American bullfrog / Ouaouaron

SPECIES ACCOUNT. American bullfrog / Ouaouaron (Lithobates catesbeianus)

Stephen C. Lougheed and Scott A. Taylor
Department of Biology, Queen’s University, Kingston, ON Canada K7L 3N6
email: steve.lougheed@queensu.ca; scott.taylor@queensu.ca

Taxonomy: Class: Amphibia. Order: Anura. Family: Ranidae. Genus: Lithobates Fitzinger, 1843. Species: Lithobates catesbeianus (Shaw, 1802). (Changed from: Rana catesbeiana based on DNA evidence from Frost et al. 2006). The species name “catesbeianus” is in honor of the English naturalist Mark Catesby (1679-1749) who explored the southeast USA for several years and was the first to publish an illustration of this species, in his Natural History of Carolina, Florida and the Bahama Islands (1731-43).

Description: This largest of North American frogs can reach 200 mm in snout vent length and sometimes exceeds 500 g. in weight (Lutterschmidt et al. 1996, Harding 1997). Bullfrogs can be distinguished from their close relative, the green or bronze frog (Lithobates clamitans) — with which they often co-occur in Canada — by the absence of a dorsolateral fold that extends from eye to pelvis. Rather bullfrogs have a ridge that extends from the eye wrapping around the prominent tympanum (Figure 1).

Bullfrogs exhibit sexual size dimorphism although there is significant overlap (Shirose et al. 1993). For example, adult males in a well studied population in Michigan averaged ~130 mm snout-vent-length while females averaged ~140 mm (Howard 1981). Ontario bullfrogs display a similar level of size dimorphism (Figure 2).

Sexes can be distinguished based on the ratio of eye to tympanum diameters and on throat colour. In mature females, the tympanum is approximately the same diameter as the eye while in larger males tympanum diameter may approach twice that of the eye, with the relation between tympanum and body size increasing allometrically (Figure 3). The large tympanum in males helps in broadcasting their loud territorial vocalizations as well as serving the obvious role in sound reception (Purgue 1997). Sexually mature males have yellow throats (in females the throat is white/cream).

The face from tympanum to snout tip is often green, but generally the colour of the dorsum and upper surfaces of the legs are quite variable, spanning brown, olive and green with varying degrees of mottling and subtle horizontal striping on the legs (Harding 1997).

The ventrum and undersides of the legs are whitish with subtle speckling. The toes of the large hind limbs are webbed but front digits are free, a typical pattern in members of the genus Lithobates. Howard (1981) provides a detailed summary of adult measurements and phenotypic differences between males and females.

Bullfrog tadpoles have close-set dorsal eyes, are extremely variable in colouration, but are typically olive green dorsally and creamy to yellow ventrally.  They are the largest tadpoles in eastern Ontario attaining lengths of 100 mm or more, and the typically take 1-2 years to develop (Altig et al. 1998). Although recently transformed frogs have relatively smooth skin, this becomes slightly bumpy and less regular with age.

Distribution: The native range of the American bullfrog extends from Central Florida west to Texas and north into Ontario, Quebec, New Brunswick and Nova Scotia. In Ontario (Figure 4), the American bullfrog is widely distributed from Essex County to the centre of Algoma District along the shore of Lake Superior.  There are substantial gaps in the current Ontario distribution of bullfrogs (e.g. parts of Essex, Kent, Huron, Perth and Middlesex Counties) as a consequence of wetland drainage over the last century. High bullfrog densities still occur in the Norfolk County/Long Point and Niagara regions and throughout Eastern Ontario and north-central Ontario (Figure 4).

NHIC American Bullfrog Ontario Distribution

American bullfrogs have been introduced into many parts of the world because of desire to farm the species for food, and in some instances for biocontrol. This has often had devastating consequences for local herpetofauna and other native wildlife (e.g. Kiesecker and Blaustein 1998, Wuab et al. 2005). Because of these introductions their range now includes western North America (Kupferberg 1997), China (Wang et al. 2007), Japan (Minowa et al. 2008), Europe (Ficetola et al. 2006, 2008), and South America (Giovanelli et al. 2008, Laufer et al. 2008). Aggressive eradication campaigns now under way will undoubtedly eliminate many of these populations (e.g. Berroneau et al. 2008).

Habitat: Breeding adults and tadpoles can be found in permanent wetlands, marshes, lake margins, and beaver ponds, typically with substantial emergent and submerged vegetation (Harding 1997).  Water bodies that do not freeze to the bottom are important for this species because of the larvae take 2 years to develop, and because adults overwinter buried in mud and other debris at the bottom. At the Queen’s University Biological Station, we have heard or seen this species throughout the property in a variety of permanent wetlands including Lake Opinicon (Cow Island Marsh, Telephone Bay), Lindsey Lake, Beaver Marsh, Two Island Lake, and Camelot Marsh.  We have also observed mature individuals in ephemeral woodland ponds and other small water bodies that dry up in the summer. Recent telemetric evidence suggests that such temporary wetlands comprise a significant component of bullfrog habitat (Gahl et al. 2009), although obviously these cannot be used successfully for breeding.

Ecology: Breeding activity typically does not begin until mid- or late May and often persists into July (Shirose et al. 1993, Harding 1997, see Figure 5). The first evidence that they are breeding are the loud territorial vocalizations of males (mnemonic for the call “jug-o-rum”). Although males can be heard calling for a 4 6 week period each spring (Shirose et al. 1993), particular calling aggregations seem to be quite short-lived (Emlen 1976). Territorial and male-male aggressive displays are common, the latter consisting of a mixture of jumps and lunges at opponents, displays using the striking yellow vocal sac, and vocalizations (Emlen 1968, 1976, Howard 1978, Ryan 1980).

Opinions on the type of mating system exhibited by American bullfrogs vary in the literature. Emlen (1976) suggests that bullfrogs lek and males choose sites for optimal display to females. In contrast, Howard (1978) suggests that males defend territories, with higher quality territories being those with fewer predators and having more desirable temperature for larval development. Regardless, female choice appears to underpin the bullfrog mating system. Females do not appear to choose males based on the pitch of their vocalizations (which relates body size) as has been suggested for other species (Bee 2002). Mating occurs by males clasping females just behind the forelimbs (see Wells 2007) for 17 155 minutes (Howard 1978), during which time females lay 5,000 20,000 eggs while the male extrudes sperm over the emerging egg mass. The fertilized eggs then form a thin floating mass on the surface of the water (Harding 1997). Eggs require about 3-5 days to hatch (Bury and Whelan 1984).

Size at sexual maturity differs between males and females. For example, a study conducted on Algonquin Park bullfrogs found that the average age at maturity for females was about 5 years (at 113 mm snout vent length) after transformation from tadpoles while it was 3 years post-transformation (at 91 mm SVL) for males (Shirose et al. 1993). For the same study, minimum age at maturity also differed between the sexes: 4 years post-transformation (at 94 mm SVL) for females, and 3 years (at 90 mm SVL) for males. Maximum life spans in wild bullfrogs are estimated to be 8-10 years, although the record for longevity in captivity is 16 years (Goin and Goin 1962).

Mature bullfrogs are generalists and will consume a range of prey spanning insects, crayfish, snakes, frogs (including individuals of their own species), fish, and even ducklings and other small birds (Harding 1997). Bullfrogs are visual ambush predators, waiting for suitable prey to cross their path. Once a prey item is sighted a bullfrog will orient toward it, and then lunge with mouth open using its fleshy, sticky tongue for capture. In turn, bullfrogs are important prey for many wildlife species including fish, herons, turtles (Chelydra serpentina), and mink (Mustela vison).  Young bullfrogs and tadpoles are most vulnerable to predation from snakes (garter and ribbon snakes, Thamnophis spp., water snake Nerodia sipedon), other frogs, fish, herons and turtles. Their large size and prodigious appetites mean that American bullfrogs can have enormous impacts on both native ecological communities (Hecnar and M’Closkey 1997, Boone et al. 2004, 2007) and those in their non-native range (e.g. Kiesecker and Blaustein 1998).

As for many ranid tadpoles, bullfrog larva are widely considered to be herbivores; however, recent evidence, including stable isotope analysis (Schiesari 2004), suggests that this is an overly simplistic view and that bullfrog tadpoles may consume considerable amounts of animal matter (Altig et al. 2007). Moreover, Pryor et al. (2005) found that tadpoles that derive much of their energy from plants and algae probably use microbe-assisted gastrointestinal fermentation to assist digestion. Bullfrog tadpoles do not transform in their first year but rather must overwinter in mud of their natal wetlands at least one year. Tadpoles may metamorphose in their second summer but more typically do so in their third at a minimum total length of 100 mm, usually in July and August (Bruneau 1980), with longer times to transformation at higher latitudes.

Although the bullfrog is considered among the most aquatic of Ontario frogs, genetic studies of eastern Ontario populations suggest that it does not move solely via aquatic corridors (Austin et al. 2004a) and that breeding aggregations separated by tens of kilometers may still experience substantial gene flow. Genetic studies also imply that dispersal is female biased (Austin et al. 2003).

Unlike some other temperate frogs, including the congeneric wood frog (Lithobates sylvaticus; see Storey 1990), the American bullfrog is not generally regarded as freeze tolerant. However, adult frogs have an array of behavioural and physiological adaptations to cope with the rigours of winter. During winter the adults bury themselves in debris or mud in the bottom of permanent water bodies that do not freeze to the bottom (Johnson 1987, Harding 1997). Radiotelemetry studies suggest that bullfrogs may remain active below the ice during their normal hibernation period (Stinner et al. 1994). Bullfrogs can tolerate substantial amounts of hypoxia (Friet and Pinder 1990) and during fall and winter have elevated levels of glucose presumably to avoid freezing and water loss (Rocha and Branco 1998).

Conservation Status: While increasing dramatically in regions of the world where it has been introduced, there have been some reductions throughout its native range caused by a number of factors including harvesting, wetland loss and pesticides (Berrill et al. 1992, Weller and Green 1997).

The following provides a summary of conservation status for the American bullfrog. This information is summarized from the National Heritage Information Centre (NHIC, http://nhic.mnr.gov.on.ca/nhic_.cfm):

GRANK (global rank across the entire range): G5 = globally secure – very common; demonstrably secure under present conditions. GRANK DATE: 1996-10-18. GRANKS are determined collectively by conservation data centres, and scientific experts.

NRANK (national rank): N5 = nationally secure.

SRANK (provincial or sub-national level): S4 = common and apparently secure in Ontario; usually more than 100 occurrences in the province.

Canada General Status: SECURE. Assessed in 2000 and 2005.

Ontario General Status: SECURE Ontario General Status Date: 01-Nov-99. For amphibians and reptiles, sranks are based largely on the Ontario Herpetofaunal Summary project (Weller and Oldham 2000).

Committee on the Status of Endangered Wildlife in Canada (COSEWIC): not listed

Research needs: The dynamics of chorusing behaviour in bullfrogs and other species remains understudied and future research should investigate interactions among males in large choruses (e.g. Simmons et al. 2008). Spatiotemporal patterns in the feeding ecology of bullfrog larva are largely unknown (Altig et al. 2007). We still require more studies of basic bullfrog ecology in non-native parts of its range to understand both its ecological impact and means of control (e.g. Govindarajulu et al. 2006, Adams and Pearl 2007). We require additional information on factors that might reduce population density in its native range (Berrill et al. 1992, Weller and Green 1997). We also lack detailed information on dispersal after metamorphosis. New analytical approaches involving molecular data and improved smaller, radiotransmitters coupled with GIS will help us to understand local gene flow and dispersal dynamics. Phylogeographic analyses have revealed two distinct mitchondrial lineages (Austin et al. 2004b, Austin & Zamudio 2008) and some have suggested that these should be elevated to distinct species. This requires further work using nuclear markers and experimental analysis on the mate recognition system. Finally, despite suggestions that bullfrogs are declining throughout much of their native range we have few data to support or refute such claims.  Thus we require coordinated road-scale monitoring of bullfrog (and other frog) populations across Ontario, complementing some “citizen monitoring” initiatives that include frogs (e.g. Frogwatch, Marsh Monitoring).

Literature & Further Reading

1. Adams, M.J. and C.A. Pearl. 2007. Problems and opportunities managing invasive bullfrogs: Is there any hope? In F. Gherardi (ed.) Invading Nature – Springer Series In Invasion Ecology. Vol. 2. Springer Netherlands. Pp. 679-693.
2. Altig, R. M.R. Whiles and C.L. Taylor. 2007. What do tadpoles really eat? Assessing the trophic status of an understudied and imperiled group of consumers in freshwater habitats. Freshwater Biol. 52: 386-395.
3. Altig, R., R. W. McDiarmid, K. A. Nichols and P.C. Ustach. 1998. A Key to the Anuran Tadpoles of the United States and Canada. Contemp. Herp. Inform. Series 2: 1-58.
4. Austin, J.D., J.A. Dávila, S.C. Lougheed and P.T. Boag. 2003. Genetic evidence for female-biased dispersal in the frog, Rana catesbeiana (Ranidae). Mol. Ecol. 12: 3165- 3172.
5. Austin, J.D., S.C. Lougheed and P.T. Boag. 2004a. Discordant temporal and geographic patterns in maternal lineages of eastern North American frogs, Rana catesbeiana (Ranidae) and Pseudacris crucifer (Hylidae). Mol. Phylogenet. Evol. 32: 799-816.
6. Austin, J.D., S.C. Lougheed and P.T. Boag. 2004b. Controlling for the effects of history and nonequilibrium conditions in gene flow estimates in northern bullfrog (Rana catesbeiana) populations. Genetics 168: 1491-1506.
7. Austin, J.D. and K.R. Zamudio. 2008. Incongruence in the pattern and timing of intra-specific diversification in bronze frogs and bullfrogs (Ranidae). Mol. Phylogenet. Evol. 48: 1041-1053.
8. Bee, M.A. 2002. Territorial male Bullfrogs (Rana catesbeiana) do not assess fighting ability based on size-related variation in acoustic signals. Behav. Ecol. 13: 109–124
9. Berrill, M., S.M. Bertram, P. Tosswill and V. Campbell. 1992. Is there a bullfrog decline in Ontario? In: Declines in Amphibian Populations: Designing a National Monitoring Strategy. C. Bishop and K. Pettit (eds.) Occasional Paper #76, Canadian Wildlife Service.
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Reviewers: James D. Austin (Univ. Florida) and Robert Montgomerie (Queen’s Univ.)