The American Bullfrog (Lithobates catesbeianus) is a widespread amphibian native to eastern North America. Its range extends from Nova Scotia to central Florida and westward across the Great Plains to the Rocky Mountains. It is commonly found in various freshwater environments.
Physical Characteristics
The American Bullfrog is the largest frog in North America. Adults typically range from 9 to 15.2 cm (3.5 to 6 inches) in length, with some reaching up to 20.3 cm (8 inches) and weighing up to 0.5 kg. Their smooth skin varies from green to brown, often with darker spots or blotches on their backs. The underside is generally white to cream, sometimes mottled with dark pigment.
A large, circular eardrum (tympanum) is visible behind each eye. In males, this tympanum is larger than the eye, while in females, it is about the same size or smaller. The American Bullfrog lacks dorsolateral folds. Their hind feet are fully webbed, with the longest toe extending beyond the webbing.
Habitat, Diet, and Vocalizations
American Bullfrogs inhabit permanent freshwater bodies such as ponds, lakes, swamps, marshes, and slow-moving rivers. They prefer warm, still, shallow waters with abundant vegetation. They are also found in human-modified environments like pools, canals, and ditches.
Their diet is carnivorous and opportunistic, consuming nearly any small animal. Prey includes invertebrates (insects, snails, worms) and vertebrates (small rodents, lizards, snakes, other frogs, birds, and bats). Bullfrog tadpoles primarily graze on aquatic plants, algae, and invertebrates.
Male American Bullfrogs produce a deep, resonant call, often described as a “jug-o-rum” or a bull’s bellow. This call serves for territorial defense and attracting mates during the breeding season. Bullfrogs also emit a short, fast squeak as an alarm call before leaping into water to escape danger.
Reproduction and Development
The breeding season for American Bullfrogs typically occurs once a year, from late spring into summer. Males arrive at breeding sites first and establish territories, often forming choruses, which attract females. Males use various calls for territorial warnings, confrontation, and mating.
During mating, the female releases a large egg mass, containing up to 20,000 eggs, in a foamy film on the surface of quiet waters. Fertilization is external by one male. Eggs hatch into tadpoles in about four days. Tadpoles are greenish-yellow with small black dots and can reach 15 cm (6 inches) in length.
The tadpole stage is extended, lasting one to three years before metamorphosis. During this period, tadpoles develop legs, their tails disappear, and lungs replace gills, preparing them for adult life. Adults reach sexual maturity after two to five years.
Ecological Role
In their native ecosystems, American Bullfrogs are both predators and prey. Their size and appetite allow them to consume a wide range of animals, contributing to the food web. However, their introduction to non-native regions has led to ecological disruptions, establishing them as an invasive species globally, including the western United States, Europe, Asia, and South America.
As invasive species, bullfrogs threaten native wildlife. They outcompete indigenous species for food and habitat due to their size and reproductive rate. Their predatory behavior can lead to declines in native amphibian and other small vertebrate populations. American Bullfrogs can also act as carriers for the chytrid fungus (Batrachochytrium dendrobatidis), a pathogen causing chytridiomycosis, a deadly disease for many amphibian species.