American Bullfrog (Rana catesbeiana*)

Photo by P. Brown

American Bullfrog. Photo by P. Brown

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Editor’s Note: We follow Amphibiaweb and Yuan et al. (2016: Systematic Biology, doi: 10.1093/sysbio/syw055) in our usage of Rana rather than Lithobates for Arizona’s ranid frogs.

The American Bullfrog is the largest (< 220 mm SVL, Thomas and Wogan [1999]) frog in the 100-Mile Circle and Arizona, and may be the largest anuran, although exceptionally large Sonoran Desert Toads probably exceed the largest American Bullfrogs in weight.  This sizable, smooth-skinned frog lacks dorsolateral folds but has a prominent fold of skin above and behind the tympanum.  The  tympanum is especially distinct and is 1-2 times the diameter of the eye. Dorsally it is olive gray, sometimes with a net-like dark reticulation, but never having round, distinct dark spots as in the leopard frogs.  The chin is mottled and the snout is rounded.  It is most similar to the Tarahumara Frog, but that species is smaller (<114 mm SVL), exhibits rougher skin, has an indistinct tympanum smaller than the eye, and it lacks mottling on the chin.  The Tarahumara Frog also has a limited distribution in the 100-Mile Circle and is a habitat specialist (see the account for that species).  American Bullfrogs, especially juveniles, may emit an alarm call – an “eeep!” when startled and as they jump into the water.  Leopard frogs and Tarahumara Frogs do not vocalize when startled (although they may emit a release call – a squawk – if grabbed by hand or by a predator).

The American Bullfrog is an invasive, introduced species in Arizona and the 100-Mile Circle.  It is native to the central and eastern United States and was brought to Arizona in the late 19th century or early 20th century as a game species.  The Arizona Game and Fish Department (AGFD) actively stocked lakes and rivers with Bullfrogs from the 1920s through at least 1982, and subsequently the frogs moved about on the landscape on their own and were probably further transported, intentionally and unintentionally, by people as well.  As a result, the American Bullfrog is widespread in the 100-Mile Circle almost everywhere permanent water is found and in most vegetation community types from arid Sonoran desertscrub upslope into pine-oak woodland.  The species is absent from the highest mountains, but that is probably more a function of availability of permanent bodies of water, rather than cold or other factors.  The American Bullfrog is not as well distributed in Sonora, probably because introduction and spread of aquatic exotic species there is a decade or two behind what has occurred in Arizona.

Breeding sites are typically deep pools with quiet, permanent water.  American Bullfrogs do not thrive in or are absent from high gradient streams, shallow flowing water, or systems prone to heavy flooding.  Breeding in the lower elevation sites typically begins in March or April and continues into the summer months.  Breeding is delayed at higher elevations.  The male advertisement call is a loud, low “jug-o-rum”.  This and other calls of the American Bullfrog can be heard here: 

Females sometimes make a softer, higher pitched croaking sound. Clutches of up to almost 48,000 eggs are laid in quiet water as thin sheets covering up to 1 m2 of the water’s surface.  The tadpoles take from several months to two years to metamorphose, and grow to (rarely) over 200 mm total length, which is larger than the tadpoles of any other species in the 100-Mile Circle.  The tadpole’s body and head are characteristically marked with small black dots that are particularly notable in larger tadpoles.  Metamorphs resemble adults but are mostly 35-45 mm SVL.  The tadpoles are primarily herbivorous.  Juvenile and adult American Bullfrogs eat most anything that moves and fits into their disturbingly large mouths.  They consume both terrestrial and aquatic animals, and may at times eat large numbers of tadpoles and small frogs of their own kind.  Because of its opportunistic feeding habitats, large size, and often high densities, American Bullfrogs are capable of eliminating populations of some species of gartersnakes, leopard frogs, and turtles in simple aquatic systems, and this introduced frog is in part responsible for the decline of native aquatic herpetofauna in the Southwest.  It also becomes infected with the pathogen that causes chytridiomycosis, a fungal skin disease that kills native Arizona frogs.  In the Bullfrog, these infections are often sub-lethal, and as a result, it may be an important carrier of the disease.   The dispersal ability of this species, up to 11 km overland, makes it a formidable threat that is not easily extinguished.

Within the 100-Mile Circle, the American Bullfrog is both a pest and a game species.  Although no longer stocked by the AGFD, it is still considered a desirable game species by many.  A fishing license is needed to take a Bullfrog in Arizona. Major eradication programs have eliminated American Bullfrogs from several areas in the 100-Mile Circle as a means to recover the Chiricahua Leopard Frog and other vulnerable native aquatic species.  Most of the literature on this species is under the name Rana catesbeiana.

Suggested Reading: 

Brennan, T.C., and A.T. Holycross. 2006. Amphibians and Reptiles in Arizona. Arizona Game and Fish Department, Phoenix, AZ.

Bury, R.B., and J.A. Whelan. 1984. Ecology and management of the bullfrog. U.S. Department of the Interior, Fish and Wildlife Service, Resource Publication 155:1-23.

Casper, G.S., and R. Hendricks. 2005. Rana catesbeiana Shaw, 1802 American Bullfrog.  Pages 540-546 in: M.J. Lannoo (editor), Amphibian Declines: The Conservation Status of United States Species.  University of California Press, Berkeley.

Schwalbe, C.R., and P.C. Rosen. 1999.  Bullfrogs – the dinner guests we’re sorry we invited. Sonorensis 19:8-10.

Rosen, P.C., and C.R. Schwalbe. 2002. Widespread effects of introduced species on reptiles and amphibians in the Sonoran Desert region.  Pages 220-240 in B. Telman (editor), Invasive Exotic Species in the Sonoran Desert Region, The University of Arizona Press and the Arizona-Sonora Desert Museum, Tucson.

Thomas, L.A., and G.O.V. Wogan. 1999. Rana catesbeiana record size. Herpetological Review 30(4):223-224.

Author:  Jim Rorabaugh

For additional information on this species, please see the following volumes and pages in the Sonoran Herpetologist: 2006 Apr:41-42; 2006 Jul:74-77.


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