B.C. Reptiles & Amphibians

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Credit: Diana Robinson

American Bullfrog

Lithobates catesbeianus

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Description

Other names: Bullfrog, Rana catesbeianus

The American Bullfrog is not native to British Columbia. It is the largest species of frog found in North America, growing up to 20 cm in body length and 750 g, with females typically being larger than males. Individuals range in colour from pale green to dark green or brown on top and typically have a white or cream belly that may be mottled. The underside of the chin is typically white, although during the breeding season males can display a bright yellow patch. The Bullfrog is distinguished by a very large tympanum that is the same size as the eye in females, and larger than the eye in males. Bullfrogs also lack dorsolateral folds, although they possess a prominent fold of skin around the tympanum. The hind legs of American Bullfrogs typically have dark cross-bands and the hind feet are heavily webbed. Bullfrog tadpoles are also the largest in North America and have long tails with a large fin. Small tadpoles (<2.5 cm) are black with gold patterns, and turn to grey-green with black flecks as they grow. Tadpoles can grow up to 15 cm but typically metamorphose when around 10 cm in length.

Listen to the Indigenous words for “frog” here!

American Bullfrog Call

The call of the American Bullfrog is deep and resonant, often described as a bass, growly “jug-o-rum”. A full chorus can be heard from half a kilometer away.

American Bullfrog Call

The call of the American Bullfrog is deep and resonant, often described as a bass, growly “jug-o-rum”. A full chorus can be heard from half a kilometer away.

Similar Species

Green Frogs, another introduced species in British Columbia, also have a large, obvious tympanum but are differentiated from Bullfrogs by their dorsolateral folds (raised skin down their back).

American Bullfrog

Green Frog

Distribution

The American Bullfrog is native to eastern Canada, but was introduced to British Columbia following World War II as a food source. Bullfrogs were successful in British Columbia and spread throughout the Lower Mainland, Vancouver Island, and the South Okanagan. Bullfrogs are found naturally throughout much of the central and eastern United States, and have been introduced to many locations in the west and globally.

Habitat

American Bullfrogs are heavily reliant on aquatic environments and thus require large, permanent water bodies for breeding and hibernation. They can be found in lakes, rivers, streams, beaver ponds, and permanent wetlands. Research has shown that Bullfrogs will readily migrate between permanent and seasonal ponds until they dry up. Bullfrogs will hibernate by digging themselves into the bottom of ponds or lakes.

Reproduction

The breeding season for American Bullfrogs is later than for most other frog species, usually occurring from mid-June to late July on warm, rainy nights. Males will call to attract females and, if a suitable mate is found, will fertilize eggs externally as they are laid by grasping onto her back in amplexus. Females will lay up to 20,000 eggs in shallow water with lots of vegetation and the eggs will spread out to form sheets near the surface. The eggs are black and develop very quickly, hatching in less than a week. Females can produce 2 clutches per year. In most areas, tadpoles will hibernate for at least one winter before metamorphosing, but in warmer climates they may metamorphose in less than 6 months. Bullfrogs in colder climates or areas with short growing seasons may take up to 5 years to reach sexual maturity and can live for up to 10 years.

Diet

American Bullfrogs are opportunistic predators, meaning they will eat anything they can capture and their diet consists of invertebrates and vertebrates. They are known for their insatiable appetite, and smaller frogs (including other Bullfrogs) are an important part of their diet. Tadpoles eat organic debris, plant tissue, and aquatic invertebrates. Young Bullfrogs primarily eat insects, but as they grow, they may eat young birds, mice, turtles, snakes, or fish.

Conservation Status

Global: G5 (2017)

Provincial: SNA

BC List: Exotic

Learn more about conservation status rankings here

Threats

Habitat loss and wetland destruction has resulted in declines or extirpation in many parts of southern Canada, especially in Ontario, where much of the landscape has been converted for intensive agriculture. Pollution from herbicides, agricultural effluent, and road salt can cause direct mortality and developmental defects. Bullfrogs are susceptible to mass mortality from pathogens such as Chytrid fungus and Ranavirus. In the past, Bullfrogs were heavily targeted for food use for their meaty legs, but this is much less common today. While American Bullfrogs may be here to stay, there are ways you can help prevent the spread. Never transport Bullfrog tadpoles or adults – if they escape, they may be able to populate new areas. If you spot Bullfrogs, report them to the Invasive Species Council of British Columbia.
Learn More
Criag-Stanfill---tympanum-close-up
Dave-Huth---leg-and-foot-close-up
Dave-Huth---tadpole-early-stage
David-Bradford---of-Jacob-Ewashen---tadpole
Matt-Morrison---in--hand-size-ref
Steve-Clare---metamorph
Todd-Pierson---green-frog-(top)-vs-bull-frog-(bottom)-no-dorsolateral-ridges
Trish-Hartmann---amplexus
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Did You Know?

Fact #1

American Bullfrogs, where they have been introduced, are believed to contribute to native species loss. In B.C., Bullfrogs have displaced Red-legged Frogs and Pacific Treefrogs in many areas on Vancouver Island. As Bullfrogs feed on smaller frogs, they may contribute to declines in other species that are not adapted to avoiding them. Methods to control Bullfrog populations are being explored. The removal of tadpoles may actually serve the opposite effect, and the timing of removal has been shown to be important.

Fact #2

If you catch an American Bullfrog with your hands, they may go limp and appear dead. Then suddenly, they will spring back to life and leap away. American Bullfrogs can leap up to 2 meters in distance!

Fact #3

American Bullfrogs do not display any sleep-like behaviour during the active season! They are active all day and night to advertise their territory to potential mates, and defend their homes from attacking intruding frogs.

Species Account Author: Marcus Atkins

References
B.C. Conservation Data Centre. 2010. Species Summary: Lithobates catesbeianus. B.C. Minist. of Environment.
Available:https://a100.gov.bc.ca/pub/eswp/ (accessed May 18, 2021).
Garner, T. W., Perkins, M. W., Govindarajulu, P., Seglie, D., Walker, S., Cunningham, A. A., & Fisher, M. C. (2006). The emerging amphibian pathogen Batrachochytrium dendrobatidis globally infects introduced populations of the North American bullfrog, Rana catesbeiana. Biology letters, 2(3), 455-459.
Govindarajulu, P. (2004). Introduced bullfrogs (Rana catesbeiana) in British Columbia: impacts on native Pacific treefrogs (Hyla regilla) and red-legged frogs (Rana aurora).
Matsuda, Brent, David M. Green, and Patrick T. Gregory. 2006. Amphibians and Reptiles of British Columbia. Handbook. Royal BC Museum, Victoria.
Ovaska, K, S. Lennart, C Engelstoft, L. Matthias, E. Wind and J. MacGarvie. 2004. Best Management Practices for Amphibians and Reptiles in Urban and Rural Environments in British Columbia. Ministry of Water Land and Air Protection, Ecosystems Standards and Planning, Biodiversity Branch
http://a100.gov.bc.ca/pub/eirs/finishDownloadDocument.do?subdocumentId=667
https://a100.gov.bc.ca/pub/eswp/search.do
https://www.naturewatch.ca/frogwatch/bullfrog/
http://www.canadianherpetology.ca/species/species_page.html?cname=American%20Bullfrog