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Canadian Herpetological Society Société d'Herpétologie du Canada |
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American Bullfrog
Lithobates catesbeianus
Family: Ranidae
Until recently, the American Bullfrog was in the genus Rana, but it is now in the genus Lithobates.
COSEWIC status:- Not Assessed
- No Status
- Least Concern
Description
The American Bullfrog is the largest frog in Canada and adults can reach 20 cm in body length. Individuals are olive, green or brown and the back may be distinctly mottled or lack patterning altogether. The belly is cream to whitish and it may be mottled on some individuals. This species lacks dorsolateral folds (folds of skin running down each side of the back), although there is a noticeable fold of skin that runs above and behind the tympanum. The rear legs have dark cross-bands and the hind feet are strongly webbed. Males are generally smaller than females and the tympanum (eardrums) is noticeably larger than the eyes, while the females’ tympanum is about the same size as the eyes. Larvae (tadpoles) have long tails with a large fin and lack front legs (newly hatched tadpoles are legless). Small larvae (< 2.5 cm) are black with gold patterning and larger larvae are greyish green to olive with black flecks and have a well developed tail fin. The larvae can grow to over 15 cm in length, but most metamorphose when they are around 10 cm in length. The call of this species is deep and resonant and often described as “jug-of-rum” or a repeated “rum-rum-rum.”
Similar Species
The American Bullfrog may be confused with the Green Frog and Mink Frog, and all of these species exhibit substantial variation in colour and pattern. Green Frogs have pronounced dorsolateral folds. Mink Frogs may lack dorsolateral folds but they typically usually have blotches rather than cross-bands on the rear legs. The webbing on the Mink Frog’s hind feet extends to the end of the 5th toe and the last digit of the longest (4th) toe, whereas the webbing on the Bullfrog’s hind feet is not that extensive.
Distribution
Click for larger image
In Canada, American Bullfrogs are native to southern Ontario, southern Quebec, New Brunswick and Nova Scotia. This species has been introduced to British Columbia where it has spread along the lower mainland and Vancouver Island. This species also occurs throughout the eastern and central United States. American Bullfrogs have been introduced to numerous locations in the western United States as well as elsewhere in the world.
Habitat
American Bullfrogs are highly aquatic and require large, permanent bodies of water for breeding and hibernation, such as lakes, rivers, streams, beaver ponds and other wetlands. Individuals can also be found in smaller or temporary aquatic habitats during the summer, including small ponds and creeks. Individuals hibernate by burying into the bottom of lakes, rivers and wetlands.
Biology
In Canada, American Bullfrogs hibernate during the winter and are active from April or early May until September or October, depending on latitude. Male American Bullfrogs use calls to attract females during the breeding season, which is in June and July. During breeding, the male grasps the female (amplexus) and fertilization occurs externally in aquatic habitats as the female lays eggs. Females lay masses of 5,000–20,000 eggs in shallow water (less than 60 cm deep) with abundant aquatic vegetation, and the eggs spread out to form sheets near the surface of the water that can be up to 1 m in diameter. The eggs are black and are surrounded by a clear jelly envelope. The eggs develop rapidly and typically hatch in less than a week, and larvae transform into frogs after 1–3 years. It can take American Bullfrogs 2–4 years to reach maturity after metamorphosis and individuals can live up to 10 years. Bullfrogs are opportunistic predators and will eat almost anything they can capture; their diet includes a wide range of invertebrates and small vertebrates, including smaller frogs. Bullfrogs do not exhibit sleep-like behaviour during the active season and individuals are active during both the day and night. Bullfrogs use calls to advertise their territories and will defend them by attacking intruding frogs of the same sex.
Threats
The destruction of wetland and shoreline habitat has resulted in declines or local extirpation in many parts of southern Canada, especially in southwestern Ontario where much of the landscape has been converted to intensive agriculture and urban uses. Pollution, such as herbicides, agricultural effluent and road salt, can be detrimental to frog populations by causing direct mortality as well as developmental deformities. Road mortality can be a significant threat when roads are within close proximity to this species’ aquatic habitat. Pathogens such as chytrid fungus and Ranavirus can cause mass mortality of frog populations. Climate change may also pose future threats to Canada’s frog populations. Bullfrogs were collected in large numbers for frogs’ legs in the past, but that practice has become less common now.