Amphibians
American Bullfrog
Lithobates catesbeianus · also called bullfrog, North American bullfrog
The American bullfrog is the biggest, loudest, and hungriest frog in North America. In its home range it is a familiar part of pond life; carried elsewhere by people, it has become one of the most destructive invasive amphibians on the planet.
Overview
Bullfrogs are native to the eastern United States and southeastern Canada, in and around permanent water — ponds, lake margins, slow rivers, marshes, and ditches with plenty of vegetation. They need water that does not dry up, because their tadpoles take one to three years to develop. They are highly aquatic, rarely moving far from the shoreline, and are active mainly at night.
Physical characteristics
A bullfrog is green to olive to brown, often mottled, with a plain (not ridged) back, powerful hind legs, and fully webbed hind feet. The most noticeable feature is the tympanum, the circular eardrum behind each eye; in males it is clearly larger than the eye, in females about the same size. Males also develop a yellow throat in the breeding season.
A mouth that eats everything
Bullfrogs are generalist ambush predators. A bullfrog sits motionless at the water’s edge and strikes at almost anything that moves and looks edible, flipping its sticky tongue at small prey and lunging mouth-first at larger animals, using its hands to stuff them in. The result is a diet that spans invertebrates, fish, other amphibians, reptiles, small mammals, and birds. This indiscriminate appetite is exactly what makes an introduced bullfrog so damaging to native wildlife.
Reproduction
Males set up calling territories along the shore and defend them with wrestling matches. A female chooses a male and lays a vast floating sheet of eggs — up to 20,000, sometimes more — which hatch within about a week. The tadpoles are large (up to 15 cm) and slow to develop, overwintering once or twice before transforming into froglets.
The invasion problem
In its native range the bullfrog is simply a common, secure species — Least Concern, stable. The conservation story is what happens when it is moved. Spread worldwide through frog-leg farming, biological “pest control,” the aquarium and pond trade, and deliberate stocking, feral bullfrogs now damage ecosystems on several continents by preying on and outcompeting native species and by acting as a symptom-free carrier of the chytrid fungus. Control programmes rely on draining ponds, trapping, and targeted removal, and rarely achieve full eradication.
American Bullfrog: common questions
How big is an American bullfrog?
It is the largest frog native to North America — a big female can be 20 cm from snout to vent, weigh around 750 grams, and measure close to 45 cm with the legs extended. Only a handful of the world's frogs, such as the African goliath frog, are larger.
What does a bullfrog eat?
Almost anything it can cram into its mouth. Bullfrogs are ambush predators that lunge at movement, and their recorded prey includes insects, spiders, worms, fish, crayfish, snails, small turtles, snakes, rodents, bats, ducklings and other birds, and large numbers of other frogs — including their own species. They pull larger prey in with their front feet and swallow it whole and often alive.
Why are bullfrogs a problem outside North America?
Where they have been introduced — the western United States, Europe, Japan, South America, and elsewhere — bullfrogs eat and outcompete native frogs, fish, and reptiles, and they carry the amphibian chytrid fungus without being harmed by it, spreading a disease that has devastated other species. Their size, huge appetite, high output of eggs, and long-lived tadpoles make them very hard to remove once established.
What is the "jug-o-rum" sound?
It is the advertisement call of the male bullfrog, a low, resonant bellow often written as "jug-o-rum" or "br-wum." Males call from the water's edge on warm nights in late spring and summer to attract females and warn off rival males, inflating the vocal sac and pumping air back and forth. The call gives the frog its name.