American bullfrog (Lithobates catesbeianus)

Also known as: bullfrog, common bullfrog
Synonyms: Lithobates catesbeiana, Rana catesbeiana
  
French: Grenouille-taureau
Spanish: Rana Toro
KingdomAnimalia
PhylumChordata
ClassAmphibia
OrderAnura
FamilyRanidae
GenusLithobates (1)
SizeMale length: 11.1 - 17.8 cm (2)
Female length: 12 - 18.3 cm (2)
Tadpole length: 4 - 15 cm (3) (4)
Weight0.5 kg (4) (5)
Top facts

The American bullfrog is classified as Least Concern (LC) on the IUCN Red List (1).

Aptly named for its deep, resonant croak (3), the American bullfrog (Lithobates catesbeianus) is North America’s largest frog species (2) (4) (5) (6).

This impressive, golden-eyed amphibian has a broad head and body (6), and its rough skin is covered in tiny, randomly arranged tubercles (2). The tips of the American bullfrog’s fingers and toes are blunt, and the webbing between the digits is well developed (2).

The colouration of the American bullfrog varies widely depending on the location (2) (7), with its upperparts ranging from bright green to olive or brownish green (3) (4) (5). A netlike pattern of brown or grey markings may be present on its back (2). The hind limbs are long and powerful (7) and have dark blotches and bands (5). The underside of the American bullfrog is much paler than the upperparts (3), with a cream or whitish belly (3) (5) that is tinged with yellow or mottled with grey (2) (3).

As well as being larger than the male (5), the female American bullfrog is also generally browner and more highly spotted (3). The throat of the male is yellow, while that of the female is white (5) (6), and a further difference between the sexes is the presence of pigmented ‘nuptial pads’ on the thumbs of the male, which are used to grip the female during mating (2).

As with the adults, the tadpoles of the American bullfrog are large (4) (5). The back of the tadpole is yellowish green, speckled with black spots, and has an arched dorsal fin, while the belly is lighter (5).

The American bullfrog is native to eastern North America (4) (5) (6) (7), including south-eastern Canada (7).

However, this species has also been introduced to western parts of the United States (3), as well as certain regions of Europe, South America, eastern Asia and the Caribbean (4) (5) (7). In some areas, introductions were made to produce frogs’ legs as a source of food (2) (4) (8) or to deploy the species as a biological control agent to deal with insect pests (8). It is thought that the American bullfrog was introduced to the UK through the deliberate release of unwanted pets and through accidental importation with aquatic plants or fish stocks (8).

The American bullfrog was introduced to California in the 1890s and Colorado in the early 1900s (5), and has since spread to all 50 states of the USA, including Hawaii (2).

The American bullfrog is a highly aquatic species, spending most of its life in or near to water (3) (7). However, during exceptionally wet weather, it may move away from permanent water bodies (3).

In all stages of its lifecycle (4), the American bullfrog is found in permanent freshwater ponds and lakes (4) (6) (7). Still waters are generally preferred (4) (5), although the American bullfrog is also often found in slow-moving streams (3) (5) (6) (7).

The American bullfrog favours bodies of water in lowlands (4), particularly shallow areas (5) where aquatic vegetation is abundant (3) (5). The tadpoles of this species, however, tend to prefer non-vegetated water bodies (5).

Cold or dry areas are usually avoided (4), as the American bullfrog requires warm water for breeding (5). This species has a higher tolerance than many other native amphibians for the elevated water temperatures often found in human-modified environments (4).

The American bullfrog is active both by day and night (5). As with many other amphibians, this species produces a toxin in its skin which makes it distasteful to most vertebrate predators (4).

The American bullfrog is a carnivorous amphibian (3) (4) (5) (6) and has a voracious appetite (4), consuming almost anything it can fit in its mouth and swallow (4) (6). It is an opportunistic predator (2) (3) and has a very varied diet, consisting of insects, earthworms, crayfish, spiders and snails (2) (3) (5). The American bullfrog catches prey using its large, sticky tongue (5), after lying in wait for its victim (3). It also consumes larger prey, including snakes, birds, turtles and frogs, as well as bats and other small mammals (3) (5) (6), and it has even been known to catch swallows flying low over the water (3). Cannibalism is prevalent in the American bullfrog, with other individuals of the same species thought to constitute up to 80 percent of its diet (2).

The tadpoles and young adults of the American bullfrog are mostly herbivorous, and feed on algae and other plant material, although they also sometimes take small aquatic invertebrates (3) (4) (5).

The breeding season of the American bullfrog starts in the spring, and continues throughout early summer. However, timings can vary somewhat depending on latitude (2), with breeding occurring from May to July in northern states, and from February to October in warmer regions (5). Males defend territories during the breeding season, and use their vocal sacs to produce a deep, sonorous call to attract mates (7). This distinctive booming call, which can be heard from almost a kilometre away (3), consists of a single note (2) and is often reported to sound like ‘jug-o-rum’ (3) (4) (5) (6). The male American bullfrog moves out into open water to call, while the female stays inshore. The female will only join the male once she is ready to lay eggs (3).

After mating, the female lays between 10,000 and 25,000 eggs (3) (6), with the size of the clutch being dependent on the size of the female (3). A clutch normally consists of one quarter of the female’s body weight (2). The eggs are jelly-coated (5) (6) and float in a film on the surface of the water (3) (4), creating a ‘raft’ which can reach a metre in diameter (5). Not long before hatching, the mass of eggs sinks (4) (5). Hatching occurs within a week of the eggs being laid (3) (4) (6), but the tadpoles can take up to two years to undergo metamorphosis (3) (4) (6), depending on the temperature of their environment (2).

Tadpoles of this species are able to overwinter in water under a cover of ice (5), while adults tend to hibernate in mud when the weather becomes cold (3) (4) (5) (6).

Other than being farmed and hunted for human consumption in some areas (4) (5) and being predated upon by raccoons, snakes and some wading birds (4) (6), the American bullfrog is not currently facing any major threats. In fact, its introduction to areas outside of its natural range has threatened many native species, particularly other amphibians, either through predation or competition (2) (4) (5) (8). As a result, the American bullfrog poses a serious challenge to conservation efforts in many regions (4), and it has been classed by IUCN’s Invasive Species Specialist Group as one of the 100 worst invasive species in the world (5).

There are also concerns that the American bullfrog may be a carrier of the chytrid fungus, which it appears to be somewhat resistant to, but which is deadly to many other amphibian species (2) (8) (9).

The American bullfrog is able to travel relatively long distances overland during rainy periods (4). This, combined with the ease with which juveniles colonise new ponds (2), leads to recolonisation occurring quickly, even where eradication programmes have been implemented. This makes American bullfrog populations very difficult to control (4).

Given that the American bullfrog is not threatened, there are currently no conservation measures aimed specifically at this species.

In the UK, where it is considered to be an invasive species, the American bullfrog has been the subject of an eradication programme. The initial population of 9,000 or more individuals has successfully been removed, and since then breeding numbers have been significantly lower. So far, more than £100,000 has been spent on control measures for and monitoring of the American bullfrog in the UK (8).

Find out more about the American bullfrog:

Learn more about amphibians and their conservation:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

  1. IUCN Red List (May, 2012)
    http://www.iucnredlist.org/
  2. AmphibiaWeb - Rana catesbeiana (May, 2012)
    http://amphibiaweb.org/cgi-bin/amphib_query?query_src=aw_search_index&table=amphib&special=one_record&where-genus=Rana&where-species=catesbeiana
  3. Marshall Cavendish (2010) North American Wildlife. Marshall Cavendish Reference, New York.
  4. Boersma, P.D. and Van Buren, A.N. (2006) Invasive Species in the Pacific Northwest. University of Washington Press, Washington.
  5. Woodward, S.L. and Quinn, J.A. (2011) Encyclopedia of Invasive Species: From Africanized Honey Bees to Zebra Mussels, Volume 1. ABC-CLIO, Santa Barbara.
  6. Day, L., Klingler, M.A. and Bloomberg, M.R. (2007) Field Guide to the Natural World of New York City. Johns Hopkins University Press, Baltimore.
  7. Burnie, D. (2011) Animal. Dorling Kindersley Ltd., London.
  8. GB Non-native Species Secretariat (NNSS): Factsheet - American bullfrog (October, 2013)
    https://secure.fera.defra.gov.uk/nonnativespecies/factsheet/factsheet.cfm?speciesId=2040
  9. GB Non-native Species Secretariat (NNSS): Information Sheet - American bullfrog (October, 2013)
    https://secure.fera.defra.gov.uk/nonnativespecies/index.cfm?sectionid=47