Lithobates catesbeianus (Shaw, 1802)

Scientific Naming

The bullfrog, scientifically named Lithobates catesbeianus (Shaw, 1802), has an olive-green base color on its dorsal (upper) surface, which may be solid or marked with grayish brown mottling and banding.

Dorsal Coloration

Its ventral (under) surface is off-white, with blotches of yellow or gray. There is often a clear color contrast between the green upper lip and pale lower lip.

Teeth Structure

Its teeth are very small and only suited for grasping prey.

Eye Characteristics

It has prominent eyes with brown irises and horizontal, almond-shaped pupils.

Tympanum Structure

The tympana (eardrums) are easily visible just behind the eyes, and are enclosed by dorsolateral skin folds.

Limb Appearance

The limbs are gray, either blotched or banded; the fore legs are short and sturdy, while the hind legs are long.

Toe Webbing

The front toes are not webbed, and all back toes have webbing between digits except the fourth toe, which is unwebbed.

Sexual Dimorphism

Bullfrogs are sexually dimorphic: males are smaller than females, have yellow throats, and have tympana larger than their eyes, while females have tympana about the same size as their eyes.

Body Length

Bullfrogs typically measure 3.6 to 6 inches (9 to 15 cm) in snout-to-vent length.

Juvenile Weight Gain

They grow quickly in their first eight months of life, usually increasing in weight from 5 to 175 g (0.18 to 6.17 oz).

Maximum Size

Large mature individuals can weigh up to 500 g (1.1 lb), and rare records note individuals reaching 800 g (1.8 lb) and 8 inches (20 cm) snout-to-vent. The American bullfrog is the largest true frog species native to North America.

Native Eastern North America Range

The bullfrog is originally native to eastern North America, where it is common in every U.S. state east of the Mississippi River. Its natural range extends from the eastern Canadian Maritime Provinces west to Idaho and Texas, and north to Michigan (including the Upper Peninsula), Minnesota and Montana; it is largely absent from North Dakota.

Introduced Western US Range

It has been introduced to Nantucket island, and to parts of the western U.S. including Arizona, California, Colorado, Hawaii, Idaho, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming, where it is considered an invasive species.

Local Ecological Impacts

Conservation concerns note that it may outcompete or prey on native reptile and amphibian species, disrupting local ecological balance.

Global Introduction Reasons

It has also been introduced to Hawaii, South America, Asia, the Caribbean, and Europe for purposes including frog farming and biological control of other species.

California Threat Impacts

It is very common on the U.S. West Coast, especially in California, where it is thought to threaten the California red-legged frog and is considered a contributing factor to the decline of this vulnerable species. It has been recorded feeding on young of several snakes, including the threatened, California-endemic giant garter snake.

Utah Control Measures

In early 2023, the Utah Department of Natural Resources began sharing social media tips for catching and cooking bullfrogs to encourage residents to help control the growing invasive population by harvesting them for food.

Additional Introduced Regions

Other introduced regions for the bullfrog include extreme southern British Columbia (Canada), almost every state in Mexico, as well as Belgium, Cuba, the Dominican Republic, Haiti, Italy, Jamaica, the Netherlands, Puerto Rico, Argentina, Brazil, China, Colombia, Japan, South Korea, Uruguay and Venezuela.

Intentional Introduction Causes

Most introductions have been intentional, either to provide a human food source or as biological control agents.

Unintentional Introduction Causes

Unintentional introductions also occur via escapes from breeding facilities, scientific research sites, or from captive/ pet releases.

Chytrid Fungus Carrier Status

Conservationists are concerned that the bullfrog is relatively immune to chytridiomycosis (chytrid fungus), a lethal fungal infection that has severely harmed many other frog species. As an asymptomatic carrier, the bullfrog may spread the fungus to more susceptible native frog species when it invades new territories.

Prey Role in Ecosystem

In ecology, bullfrogs are an important prey source for many birds (especially large herons), North American river otters (Lontra canadensis), predatory fish, and occasionally other amphibians.

Adult Predator Range

Predators of adult American bullfrogs range from 150 g (5.3 oz) belted kingfishers (Megaceryle alcyon) to 500 kg (1,100 lb) American alligators (Alligator mississippiensis).

Egg and Larvae Predation

Bullfrog eggs and larvae are unpalatable to many salamanders and fish, but the high activity level of tadpoles makes them noticeable to predators not deterred by their bad taste.

Human Hunting for Food

Humans hunt bullfrogs as game and eat their legs.

Threat Escape Behaviors

When threatened, adult bullfrogs escape by splashing and leaping into deep water. Trapped individuals may squawk or let out a piercing scream that can startle attackers enough to allow escape.

Group Danger Response

An attack on one bullfrog alerts nearby other bullfrogs to danger, and all will retreat to the safety of deep water.

Snake Venom Resistance

Bullfrogs may be at least partially resistant to the venom of copperhead (Agkistrodon contortrix) and cottonmouth (Agkistrodon piscivorus) snakes, though these species, along with northern water snakes (Nerodia sipedon), are known natural predators of bullfrogs.

Invasive Competitive Traits

As an invasive species, multiple traits of L. catesbeianus boost its competitive ability. Its generalist diet lets it feed in many different environments.

Juvenile Predation Adaptation

Stomach content analysis found that adult bullfrogs regularly eat predators of bullfrog young, including dragonfly nymphs, garter snakes, and giant water bugs. This reduces the effectiveness of natural ecological population controls on bullfrog juveniles in invaded areas.

Antipredator Defense Immunity

L. catesbeianus also shows immunity or resistance to the antipredator defenses of other organisms: stomach content analysis from New Mexico bullfrog populations shows they regularly eat wasps with no conditioned avoidance of stingers; along the Colorado River, stomach contents show they can tolerate the uncomfortable spines of stickleback fish; there are also reports of American bullfrogs eating scorpions and rattlesnakes.

Niche Shift Capability

Analysis of the American bullfrog's realized niche at various sites in Mexico, compared to niches of endemic Mexican frogs, suggests the American bullfrog is capable of niche shift, and can threaten many endemic Mexican frog species even when they are not currently competing directly.

Human Food Use

For human use, the American bullfrog serves as a food source, especially in the Southern and some Midwestern areas of the United States.

Traditional Night Hunting Method

Traditional hunting methods involve paddling or poling silently through ponds or swamps at night via canoe or flatboat; once a frog's call is heard, a light is shined on the frog to temporarily stop it from moving, and it will not jump into deeper water if approached slowly and steadily. When close enough, the frog is gigged with a multiple-tined spear and brought aboard.

Land Hunting Methods

Bullfrogs may also be stalked on land, with great care taken not to startle them. In some U.S. states, breaking the skin during capture is illegal, so grasping gigs or hand capture are used.

Culinary Preparation

Like most frogs, only the hind legs are commonly eaten; when cooked, they resemble small chicken drumsticks, have a similar flavor and texture, and can be prepared the same way.

Commercial Farming Challenges

Commercial bullfrog farming in near-natural enclosed ponds has been attempted, but faces many difficulties. Though pelleted feed exists, bullfrogs usually will not eat artificial diets, and providing enough live prey is challenging.

Additional Farming Risks

Disease is also a common problem even when sanitary conditions are carefully maintained. Other challenges include predation, cannibalism, and poor water quality.

Farm Escape Risks

The large, powerfully leaping frogs inevitably escape, and can then harm native frog populations.

Global Frog Leg Trade

Countries that export bullfrog legs include the Netherlands, Belgium, Mexico, Bangladesh, Japan, China, Taiwan, and Indonesia; most of these frogs are wild-caught, though some are captive-raised. The United States is a net importer of frog legs.

Great Depression Farming Scheme

During the Great Depression, businessman Dr. Albert Broel promoted bullfrog farming as a way for people facing economic hardship to earn extra money by raising and selling bullfrogs. He published instructional manuals including Frog Raising for Pleasure and Profit, and offered in-person training, plus a guarantee to purchase all adult frogs that independent breeders raised to adult weight.

Scheme Legacy

The scheme was ultimately unsuccessful for the same reasons listed above, but its business model of outsourcing livestock raising to independent farmers is still used today, especially in the poultry industry.

Educational and Cultural Use

The American bullfrog is also used as a dissection specimen in many biology and anatomy classes in schools around the world, and it is the official state amphibian of Missouri, Ohio, and Oklahoma.