American Bullfrog

Rana catesbeiana
Wednesday, July 7, 2010

The American Bullfrog is the largest frog in this area of the US. It is a true frog, of the family Ranidae, and is commonly found in this area.  

Bullfrogs are typically greenish to brownish, with mottled backs and pale, mottled undersides. They can grow to more than 20 cm, although the average is between 9 and 15 cm for a full-grown bullfrog. Discrimination between sexes is made possible by examining the tympanum, or external ear, of the mature frog visible as a darker ring below its eye. Male frogs’ tympanum will be much larger than their eye; females’ will be the same size or smaller.

Bullfrogs are mainly aquatic creatures, and tend to dwell in permanent, still or slow-moving waters. They prefer warm waters, often on pond borders or shallow rivers. Bull frogs are easily identifiable by their calls, a deep chug-a-rum, chug-a-rum. These calls can be heard from over a kilometer away.

The bullfrog is a species native to most of the eastern US, ranging as far out as Wisconsin. However, it has been introduced, accidentally or on purpose, to many western states so that its native range is now confused. The bullfrog has also been introduced to Europe, South America, and Asia, in some cases because of their suitability to hunt for the frog leg market.Bullfrogs spend a great deal of time “sitting and waiting” for their prey. They eat most things typical of ranids: larvae, minnows, tadpoles, insects, worms, salamanders, etc. However, they have also been found to eat small snakes, turtles, birds, and even bats. They are also cannibalistic and will eat one of their own when the opportunity presents itself. The tadpoles generally graze on aquatic plants.

Conversely, smaller bullfrogs are commonly eaten by snakes, birds, and turtles, as well as raccoons and larger birds such as herons, among a wide range of area-specific predators. Most fish ignore the tadpoles because of their bad taste.

When mating, bullfrogs, as with all ranid frogs, fertilize the eggs externally. At some point during the breeding season, which usually begins in late spring or early summer, the mating female will deposit as many as 20,000 eggs inside a semi-transparent protective film at the same time as the male releases sperm into the water. The fertilized eggs will hatch in 3-5 days, and the tiny tadpoles will emerge with gills and a tail. They take anywhere between a few months to 3 years to transform into frogs, depending on the temperatures of the area, after which time they take another 2 years to become sexually mature. Bullfrogs can live for around 8-10 years in the wild, although one specimen in captivity reached age 16. Adult bullfrogs breathe air, although they still spend a lot of time in the water.

This bullfrog is an adult female, captured by Leleña after it had become trapped in a window well behind Lewis. The froglet, tadpole, and eggs were collected in the pond.

Article by Hazel Galloway

Images of this organism can be viewed here, courtesy of Animal Diversity Web (University of Michigan Museum of Zoology). 


  • Bullfrog