog farms. Consequences of the introduction of non-native amphibians to native herpetofauna can be severe. The American bullfrog has been held responsible for outbreaks of the chytrid fungus found to be responsible for declining amphibian populations in Central America and elsewhere. They are also important predators and competitors of endangered native amphibians and fish. The control of this invasive in Europe partly relies upon increasing awareness, monitoring and education about the dangers of releasing pets into the wild. Strict laws are also in place to prevent further introductions. Eradication is achieved largely by physical means including shooting, spears/gigs, bow and arrow, nets and traps.
L. catesbeianus are a large frog reaching up to 20cm in snout-to-vent length (SVL) and up to 800g in weight. They have a robust body with a wide flat head and smooth skin with no wrinkles, warts or spikes (Flores, 2005). Dorsal colour is pale green to dark olive and can have brown spots. Ventral side is white, grey or yellowish (LeClere, n.d.). As sexual maturity approaches in males the upper abdomen temporarily turns yellowish in colour (Flores, 20005). American bullfrogs have conspicuous tympanic membranes (eardrums). Mature males have tympanums twice the diameter of the eye, while mature females have tympanums about the same diameter as the eye (National Research Council, 1974). Males are also slightly smaller than females and have darkly pigmented thumb pads in contrast to the more delicate streamlined thumb of the female (National Research Council, 1974). Bullfrogs, in contrast to the similar green frog, do not have dorsolateral ridges (National Research Council, 1974; LeClere, n.d.). Tadpoles are greenish yellow with small spots, growing up to 15 cm.
Rana clamitans, Rana ridibunda
lakes, riparian zones, water courses, wetlands
Bullfrogs are described as occupying a wide range of aquatic habitats including lakes, ponds, swamps, bogs and backwaters (Conant, 1975; Stebbins, 1985 in Adams et al., 2003), as well as reservoirs, marshes, brackish ponds (in Hawaii), streams (Santos-Barrera et al. 2009), irrigation ponds and ditches (Govindarajulu, 2004). Adult bullfrogs prefer warmer, lentic habitats such as vegetated shoals, sluggish backwaters and oxbows, farm ponds, reservoirs, marshes, and still waters with dead woody debris and dense and often emergent vegetation (George, 1940; Holbrook, 1842; Bury & Whelan, 1984 in Casper & Hendricks, 2005). In a seasonal marsh in Sonoma County, California, bullfrogs selected spikerush habitat in winter and spring, and aquatic buttercup in summer (Cook & Jennings 2007). Bullfrogs bred in spring and summer in deep areas with dense cover, predominantly smartweed (Cook & Jennings, 2007). Areas having high precipitations during both summer and winter, high maximum temperature, high human pressure, and intermediate minimum annual temperature were those with the highest predicted suitability for bullfrogs (Ficetola et al. 2007b).
Several authors suggest that bullfrogs may have a preference for highly artificial and highly modified habitats, such as millponds, livestock grazing ponds and reservoirs (Wright & Wright, 1949; Bury & Luckenbach, 1976; Jennings, 1988, Zampella & Bunnell, 2000 in Adams et al., 2003; Doubledee et al., 2003; Ficetola et al., 2007b). Hayes and Jennings (1986, in Cook & Jennings 2007) pointed out that human-driven habitat modification, such as changes in hydrology from seasonal to permanent water, removal of emergent vegetative cover, and elevation of water temperatures from increased sunlight all favor the establishment of bullfrogs. Yiming et al. (2005) concluded from their study that the ease with which bullfrogs have invaded islands of the Zhoushan archipelago relative to the mainland has little to do with biotic resistance but results from variation in factors under human control. Habitats that are highly modified by human activity are typically characterised by a decrease or complete lack of habitat complexity (Doubledee et al., 2003). In such environments bullfrogs are expected to have high attack rates (the attack rate is a measure of bullfrog search efficiency, specifically the length of shoreline that is kept clear of prey items by a bullfrog in a give time interval). Bullfrogs are expected to be less efficient at keeping a complex shoreline choked with cat-tails and bulrushes clear of prey items than they would be along a shoreline devoid of such vegetation (Doubledee et al., 2003). In other words human-modified habitats probably enhance habitats for bullfrogs by providing optimum conditions for bullfrogs to find and devour their prey.
Bullfrogs are sometimes found in temporary water bodies hundreds of meters from permanent water (Santos-Barrera et al. 2009). However, they are typically found in permanent ponds, whereas most other amphibians inhabit temporary ponds. Pond hydroperiod is known to regulate some amphibian communities (e.g., Semlitsch et al. 1996, in Boone 2004) and may mediate the impact bullfrogs have on amphibians. Bullfrogs are often excluded from temporary ponds because they have larval periods exceeding one year, although they can reach metamorphosis in a single season (MDB Pers. Obs.; Pechmann et al., 2001 in Boone 2004). Bullfrogs will hibernate at the bottom of water bodies or in secluded places on land (CABI Bioscience, 2005).
Disease Transmission: Chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis, is an emerging disease of amphibians responsible for population declines and even extinctions globally (Hanselmann et al., 2004). Introduced populations of Lithobates catesbeianus can harbour reservoirs of the fungal agent without showing significant clinical disease symptoms themselves (Hanselmann et al., 2004).
Native North American amphibians which may be affected by bullfrogs include: the Pacific chorus frog (Pseudacris regilla); the Northern red-legged frog (Rana aurora); the plains leopard frog (Lithobates blairi); the northern leopard frog (Lithobates pipiens); the lowland leopard frog (Lithobates yavapaiensis); and the entire suite of central Californian amphibians (Kraus, 2009).
Similar declines in native species concurrent with the introduction of bullfrogs have been noted in Europe, in Germany (C.R. Boettger, 1941; Thiesmeier et al., 1994 in Kraus 2009), Florence (Italy) (native Rana; Touratier. 1992b in Kraus 2009) and in the Aquitaine of southwestern France (native fish; Touratier, 1992a in Kraus, 2009).
Threat to Endangered Wildlife: In the USA the bullfrog is known to prey on the following endangered amphibians: Amargosa Toad (Anaxyrus nelsoni) (Jones et al., 2003 in Kraus, 2009); California tiger salamander (Ambystoma californiense); Chiricahua leopard frog (Lithobates chiricahuensis); the California red-legged frog (Rana draytonii); and the Oregon spotted frog (Rana pretiosa)
Ecosystem change: Several field studies portray tadpoles as ‘‘ecosystem engineers’’ that alter the biomass, structure and composition of algal communities (Dickman, 1968; Seale, 1980; Osborne & McLachlan, 1985; Kupferberg, 1997a; Flecker et al., 1999; Peterson & Boulton, 1999 in Pryor, 2003).
Modification of Nutrient Regime: High food intake (Wassersug, 1984 in Pryor, 2003) and high population densities (up to thousands of individuals per m²; Alford, 1986, in Pryor, 2003) suggest that tadpoles have considerable impact on nutrient cycling and primary production in freshwater ecosystems.
Predation: Tadpoles of L. catesbeianus feed upon eggs and larvae of the endangered Razorback Sucker (Xyrauchen texanus) in laboratory conditions (Mueller et al., 2006 in Kraus, 2009), and their densities in artificial habitats can depress fish larvae recruitment (Kraus, 2009).
Competition: Introduced bullfrogs compete with endemic species (Kupferberg, 1997; Kiesecker & Blaustein, 1997 in Hanselmann et al., 2004). Unlike many other frogs, bullfrogs can coexist with predatory fish (Hecnar, 1997 in Casper & Hendricks, 2005), giving bullfrogs a competitive advantage.
Interaction With Other Invasive Species: In Oregon, the invasion of bullfrogs appears to have been facilitated by the presence of the non-native sunfish (Adams et al., 2003).
For a detailed account of the environmental impacts of L. catesbeianus please read: Lithobates catesbeianus Impacts Information.
Bullfrogs have been intentionally introduced to new habitats as a food resource for humans (Moyle, 1973; Jennings & Hayes, 1985) or for biological control of insects (Jennings & Hayes, 1985; Lawler et al., 1999). They have also been introduced to some areas for farming. However frogs are difficult to farm and farming operations are often unsuccessful (Helfrich et al., 2009; Laufer et al., 2008).
At least 322 species of reptiles and amphibians have been naturalised across the globe; of these 14 species have had ecological impacts demonstrated or reasonably inferred (Kraus, 2009) and of these only three (brown tree snake (Boiga irregularis), cane toad (Rhinella marina) and L. catesbeianus) are even moderately well studied (Kraus, 2009).
Hobson and colleagues (1967 in Casper & Hendricks, 2005) reported that adult bullfrogs have a state of rest characterised 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 in Casper & Hendricks, 2005). Bullfrogs usually squawk when fleeing, often setting off a mass bullfrog exodus from a shoreline into deeper waters (Schwalbe & Rosen, 1999 in Casper & Hendricks, 2005). Bullfrogs may emit a piercing scream when seized, which may startle a predator enough to allow escape (Harding, 1997; Casper & Hendricks, 2005). It has been suggested that bullfrog larvae may be able to recognise cues of novel predators, which could contribute to their success as a native species in a region (Pearl et al., 2003).
The bullfrog is mainly nocturnal, but a loud deep pitched bellow, can be heard during the day (CABI Bioscience, 2005). The mating call has one note that lasts 0.8 seconds at a frequency of 1.0 kHz (Encyclopedia of Life, 2009). To hear the mating call of the American bullfrog please go to Sounds of North American Frogs: The Mating Call of the Bullfrog or AmphibiaWeb.
Currie and Bellis (1969 in Casper & Hendricks, 2005) found bullfrogs to have a mean activity radius of 2.6 meters in an Ontario pond. Their data suggest that home range size is reduced at higher densities and increases as the size of an individual increases, and that males have a larger home range than females.
Finally, the ongoing climatic changes at global scale can modify the suitability of some areas; for example, global warming can cause an expansion of suitable areas towards higher latitude (Roura-Pascual et al., 2004; Thuiller et al., 2007 in Ficetola et al., 2007b).
Native Range: Its native range is from Nova Scotia and New Brunswick, southern Quebec (Canada), south to eastern North America (except southern Florida), southward through Veracruz (Mexico) (IUCN 2009). It is also found in Sinaloa, and probably other areas in Mexico, as well (IUCN 2009).
Known Introduced Range: Lithobates catesbeianus has been introduced in over 40 countries and four continents over the last century (Lever, 2003 in Ficetola et al., 2007b). Bullfrogs are broadly invading western United States and have also been introduced in Mexico, western Canada, Hawaii, Japan, Italy, the Netherlands, Cuba and Jamaica (Green, 1978; Stebbins, 1985; Stumpel, 1992 in Adams et al., 2003).
Introduction pathways to new locations
Aquaculture: Bullfrogs will disperse from artificial water bodies into natural water bodies using seasonal water corridors (Govindarajulu, 2004). Dispersals of at least 3.2 km from the home pond have been recorded, with dispersal distances of seven to eight kilometers likely (Schwalbe & Rosen, 1999 in Casper & Hendricks, 2005). Maximum movement distances of up to 1600 meters (mean 402 meters), as well as homing, were reported by Ingram and Raney (1943 in Casper & Hendricks, 2005) in New York.
Biological control: In some cases, bullfrogs have been deliberately introduced to control agricultural insect pests. This pathway has been of limited importance in the second half of the 20th century.
For ornamental purposes: In British Columbia, Canada they are sold in aquatic garden supply stores for the enhancement of ornamental ponds.
Landscape/fauna "improvement": They have been deliberately introduced as an aesthetically pleasing wildlife.
Natural dispersal: Movements of 2.8 km and more have been documented but bullfrogs are generally philopatric (Bury & Whelan, 1984 in Adams et al., 2003).
Other: Anthropogenic changes, for example, creating wetlands with longer hydroperiods (e.g., farm ponds, sediment basins, golf course ponds), chemical contamination, and fish introduction, may facilitate the movement of invasive species like bullfrogs by increasing the amount of suitable habitat (Boone, 2004).
Pet/aquarium trade: Translocations into private wetlands as a pet or source of food (Albertini 1970; Yiming et al. 2006, in Ficetola et al. 2007a) can substantially increase the rate of expansion of this invasive species. Interviews of local people confirmed that, within a study area in France, translocations were performed at least in one case (Ficetola et al. 2007a).
Smuggling: Some bullfrogs continue to be smuggled into some European Union states (following legislation prohibiting the import of the species); arrests aiming to curtail this activity have been made and tens of thousands of animals have been seized (Fiore & Avanzo, 2002; Dupré et al., 2006 in Kraus, 2009).
Stocking: Species such as R. catesbeiana have been introduced into new locations with the intention of establishing new food sources for human consumption (Kraus, 2009). Although this pathway has been of limited importance in the second half of the 20th century although it is still important in many developing counties (Kraus, 2009).
Local dispersal methods
Natural dispersal (local): Bullfrogs will disperse from artificial water bodies into natural water bodies using seasonal water corridors (Govindarajulu, 2004). Dispersals of at least 3.2 km from the home pond have been recorded, with dispersal distances of seven to eight kilometers likely (Schwalbe & Rosen, 1999 in Casper & Hendricks, 2005). Maximum movement distances of up to 1600 meters (mean 402 meters), as well as homing, were reported by Ingram and Raney (1943 in Casper & Hendricks, 2005) in New York.
For a detailed account of the management and control options to prevent the spread of L. catesbeianus please read: Lithobates catesbeianus (American Bullfrog) Management Information. The information in this document is summarised below.
Careful monitoring is necessary for the early detection and management of newly established frog populations (Ficetola et al. 2007a). The successful eradication of Lithobates catesbeianus from Great Britain (Fisher & Garner, 2007) is an exception to the general pattern of eradication failure because the programmes were placed in the hands of conservation-management professionals (Kraus, 2009).
Preventative Measures: The presence of the emerging Batrachochytrium dendrobatidis chytrid fungal pathogen (which is responsible for declines in amphibians world-wide) suggests that trade and introduction of amphibians should be monitored (Hanselmann et al., 2004). Hanselmann and colleagues suggest that traded amphibians be made subject to veterinary surveillance and quarantine guidelines developed by the International Union for the Conservation of Nature (IUCN) and the Office Internationale des Epizooties (OIE) (Cunningham et al. 2001, in Hanselmann et al. 2004). In relation to control of the chytridiomycosis disease in amphibians, please see Control Strategies for Diseases in Wild Amphibians.
European legislation prohibits new introductions of L. catesbeianus and environmental agencies promote eradication plans (Ficetola et al. 2007a), however, a coordinated effort in the European Union to address invasive species of any kind has not been made (Kraus 2009). Actions are currently are restricted to isolated activities by member states (Kraus, 2009). Genovesi & Scalera (2007 in Kraus, 2009) have proposed a coordinated system of lists covering approved, prohibited, or requiring further study for importation. Doing so would make prevention programs for alien herpetofauna in the European Union more proactive (de Groot & Gerrits, 2002 in Kraus, 2009).
L. catesbeianus is listed as an A1 species by the Belgium Invasive Species Forum (BISF) meaning it represents a high environmental hazard and is present in isolated populations (Etienne et al., 2007). Please see BISF definitions for more information. According to the Invasive Species Environmental Impact Assessment (ISEIA) this species earns a score of 12 (out of 12) which puts it in the black list (A category) (Etienne et al. 2007).
Physical Control: Adult L. catesbeianus may be killed by shooting, spears/gigs, bow and arrow, clubs, nets, traps, angling or by hand. They can be located using torchlight which also temporarily stuns them (P. Veenvliet Pers. Comm. 2003). L. catesbeianus can be controlled using a reptile-proof fence to catch the neonates and traps in the ground to catch them as they leave the pond. Collecting egg masses can be effective in combination with killing frogs and tadpoles.
Knowledge and Research: Studies of actual and potential ecological impacts should be conducted (Santos-Barrera et al. 2009).
Education and Awareness: Ficetola and colleagues (2007a) suggest the promotion of educational programs to reduce the risk of new introductions in Europe.
Bullfrog tadpoles are mainly herbivorous and consume algae, aquatic plant material and some invertebrates (Treanor & Nichola, 1972; Bury & Whelan, 1984 in Casper & Hendricks, 2005). Their efficient gill filters allow them to feed on an impressive diversity of algal species (Kenny, 1969, Wassersug, 1972 in Pryor, 2003) and their labial teeth (which bear a striking resemblance to the radulae of herbivorous snails; Stenick & Watling, 1982 Pers. Obs. in Pryor, 2003) allow them to graze periphyton (Wassersug, 1984; Kupferberg et al., 1994; Kupferberg, 1997a; Altig & McDiarmid, 1999 in Pryor, 2003). Bullfrog tadpoles will also prey on the tadpoles of other species (Kiesecker & Blaustein, 1997 in Blaustein & Kiesecker, 2002).
Adult bullfrogs are gape-limited opportunistic predators that employ a sit-and-wait approach to feeding (Bury & Whelan, 1984, Schwalbe & Rosen, 1988 in Casper & Hendricks, 2005). Bullfrogs essentially eat whatever they can fit into their mouths (Roach, 2004), including crayfish, dragonfly nymphs, aquatic hemipterans and water beetles and small vertebrates such as fish, frogs, turtles, snakes, birds, bats, and weasels (Hirai, 2004 and references therein). They have also been known to eat other bullfrogs. In fact, in southern Arizona the most common vertebrate found in bullfrog intestines were other bullfrogs (C. Schwalbe, pers. comm. in Roach, 2004).
Introduced crayfish red swamp crayfish (Procambarus clarkii) make up the majority of the bullfrogs' diets in many regions (eg. Colusa National Wildlife Refuge, California (USA); Wylie et al. 2003). Stomachs analysis of adult bullfrogs (N=100) collected on the lower Colorado River, Arizona-California by Clarkson and deVos (1986) revealed crayfish (P. clarki) accounted for the highest volume of food material. Wolf spiders (Lycosidae) were the next most voluminous taxon. Other relatively frequently occurring items included long-horned earwigs, sowbugs, German cockroaches, crickets and plant material. Found less frequently were diverse items such as a young muskrat, rattlesnake, kingsnake, and soft-shelled turtle (carapace length 45 millimeters), several species of fish, an Asiatic clam, a scorpion, and numerous insect species, primarily coleopterans.
Hirai (2004) analysed stomach contents of bullfrogs in the Mizorogaike Pond (Kyoto, Japan) and found that adult bullfrogs feed predominantly on crayfish (P. clarkii), and juveniles feed on a diverse range of arthropods. Adult bullfrogs (N=42) were found to contain diverse arthropods and slugs, along with plant materials and minerals. Arthropoda contained five classes (Insecta, Arachnida, Crustacea, Chilopoda, and Diplopoda). Insecta contained seven orders. The most important and one of the largest food items was the crayfish (43.0 to 89.3 millimeters). Beetles, spiders and hemipterans were also commonly consumed. Aquatic prey were as follows: three water-striders, one dragonfly nymph, 62 crayfish and one freshwater shrimp. Food of the juvenile bullfrog (N=79) included a wide variety of anthropods, slugs, pond snails, leeches, earthworms and fish. Arthropoda contained five classes. Insecta contained nine orders. The two most frequently consumed food items were ants and spiders. Ants and spiders were numerically important, but not volumetrically because of their small body size. Dipterans, caterpillars, beetles, and woodlice were also common food items. Volumetrically, crayfish were again the most important food item. The second largest proportion was occupied by beetles As aquatic prey, one water beetle, 13 maggots, four waterstriders, six nymphal dragonflies, two crayfish, 11 freshwater shrimps, seven amphipods, 18 pond snails, one leech and one bluegill were found.
Sexual maturity in bullfrogs usually occurs at one to two years in males, and at two to three years in females (Howard 1981, in Casper & Hendricks 2005). Bullfrogs breed in the vegetation-choked shallows (Pope 1964a, in Casper & Hendricks 2005) of permanent bodies of water. Bullfrog breeding is restricted to warmer periods during spring and summer (Cook & Jennings 2007). In southwest France, the breeding period begins in May and lasts until early September and tadpole development takes one to two years (Lorvelec & Détaint 2006).
Bullfrogs are highly territorial and have a polygynous mating system, with the largest males in a population controlling the highest quality oviposition (egg-laying) sites (Howard 1978a b, in Casper & Hendricks 2005). Males defend roughly circular territories two to five meters in diameter (Harding 1997, in Casper & Hendricks 2005). Male bullfrogs will aggressively defend sites by pushing, shoving and biting (Ryan 1980, in Casper & Hendricks 2005). Most physical encounters are won by larger males by engaging in wrestling, shoving, and pouncing (A.P. Blair 1963, Durham & Bennet, 1963, in Casper & Hendricks 2005).
Females select a mate by entering his territory (Ryan 1980, in Casper & Hendricks 2005). Female mate choice becomes more discriminating with age, with older females consistently selecting the oldest, largest males as mates (Howard 1978a, in Casper & Hendricks 2005). Older females sometimes vocalize within male choruses, which may elicit higher levels of male to male competition and assist females in selecting high-quality males (Judge et al. 2000, in Casper & Hendricks 2005). Embryo mortality depends on female choice of the oviposition site, the best of which are controlled by the largest males and where water temperatures do not exceed 32°C, over which developmental abnormalities occur (Howard 1978b, in Casper & Hendricks 2005). Bullfrog embryos have the highest critical thermal maximum of any North American frog at 32°C and the lower temperature limit for normal embryonic development is 15°C (Moore 1942, in Cook & Jennings 2007).
Longevity for wild bullfrogs is estimated to be eight to 10 years, although a captive specimen survived for nearly 16 years (Oliver 1955a, Goin & Goin 1962, in Casper & Hendricks 2005).
Eggs: Eggs are laid in thin sheets on the water surface, covering 0.5 to 1 m², and hatching in three to five days (Bury & Whelan 1984, in Casper & Hendricks 2005). The egg batch forms a floating raft attached to vegetation (CABI Bioscience 2005). Bullfrogs are extremely prolific, producing up to 20 000 eggs per clutch (Schwalbe & Rosen 1999, in Casper & Hendricks 2005). The number of zygotes sired by successfully mating males ranged from 5000 to 59 000 (mean 11 000), with the number of resultant hatchlings ranging from 300 to 29 000 (mean 5600; Howard 1978b, in Casper & Hendricks 2005). Females may lose up to 27% of their body mass during oviposition (Judge et al. 2000, in Casper & Hendricks 2005).
Larvae/Metamorphosis: Tadpoles favor warm water environments (24°C to 30°C; Brattstrom 1962b, in Casper & Hendricks 2005). The time to metamorphosis varies from a few months (in the south) to three years (in Michigan and Nova Scotia) (Collins, 1979; Bury and Whelan, 1984). The length of the larval period is negatively correlated with mean length of the frost-free period (Collins 1979, Crawshaw et al. 1992, in Casper & Hendricks 2005).
This species has been nominated as among 100 of the World's Worst invaders
Reviewed by: Stan A. Orchard, R.P.Bio.
Principal sources: Scalera, 2007; Kraus, 2009; Santos-Barrera et al., 2009
Compiled by: John J. Crayon, U.S.Geological Survey, Western Ecological Research Center, Department of Biology, University of California, Riverside, California, USA & IUCN/SSC Invasive Species Specialist Group (ISSG)
Last Modified: Thursday, 3 December 2009