American Bullfrog Lithobates catesbeianus

Description: Adult Morphology: A large, green to brown frog with no dorso-lateral ridge. Dorsal coloration varies from dark brown to uniform green and ventral coloration is lighter with gray mottling. The throat of breeding males is usually bright yellow. The tympanum of males is quite large, while that of females is closer to eye-size.

Size: This is the largest frog in North America with adults reaching lengths of 6-8 in, though most adults are 4-5 in (95-145 mm) in length.

Larvae: Bullfrog tadpoles are similar to those of green frogs (Lithobates clamitans), but do not grow as large. Bullfrog tadpoles grow to 4 in long and have finer mottling on the tail fins, often restricted to the dorsal half of the fin.

Eggs: Eggs are deposited as a film on the surface, and as many as 10,000 eggs may make up a single mass.

Similar Species: Green frogs are often confused with bullfrogs, but green frogs have a dorso-lateral fold that bullfrogs lack and green frogs also have more distinct banding on their hind legs. In general, bullfrogs grow much larger than green frogs, but this is only helpful in identifying large adults.

Distribution: American bullfrogs are found throughout North America, although its native range is restricted to the eastern half of the continent. Bullfrogs have been introduced in western North America and in other countries where it has become an aggressively invasive species, eating and competing with native wildlife. Bullfrogs will likely continue expanding their non-native range with the help of accidental and purposeful introductions and are on their way to having a global distribution.

Bullfrogs are widespread and abundant throughout Indiana and are found in most permanent aquatic habitats.

Activity: Bullfrogs are active year round in southern Indiana, though they are generally inactive during cold weather. They are most active during their summer breeding season.

Breeding Season: This species breeds in Indiana from late May to early August and can be heard calling both day and night. Unlike spring breeding frogs, this species rarely exhibits concentrated, breeding congregations and can instead be heard calling almost any time during the summer both day and night.

Taxonomy: American bullfrogs (Lithobates catesbeianus) belong to the family ranidae which includes eight species of frogs in Indiana. All Indiana ranids were formerly placed in the genus Rana, but have since been moved to the genus Lithobates. Bullfrogs belong to the Aquarana species complex and their closest relatives in Indiana are green frogs (L. clamitans), which are similar in appearance and often mistaken for this species. These two frogs are both summer breeders that require permanent wetlands or streams for breeding.

Ecology: Habitat: Bullfrogs inhabit a wide array of aquatic situations, though permanent water is necessary for breeding. They prefer slow or still warm water and are often found in swamps, slow rivers, ponds, and lakes. During the spring, bullfrogs spend a considerable amount of time in and around ephemeral wetlands where they feed heavily on aquatic invertebrates and other breeding amphibians. They are seen throughout the summer sitting out along the shoreline of ponds and slow portions of rivers both day and night. When approached, bullfrogs will dive into the water and disappear, often under leaf litter or into vegetation. Though large bodies of water seem to be their preferred habitat, bullfrogs are also commonly found in small, rocky streams (especially juveniles).

Diet: Bullfrogs are voracious predators and will eat anything smaller than their mouth. Insects and crayfish make up a large part of the diet, though vertebrates like other frogs, tadpoles, small turtles, snakes, and small mammals are also eaten.

Reproduction and Life History: Bullfrogs breed in shallow, warm, vegetated waters and have a prolonged summer breeding season during which the males are highly territorial. Male bullfrogs defend small sections of the wetlands they breed in by vocalizing and aggressively chasing out intruders. The males are able to differentiate between the call of a neighbor and the call of an intruder and respond accordingly. They breed exclusively in permanent water because larval development takes one to three years. Bullfrogs are well adapted for breeding in wetlands inhabited by fish and their larvae are apparently distasteful and possibly toxic to some fish species. Unlike most other amphibians, bullfrogs are tolerant of polluted waters and will readily breed in urban ponds, lakes, and drainages.

Call: Bullfrogs have a deep, resonating call, which is often repeated in several minute intervals. Its call is described phonetically by many as "jug-o-rum."

Conservation: These frogs are abundant throughout the state, tolerating polluted waters and readily colonizing manmade ponds. They are considered a game species in Indiana and are harvested for their legs. This localized harvest pressure likely has little effect on populations. In some cases, these large, aggressive frogs may exclude or compete with other Anurans. Bullfrogs have also been documented as carriers of both Batrachochytrium dendrobatidis (chytrid fungus) and Ranavirus though in most cases they seem to be generally unaffected by the diseases. Between their potential as disease vectors, their aggressive predatory habits, and their tendency to utilize a wide range of aquatic habitats; the bullfrog is a devastating invasive species outside its native range and should never be released into an area it is not known to occur in. Bullfrogs are also known to feed on fish in hatchery ponds. The larvae in these ponds may be able to metamorphose within their first year fed on a diet of fish food.

Literature

Barrasso, D. A., R. Cajade, S. J. Nenda, G. Baloriani, and R. Herrera. 2009. Introduction of the American Bullfrog Lithobates catesbeianus (Anura: Ranidae) in Natural and Modified Environments: An Increasing Conservation Problem in Argentina. South American Journal of Herpetology 4.1:69-75.

Bee, M. A. and H. C. Gerhardt. 2001. Neighbour–stranger Discrimination by Territorial Male Bullfrogs (Rana catesbeiana): I. Acoustic basis. Animal Behaviour 62.6:1129-1140.

Brodman, R. 2003. Amphibians and Reptiles from Twenty-three Counties of Indiana. Proceedings of the Indiana Academy of Science 112.1:43-54.

Conant, R. and J. T . Collins. 1998. Reptiles and Amphibians of Eastern and Central North America. Third Edition, Expanded. Houghton Mifflin, New York, New York.

Corse, W. A. and E. Metter. 1980. Economics, Adult Feeding and Larval Growth of Rana catesbeiana on a Fish Hatchery. Journal of Herpetology 14.3:231-238.

Finkler, M. S. 2013. Lithobates catesbeianus. Bullfrog. Geographic distribution. Herpetological Review 44(3):472.

Gahl, M. K, A. J. K. Calhoun, and R. Graves. 2009. Facultative Use of Seasonal Pools by American bullfrogs (Rana catesbeiana). Wetlands 29.2:697-703.

Hillis, D. M. and T. P. Wilcox. 2005. Phylogeny of the New World True Frogs (Rana). Molecular Phylogenetics and Evolution 34.2:299-314.

Kruse, K. C. and M. G. Francis. 1977. A Predation Deterrent in Larvae of the Bullfrog, Rana catesbeiana. Transactions of the American Fisheries Society 106.3:248-252.

Minton, S. A. Jr. 2001. Amphibians and Reptiles of Indiana. Indiana Academy of Science, Indianapolis, IN.

Distribution Map
Distribution map of American Bullfrog (Lithobates catesbeianus)
Photographs