The bullfrog is the largest true frog found in North America. The average length is 10 - 17 cm. The colour varies from brownish to shades of green, often with spots or blotches of a darker colour about the back. The hind feet are fully webbed. The sex of an adult bullfrog can be easily determined by examining the size of the tympanum (the external ear of the frog) relative to that of the eye. The tympanum is a round circle located on the side of the head near the eye, and in males it is much larger than the eye. In females the tympanum is as large or smaller than the eye. Also, during the breeding season the throat of the male bullfrog is yellow, whereas the female's is white.

The American Bullfrog must live in water and is therefore usually found near some source of water, such as a lake, pond, river, or bog. Warm, still, shallow waters are preferred. Bullfrogs are becoming increasingly common in areas that have been modified by humans. Increased water temperatures and increased aquatic vegetation, which are common factors of lakes polluted by humans, favour bullfrogs by providing suitable habitats for growth, reproduction, and escape from predators. Bullfrogs have a much higher critical thermal maximum than most other frogs, meaning that they are able to thrive in higher water temperatures. Bullfrogs have a longer breeding season and a higher rate of premetamorphic survivorship, which also allows them to be more successful than other frogs. Colorado, among many other places, is experiencing problems due to the introduced bullfrog population. Bullfrogs may have been introduced accidentally to trout streams and lakes during the Colorado Divisions of Wildlife fish stocking operations. Bullfrogs occasionally invade fish hatchery ponds and their larvae are caught along with the fishes routinely stocked in ponds and reservoirs. Most fish are adverse to eating bullfrog tadpoles due to their undesirable taste.

Biomes: freshwater lakes and rivers

The American Bullfrog is found from Nova Scotia to central Florida, from the East coast to Wisconsin, and across the Great Plains to the Rockies. The natural western limits are now confused due to the introduction of this species into places as far west as California, Hawaii and Mexico. It is known that bullfrogs were introduced to areas of California, Colorado and Hawaii in the late 1800's.

Males attract females for mating with their calls. Breeding takes place from February to August. Fertilization is external, with the females depositing as many as 25,000 eggs in a foamy film in quiet protected waters. Fertilization is usually but not always by one male. About four days after fertilization, spotted tadpoles emerge from the floating egg mass. The tadpoles have gills and a tail, which eventually disappears as the tadpole transforms into a full grown bullfrog. Tadpole development is quite slow; it may take as long as three years to begin transformation from the tadpole stage into the adult stage. Adults reach sexual maturity after an additional two to three years.

Bullfrogs are predators. They usually feed on snakes, worms, insects, crustaceans, frogs, and tadpoles. They are cannibalistic and will not hesitate to eat their own kind.

The bullfrog prefers warm weather and will hibernate during the cold. A bullfrog may bury itself in mud and construct a small cave-like structure under water for the winter. The call of a male bullfrog has a low frequency and can be heard over a distance of one kilometre. The sound is often described as a low rumbling "jug-o-rum" or "brr-uum". Adult males are very aggressive and defend their territories, which can range from 3 - 25 meters of shoreline, by physically wrestling with others. Their hunting style is 'sit and wait.' Bullfrogs wait for some type of prey to come by, then with a flash of the tongue, they grab it and bring it back into the mouth.

Bullfrogs are dealing fairly well with the changes in the environment that have occurred due to humans, and they are becoming increasingly common in areas that have been modified by humans. In many areas, such as California, bullfrogs are driving other frog populations to extinction. An interesting reason to explain why bullfrogs in California might have an advantage over other species native to that state is that bullfrogs evolved with a diverse predatory fish fauna in eastern North America. For many years in California people have been introducing new fish species that are predators of frogs. Bullfrogs have evolved mechanisms to avoid predation by fish, such as less palatable eggs and tadpoles, and tadpoles that are not active much of the time, which reduces their exposure to predators. Native frog species of California are also suffering a decline because bullfrogs are intense predators on frog populations.

Rana clamitans - Green Frog

Rana grylio - Pig Frog

The bullfrog helps to control insect pests. Also, bullfrogs are important for medical research because their skeletal, muscle, digestive, and nervous systems are similar to those of higher animals. They are often hunted for meat (frog legs). Introduced bullfrogs may be driving native frogs to extinction in some areas. Hawaiian name for the Bullfrog is Poloka lana.

McKeown, Sean. A Field Guide to Reptiles and Amphibians in the Hawaiian Islands. Diamond Head Publishing Company, 1996

Discovery Channel. reptiles & amphibians: An Explore Your World Handbook. New York: Discovery Books, 2000

Behler, John. Familiar Reptiles and Amphibians of North America. New York: Alfred A. Knopf, 1988

Conant, Roger. Field Guide to Reptiles and Amphibians of Eastern and Central North America. Boston: Houghton Mifflin Harcourt, 1998