Introduced Species Summary Project

North American Bullfrog (Rana castesbeiana)

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Common Name:  (North American) Bullfrog

Scientific Name:  Rana catesbeiana                                                          


         Kingdom:  Animalia

         Phylum or Division:  Chordata
         Class:  Amphibia Linnaeus
         Order: Anura
         Family:  Ranidae
         Subfamily:  Rafinesque

Identification:  The North American bullfrog is the largest frog in North America.  It can reach over 6 inches in length, with males weighing up to one pound.  One of its unique physical characteristics is the absence of “dorsolateral ridges”.  Many frogs have dorsolateral folds which are lines of raised glandular skin in an area between the back and the sides (see Figure 2).   In North American frogs, there are usually two ridges running parallel to the midline of the body.  The bullfrog is an exception in that its dorsal folds start behind the eyes and go back and down around the ears or the “tympanic membrane” (ear drum).  Short folds of skin extend from over the eye to behind this ear drum.  In males, the ear drum is larger than the eye, while in females it is about the same size or smaller.  Thus, the dorsolateral features and the tympanic membrane help identify the bullfrog and distinguish it from other frogs.  Other features include the following.  The bullfrog’s hind feet are completely webbed except for the last joint of the largest toe.  It can range in color from dark olive to pale green, but may be brownish.  Tadpoles have a light ventral surface (belly, front side) and the dorsal surface (back area)  is greenish to yellow with numerous black spots.  The bullfrog’s call is a deep “jug-o-rum” or “br-wum” bellow, made day and night, and can be heard up to ¼ mile away.  In breeding season, the throat of the male is yellow, whereas the female’s is white.  Unlike other frogs, it spends most of its time in water from where it also does most of its hunting.

           Figure 2:  Green frog with dorsolateral folds


Original Distribution:  The North American bullfrog is native to the central and eastern United States and the southern portions of Ontario and Quebec. 

Current Distribution:  North American bullfrogs are now found from Nova Scotia to Central Florida, from the East coast of the U.S. to Wisconsin, and across the Great Plains to the Rockies.  The natural western limits of this species have reached as far west as California and Mexico.   It is also found in Hawaii, the Caribbean, South America, Europe and Asia.

            Figure 3: U.S. Distribution of bullfrog

Site and Date of Introduction:  Several theories exist, but it is believed to be introduced from its original habitat in North America in the early 1900s in Colorado and California.   

Mode(s) of Introduction:  There seem to be several modes of introduction.  Its original invasion appears to have been through accidental introduction to trout streams and lakes during the Colorado Divisions of Wildlife fish stocking operations in the early 1900s.  Bullfrogs occasionally invade fish hatchery ponds and their larvae are caught along with fishes routinely stocked in ponds and reservoirs.   Other modes of introduction include: (1) via the aquarium trade: as released pets no longer suitable for home aquariums; (2) landscape/fauna “improvement”: bullfrogs are deliberately introduced as a harvestable game animal (frogs legs) or aesthetically pleasing wildlife; (3) pest control: bullfrogs have been deliberately introduced to control agricultural pests; (4) self-propelled: bullfrogs are capable of considerable overland travel and are known to disperse throughout entire watersheds, provided they have interspersed patches of suitable habitat (similar to the corridor concept); and (5) aquaculture: bullfrogs are often imported into a country or area for commercial production as food (frog legs), and as noted above can inadvertently be introduced along with fishes raised in hatcheries where bullfrog larvae are abundant.

Reason(s) Why it has Become Established:  Most fish appear to be averse to eating bullfrog tadpoles because of their undesirable taste, so the bullfrog has been able to thrive in freshwater lakes and rivers without these fish predators.   Bullfrogs are predators and usually feed on snakes, worms, insects, crustaceans, frogs, and tadpoles, so it has a varied menu and diet.  They are also  cannibalistic and will not hesitate to eat their own kind.  Bullfrogs easily adapt to environments modified by human beings.  For example, increased water temperatures and resulting aquatic vegetation, which are common factors in lakes polluted by humans, favor bullfrogs by providing suitable habitats for growth, reproduction and escape from predators.  Bullfrogs also have a longer breeding season and a higher rate of “pre-metamorphic survivorship” (tadpoles), which also allows them to be more successful than other frogs.  One possible reason the bullfrog has thrived in California is that, in eastern North America (its native habitat) bullfrogs evolved with a diverse predatory fish fauna, so that it has evolved mechanisms that enable it to avoid predation by fish in various environments. 

Ecological Role:  As it consumes a wide variety of animals and insects, its main ecological role is in controlling populations of insect pests and, where abundant, other animals such as snakes and mice to control their populations.

Benefit(s):  North American bullfrogs help to control insect pests and are important for medical research because their skeletal, muscle, digestive, and nervous systems are similar to those of higher animals.  They are often hunted or bred for meat (frog legs). 

Threat(s):  They may be driving native frog populations and species to extinction in some areas in the U.S. and Canada, negatively affecting biodiversity.  Adults consume any animal that can be swallowed, such as snakes, birds, fish, crawfish and other frogs.  To the extent that these animals may be endangered in certain habitats shared by the bullfrog, extinction could ensue.  Another threat is that bullfrog larvae can have a significant impact on algae, which can affect the aquatic community structure.

Control Level Diagnosis:  Medium Priority: although the population may be increasing in some regions, it may actually be declining or sustainable in others, particularly where it is regularly harvested as game (e.g., Rhode Island).  The invasion problem may be more acute in the western U.S., particularly in California where Highest Priority may be appropriate in certain areas where other frog populations are put in danger of extinction.   Therefore, I would recommend that the bullfrog population be more carefully and selectively controlled in some regions of the U.S., while in others, natural or anthropogenic controls may already be in place (harvesting, fish introduction, collection).   Care should be taken to avoid sweeping control measures where they are not appropriate. 

Control Method:  In California, there have been attempts to control bullfrog populations by introducing new fish species that are their predators.  However, as mentioned above, bullfrogs have evolved mechanisms to avoid predation by fish, such as unpalatable eggs and tadpoles, as well as tadpoles that have evolved to be inactive much of the time, which reduces their exposure to predators.  Some control measures are already effectively in place.  For example, in the U.S., both native and introduced populations are often managed as fisheries which are held to low sustainable levels by harvesting pressure.  Adults can be taken by a variety of methods, including shooting, spears/gigs, bow and arrow, clubs, nets, traps, angling, and by hand.  Collection at night is facilitated by the frogs’ immobilization in response to bright light.  Larvae may be killed by water-borne poisons that are also used for fish.  Collecting egg material can also be an effective control method. 


1.                  Global Invasive Species Database, Rana catesbeiana (amphibian),

2.        , Species Profile, Bullfrog,

3.                  Integrated Taxonomic Information System (ITIS) Report, Rana catesbeiana,

4.                  United States Department of Agriculture (USDA), Forest Service,

5.                  United States Department of Agriculture (USDA) Southern Regional Aquaculture Center (SRAC) Publication No. 436, “Bullfrog Culture”, March 1999, by C. Greg Lutz and Jimmy L. Avery.

6.                  United States Geological Survey, Northern Prairie Wildlife Research Center,

7.                  University of Michigan’s Animal Diversity Web,

8.                  University of Victoria, “Natural History of the American Bullfrog Rana catesbeiana in British Columbia”, 3.htm.

Photos and Map:  From U.S. Geological Survey, Northern Prairie Wildlife Research Center, www.npwrc.usgs/gov/narcam/idgide/bullfrog.htm


Author: Martin Murphy
Last Edited:  February 17, 2003

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