American Bullfrog (Lithobates catesbeianus): Facts and Range

Appearance: how to recognize a bullfrog

A bullfrog is a big, broad-headed frog with smooth skin. The back is olive to bright green, plain or mottled and banded with gray or brown, and the belly is off-white with yellow or gray blotches. The upper lip is often bright green, while the lower lip is pale.

Two features help separate it from look-alikes. First, there is no ridge of skin (a dorsolateral fold) running down each side of the back, as there is in the similar green frog. Instead, a fold curves from the eye around the eardrum and ends near the front leg. Second, the hind feet are broadly webbed, which suits a frog that spends most of its life in the water.

The eye is prominent, with a brown iris and a horizontal pupil, and behind it sits a large, round eardrum, the tympanum. This eardrum is the easiest way to sex an adult. In males it is clearly larger than the eye, and breeding males have a yellow throat. In females the eardrum is about the size of the eye or smaller, and the throat is white or pale.

How big do American bullfrogs get?

Adults usually measure 9–15 cm (3.6–6 in) from snout to vent, the standard way to measure frogs without the legs. Large individuals reach about 20 cm (8 in), and with the hind legs stretched out a big bullfrog can be well over 30 cm (12 in) long. Unlike many animals, females tend to be larger than males.

A large adult weighs up to about 500 g (1.1 lb), and a few exceptional frogs have been recorded at around 800 g (1.8 lb). Young bullfrogs grow fast: in their first months after metamorphosis they can go from a few grams to well over 100 g. For a full size guide, including giant tadpoles and comparisons with the world’s biggest frogs, see How big do bullfrogs get?

Habitat and range

Bullfrogs need permanent water that stays warm in summer. Their favorite places are large ponds, lakes, marshes, swamps, reservoirs and slow backwaters with plenty of plants along the shore. They also thrive in human-made waters such as farm ponds, irrigation canals, ditches, golf course ponds and garden pools. Warm, still, weedy water suits them better than the cold, clear water preferred by many native frogs.

The native range covers eastern and central North America, from Nova Scotia and southern Quebec south to Florida and the Gulf Coast, and west across the Great Plains toward the Rocky Mountains. Bullfrogs are found in every US state east of the Mississippi River.

Over the past 120 years people have moved bullfrogs far beyond that range, mostly for frog farming and the frog-leg trade, and sometimes as pond pets. They are now established in the western United States (especially California), British Columbia, Hawaii, Mexico, the Caribbean, several South American countries, Japan, China, South Korea and other parts of Asia. In Europe there are populations in Italy, France, Belgium, Germany, the Netherlands, Spain and Greece.

What do bullfrogs eat?

The American bullfrog is a sit-and-wait predator that eats almost any animal it can catch and swallow. Insects, beetles, snails, worms and crayfish make up much of the diet, but bullfrogs are unusual among North American frogs in how many vertebrates they take: fish, tadpoles, other frogs, salamanders, small turtles, small snakes, young birds, mice and even bats have been found in their stomachs. Bigger bullfrogs eat bigger prey, and they readily eat smaller bullfrogs.

A hunting bullfrog sits still at the water’s edge until prey moves, then lunges with its mouth open. The sticky tongue flicks out and the powerful jaws close around the victim, and large prey is pushed in with the front feet. Like other frogs, it swallows food whole. Tadpoles are very different: they mainly graze on algae, plant material and tiny particles in the water. More detail is in What do bullfrogs eat?

Behavior and calls

Bullfrogs are active by day and by night, especially in warm, humid weather, and they spend most of their time in or right beside water. They are wary: at the first sign of danger they leap into deeper water with a loud splash and often a high-pitched squawk. A frightened or seized bullfrog may also give a loud scream that can startle a predator.

In the breeding season males become territorial. Each defends a patch of shoreline, often several meters long, and calls from it at night. The advertisement call is a deep drone usually written as jug-o-rum, which can be heard more than 1 km (0.6 mi) away on a still night. Males also have separate threat calls for rivals and wrestle each other, chest to chest, when calls and postures are not enough. Dominant males float high in the water with their lungs inflated to show off their yellow throats, while subordinate “satellite” males sit low and quietly nearby. To learn more about frog calls in general, see Why do frogs croak?

In winter bullfrogs become inactive and spend the cold months at the bottom of ponds or in mud and leaf litter along the edge. In the far south of the range they may stay active nearly all year.

Reproduction and life cycle

Bullfrogs breed later than most frogs, when the water has warmed up: usually from May to July in the north and over a much longer season, roughly February to October, in the south. Males gather in loose choruses and females visit briefly to choose a mate, often favoring males with good territories.

During mating the male clasps the female from above, and she lays a floating film of eggs among water plants while he fertilizes them. A single clutch can hold up to about 20,000 eggs, and the egg mass can cover an area of 0.5–1 square meter (5–11 square feet). The eggs hatch in about 3–5 days in warm water.

The tadpole stage is long. In the warm south, tadpoles may change into frogs within a few months, but in the north they usually take one to three years, spending winters in the pond. As a result they grow huge, often reaching about 15 cm (6 in) including the tail. Young frogs become able to breed at around 3–5 years old in northern areas, sooner in warm regions.

Lifespan

Wild bullfrogs commonly live about 7–10 years if they survive the dangerous tadpole and juvenile stages, which few of them do. The longest recorded lifespan is about 16 years, for a frog kept in captivity.

Predators and threats

Adult bullfrogs are eaten by large herons and egrets, belted kingfishers, raccoons, river otters, water snakes, snapping turtles, large fish and, in the southeast, American alligators. Tadpoles and froglets have even more enemies, including dragonfly larvae and predatory water bugs, although many fish avoid bullfrog tadpoles because of their unpleasant taste.

Locally, bullfrogs can suffer from pollution, drained wetlands and diseases. Overall, though, the species copes well with human-changed habitats and even benefits from warm, nutrient-rich ponds.

Conservation status

The IUCN Red List classifies the American bullfrog as Least Concern (assessed in 2022), reflecting its wide native range and large, stable populations. The IUCN currently lists it as Aquarana catesbeianus; other names in use for the same species are Lithobates catesbeianus and the older Rana catesbeiana, as scientists still debate how to divide the true frogs into genera.

The bigger conservation story is the harm bullfrogs do outside their native range. They eat and outcompete native frogs, snakes and turtles, and they can carry the chytrid fungus (Batrachochytrium dendrobatidis) without falling ill, spreading it to more vulnerable species. The bullfrog appears on the list of 100 of the World’s Worst Invasive Alien Species, and since 2016 it has been on the European Union’s list of invasive alien species of Union concern, which bans importing, keeping, breeding or releasing it. The full picture is in Are bullfrogs invasive?

Myths and facts

Myth: Bullfrogs only eat insects. Many people picture frogs eating flies, but bullfrogs regularly eat crayfish, fish, other frogs and occasionally small birds, snakes and mammals.

Myth: All big green frogs in a pond are bullfrogs. In much of eastern North America the green frog (Lithobates clamitans) is just as common. It is smaller, rarely over 10 cm (4 in), and has a ridge of skin down each side of the back, which the bullfrog lacks. Its call is a single twang like a loose banjo string, not a deep drone.

Fact: Bullfrogs eat by blinking. Like many frogs, a bullfrog can pull its eyeballs down into its head while swallowing, which helps push food toward the throat.

Relationship with people

Bullfrogs have long been part of North American life. Their legs are eaten in the southern United States and elsewhere, and bullfrog farming has been tried in many countries, which is the main reason the species spread so widely. For generations, bullfrogs were also a standard animal in school biology lessons and in research on nerves and muscles.

The bullfrog is the state amphibian of Missouri, Ohio and Oklahoma, and its call is one of the classic sounds of a summer night in eastern North America. In Europe and the western United States, however, wildlife agencies ask people not to buy, move or release bullfrogs or their tadpoles, so that the species does not spread further.