Description: The bullfrog (Rana catesbeiana) is the largest frog in the United States, with lengths ranging from 3.5 to 8 inches.1 Coloration on the back is olive, green or brown. Darkish bands and blotches are displayed on the legs, and there are usually dark spots on the back. A fold of skin extends from the eye around the eardrum. Male bullfrogs have a yellow throat and a swollen and darkened thumbcase. Also, the eardrums on males are larger than the eyes, while the female's eardrums are the same size as the eyes.1
Habitat: The bullfrog is highly aquatic, remaining in or near a permanent water source. It will enter marshes, ponds, lakes and streams in habitats ranging from desert oases to pine forests. It is usually found where there is quiet water with thick growth of cattails or other aquatic vegetation.1
Total Range: The bullfrog's native range extends from the Atlantic Coast eastward to eastern Colorado and eastern Mexico, and from southern Colorado south to northeastern Mexico. This species is not native west of the Rocky Mountains, but has been successfully introduced in many localities.1
Reproduction: In the western United States, bullfrogs breed from February through July.1 Spawning occurs in standing water, mostly during the night. There are 4,000 to 20,000 eggs deposited in water approximately 1.5 to 5 feet deep.2 Bullfrogs reach sexual maturity about one to three years after metamorphosis from the tadpole stage.
Diet: Adult bullfrogs will eat just about anything small enough for it to catch and swallow, including snakes, turtles, bats, birds and other frogs including their own young.3 Juvenile bullfrogs feed mainly on insects.2
Threat to Native Species and/or Environments: Bullfrogs have been widely introduced in the western United States as early as 1900 for sport, food and inadvertently during fish stockings. In the absence of effective predators, bullfrog populations can explode into unusually high densities, much higher than in their native range. Their presence has been blamed for the decline of native leopard frogs and the Mexican garter snake (Thamnophis eques), whose young fall prey to adult bullfrogs.
Although normally aquatic, juveniles can travel a mile during the monsoon season to colonize new ponds or other permanent water sources. This allows them to expand their range from the source where they were introduced. The large amount of eggs within each egg mass laid by the females allows the bullfrog to quickly establish itself within the new territory.3
Heavy trapping of adults and collection of their egg masses may eliminate or reduce bullfrog populations, but can be quite time consuming and expensive. If any manage to escape, they can quickly replace their numbers to what their were before these activities took place.3 Prevention through public education is probably the best and most effective measure to combat the expansion of bullfrogs into the native environment.
Bullfrogs in Pima County: Bullfrogs occur within stock ponds and possibly within urban lakes and gardens in eastern Pima County. Populations have been recently sighted within Cienega Creek Natural Preserve, which is home for the Lowland and Chiricahua leopard frogs, the latter being a federal "candidate" species under the Endangered Species Act.
It is legal to purchase live bullfrogs at a pet shop, but not to collect them alive from the wild. A fishing license is required to legally kill them.4
References
1 Stebbins, R. C. 1985. A field guide to western reptiles and amphibians. Second edition, revised. Houghton Mifflin Company, Boston.
2 New Mexico Department of Game and Fish et al. 1998. BISON-M: New Mexico Species List/Species Accounts. http://www.fw.vt.edu/fishex/states/nm.htm
3 Miera, V. May-June 1999. Simple Introductions-Major Repercussions: The story of bullfrogs and crayfish in Arizona. Wildlife Views. pp. 25-27.
4 Sredl, Mike, Arizona Game and Fish Department, Personal communication, September 2, 1999.