Bullfrogs are the largest real frog found in North America, weighing up to 0.5 kg and measuring 460 mm in length. Their average length is 100-175 mm. Their color varies from brownish to shades of green, often with spots or blotches of a darker color around their backs. Their back feet are fully webbed. The sex of an adult bullfrog can be found out easily by examining the size of the tympanum (the external ear of the frog) relative to size of the eye. The tympanum is a round circle located on the side of the head near the eye, and in males it is much larger than the eye. In females the tympanum is as large or smaller than the eye. Also, during the breeding season the throat of the male bullfrog is yellow, and the female's is white.
North American bullfrogs (Lithobates catesbeianus) are only native to the Nearctic region. They are found from Nova Scotia to central Florida, from the East coast to Wisconsin, and across the Great Plains to the Rockies. The natural western limits of this species are now confused due to their introduction into places as far west as California and Mexico. It is known that bullfrogs were introduced to areas of California and Colorado in the early 1900's. The species has also been introduced (accidentally or on purpose) into southern Europe, South America, and Asia.
North American bullfrogs need to live in water and are therefore usually found near some source of water, like a lake, pond, river, or bog. Warm, calm, shallow waters are their favorite places. Bullfrogs are becoming much more common in areas that have been changed by humans. Increased water temperatures and increased amounts of water plants, which are common signs of lakes that have been polluted by humans, favor bullfrogs by providing good habitats for growth, reproduction, and escape from predators.
About four days after fertilization, spotted tadpoles emerge from the floating egg mass. The tadpoles have gills and a tail, which eventually disappears as the tadpole transforms into a froglet. Tadpole development is quite slow; it may take between one to three years to begin transformation from the tadpole stage into the adult stage. Adults reach sexual maturity after an additional two years.
Breeding takes place in May to July in the north, and from February to October in the south. Fertilization is external, with the females depositing as many as 20,000 eggs in a foamy film in quiet, protected waters. Fertilization is usually, but not always, by one male. Tadpoles emerge about four days after fertilization. These tadpoles may remain in the tadpole stage for almost 3 years before transforming into frogs. Adults reach sexual maturity after 3 to 5 years.
Females provide the eggs with yolk before they are laid. There is no parental involvement in offspring after the eggs are laid. Newly hatched tadpoles can take care of themselves right away.
The average bullfrog lives seven to nine years in the wild. The record lifespan of an animal in captivity is 16 years.
North American bullfrogs prefer warm weather and will hibernate during cold weather. A bullfrog may bury itself in mud and construct a small cave-like structure for the winter. Their hunting style is 'sit and wait.' Bullfrogs can wait for a long time for some type of prey to come by, then, with a flash of the tongue, they grab it and bring it back into their mouths. Bullfrogs are active both during the day and at night; they are most active when the weather is moist and warm.
Adult males are very aggressive and defend their territories, which can range from 3 to 25 meters of shoreline, by physically wrestling with others.
The call of a male bullfrog has a low frequency and can be heard for over one kilometer. The sound is often described as a low rumbling "jug-o-rum". Bullfrogs also have a good sense of vision and sense vibrations. See a video of a bullfrog calling here: http://www.midwestfrogs.com/.
Bullfrogs are predators. They usually eat snakes, worms, insects, crustaceans, frogs, tadpoles, and aquatic eggs of fish, frogs, insects, or salamanders. They are cannibalistic and will not hesitate to eat their own kind. There have also been a few cases reported of bullfrogs eating bats. Bullfrog tadpoles mostly graze on aquatic plants.
Humans hunt bullfrogs for frog legs, but they have a limited hunting season in most states. Bullfrogs are also eaten by a wide variety of other animals, depending on the region. These include herons, such as great blue herons and great egrets, turtles, water snakes, raccoons, and belted kingfishers. Most fish are averse to eating bullfrog tadpoles because of their undesirable taste.
Introduced bullfrogs may be driving native frogs to extinction in some areas. Colorado, among many other places, is experiencing problems due to the introduced bullfrog population. Bullfrogs may have been introduced accidentally to trout streams and lakes during the Colorado Divisions of Wildlife fish stocking operations. Bullfrogs occasionally invade fish hatchery ponds and their larvae are caught along with the fishes that are routinely stocked in ponds.
North American bullfrogs help to control insect pests. They are important for medical research because their skeletal, muscle, digestive, and nervous systems are similar to those of other animals. They are often hunted for meat (frog legs).
Bullfrogs are dealing fairly well with the changes in the environment that have occurred due to human influences, and they are becoming increasingly common in areas that have been modified by humans. Bullfrogs have a much higher critical thermal maximum than most other frogs, meaning that they are able to thrive in higher water temperatures. Bullfrogs have a longer breeding season and a high rate of surviving through the tadpole stage, which allows them to be more successful than other frogs. In many areas, such as California and Colorado, bullfrogs are driving other frog populations to extinction. An interesting reason to explain why bullfrogs in California might have an advantage over other species that are native to that state is that bullfrogs evolved with many different kinds of fish in eastern North America. For many years in California people have been introducing new fish species that are predators of frogs. Bullfrogs have evolved ways to avoid being eaten by fish, such as tadpoles that are not active much of the time which reduces their exposure to predators. Native frog species of California are also suffering a decline because bullfrogs are intense predators on frog populations.
Bullfrogs are well known for their enormous legs. They are some of the best jumpers in the world and are used in frog racing in some parts of the United States.
Allison Poor (editor), University of Michigan-Ann Arbor.
Sandra Bruening (author), University of Michigan-Ann Arbor, Cynthia Sims Parr (editor), University of Michigan-Ann Arbor.
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