American Bullfrog

Category: Wildlife

For most people, the American bullfrog is the archetype for all frogs, aquatic, green, and loud. Oddly, you can tell the difference between adult males and females from the size of their ears, also called tympanum. In males, the tympanum is much larger than their eyes, while it is the same size or smaller than the eyes in females. Unlike many other types of frogs, they are active during the day as well the night and the deep, guttural calls of the males can be heard at any hour.

American Bullfrog

American Bullfrog

Scientific & Common Names

Kingdom - Animalia

Phylum - Chordata

Class - Amphibia

Order - Anura

Family – Ranidae

Genus - Rana (Lithobates is also used)

Species - Rana catesbeiana (Litobates catesbeianus is also commonly used)

Common names - American Bullfrog, Bullfrog

Characteristics

Bullfrogs are the largest native frogs found in the US and can reach sizes of six inches and weigh over a pound. They are traditionally thought of as green, but they can come in medium to dark brown as well. They are an aquatic species and will only leave a territory due to a threat or loss of prey. They prefer still waters, and they can be found in anything from large lakes to shallow, marshy areas. They have been kept in captivity for sixteen years, but their lifespan in the wild is 8-10 years.

Breeding

During breeding season, the throats of males become bright yellow and they form large choruses when they congregate to sing while attempting to attract females. When grouped together, the more aggressive males will fight for dominance and the chance to mate. Once a female has chosen her mate, she will deposit her eggs in his territory and he will fertilize them. The thousands of eggs will float at the top of the water in one transparent, jellied mass. A few days later, up to 6500 tadpoles will emerge as long as the temperature has remained with the range necessary for their development. If the temperature drops too low, the eggs will not develop and if temperatures reach close to 100°F, the tadpoles will mature, but there can be serious physical abnormalities. They initially are completely aquatic and breathe using gills, and they slowly transform from their legless, fishlike form while remaining underwater. The gills disappear and legs form as they begin to transition to an amphibious lifestyle. This transformation can take only a few months or, in the colder areas, several years.

Behavior

Young bullfrogs have been recorded moving six miles in just a matter of a few months, probably to avoid predation from larger bullfrogs who are both highly territorial and cannibalistic. Aside from their own kind, the carnivorous bullfrogs will eat just about anything they can fit into their mouths, including invertebrates, lizards, small mammals, and birds. They have even been shown to eat scorpions and snakes. Prey that is too large to easily fit into their mouths is sometimes stuffed in using their front legs like hands. Although they prefer the night, they will become active during the day if the conditions are tolerable.

History

There is fossil evidence tracing ancient frogs (proto-frogs) from the early Triassic period, but their lineage may go back to the Permian period (265ma). Of the 7300 species of amphibians, almost 6500 are frogs and toads, the rest are newts, salamanders, and caecilians. Although bullfrogs have no current threats, many other species are on the brink of extinction and they are susceptible to pollutants, bacterial infections, and changes in their environments. The fungal infection Chytridiomycosis ,caused by an aquatic fungal pathogen, Batrachochytrium dendrobatidis (Bd),could be responsible for the loss of hundreds of species of frogs. For their environments, frogs are the equivalent of canaries in the coal mine and they can indicate when there is a serious problem.

Present Status

Originally native to the eastern US west to the Rocky Mountains and parts of Canada, American bullfrogs are now one of the most invasive amphibians in the world. They can be found in all the western US states, Hawaii, the Caribbean, Europe, Asia, and South America. Attempts have been made at eradication, but no controls have proven effective.

References

  1. Peterson Field Guides Reptiles and Amphibians (Eastern/Central North America) by Roger Conant & Joseph T. Collins

  2. No Turning Back (The Life and Death of Animal Species} Richard Ellis

  3. North American Amphibians: Distribution and Diversity by David M. Green, Linda A. Weir , Gary S. Casper , Michæl J. Lannoo

  4. Amphibian Declines: The Conservation Status of United States Species by Michæl J. Lannoo

  5. Herpetology, Fourth Edition: An Introductory Biology of Amphibians and Reptiles by Laurie J. Vitt and Janalee P. Caldwell

  6. Frogs of the United States and Canada, 2-vol. set by C. Kenneth Dodd Jr.

  7. American Bullfrogs as Invasive Species: A Review of the Introduction, Subsequent Problems, Management Options, and Future Directions

  8. Nathan P. Snow and Gary Witmer USDA APHIS Wildlife Services, National Wildlife Research Center, Fort Collins, Colorado