Description: The dorsal (upper) surface of the bullfrog has an olive-green basal color, either plain or with mottling and banding of grayish brown. The ventral (under) surface is off-white blotched with yellow or gray. Often, a marked contrast in color is seen between the green upper lip and the pale lower lip. The teeth are tiny and are useful only in grasping. The eyes are prominent with brown irises and horizontal, almond-shaped pupils. The tympana (eardrums) are easily seen just behind the eyes and the dorsolateral folds of skin enclose them. The limbs are blotched or banded with gray. The fore legs are short and sturdy and the hind legs long. The front toes are not webbed, but the back toes have webbing between the digits with the exception of the fourth toe, which is unwebbed.
Bullfrogs are sexually dimorphic, with males being smaller than females and having yellow throats. Males have tympana larger than their eyes, whereas the tympana in females are about the same size as the eyes. Bullfrogs measure about 3.6 to 6 inches in snout–to–vent length. They grow fast in the first eight months of life, typically increasing in weight from 0.18 to 6.17 oz., and large, mature individuals can weigh up to 1.1 lb. In some cases bullfrogs have been recorded as attaining 1.8 lb. and measuring up to 8 in from snout to vent. The American bullfrog is the largest species of frog in North America.
Habitat: American bullfrogs inhabit ponds, swamps, lakes, reservoirs, marshes, brackish ponds (e.g., Hawaii), stream margins, and irrigation ditches, especially sites with abundant floating, emergent, or submerged vegetation. They may disperse from water in wet weather and sometimes are found in temporary waters hundreds of meters from permanent water. Bullfrogs sometimes make seasonal migrations to and from ephemeral bodies of water, or between adjacent permanent aquatic habitats. Individuals often remain in one pond or in a cluster of adjacent ponds throughout a season but may move up to a mile or more from one year to the next. Breeding sites are primarily permanent slow- or nonflowing bodies of water.
Range: The bullfrog is native to eastern North America. Its natural range extends from the Atlantic Coast, north to Newfoundland, to as far west as Oklahoma and Kansas. It is not found on offshore islands near Cape Cod and is largely absent from Florida, Colorado, Nebraska, South Dakota, and Minnesota. It has been introduced into Nantucket island, Arizona, Utah, other parts of Colorado and Nebraska, Nevada, California, Oregon, Washington, and Hawaii. In these states, it is considered to be an invasive species and concern exists that it may outcompete native species of amphibians and upset the ecological balance. It is very common on the West Coast, especially in California, where it is believed to pose a threat to the California red-legged frog, and is considered to be a factor in the decline of that vulnerable species.
Diet: Bullfrogs are voracious, opportunistic, ambush predators that prey on any small animal they can overpower and stuff down their throats. Bullfrog stomachs have been found to contain rodents, small lizards and snakes, other frogs and toads, amphibians, crayfish, small birds, scorpions, tarantulas and bats, as well as the many types of invertebrates, such as snails, worms and insects, which are the usual food of ranid frogs. These studies revealed the bullfrog's diet to be unique among North American ranids in the inclusion of a large percentage of aquatic animals, such as fish, tadpoles, ram's horn snails, and dytiscid beetles. Bullfrogs are able to capture large, strong prey because of the powerful grip of their jaws after the initial ranid tongue strike. The bullfrog is able to make allowance for light refraction at the water-air interface by striking at a position posterior to the target's perceived location.
Reproduction: The bullfrog breeding season typically lasts two to three months. A study of bullfrogs in Michigan showed the males arriving at the breeding site in late May or early June, and remaining in the area into July. The territorial males that occupy sites are usually spaced some 9.8 to 19.7 feet apart and call loudly. At least three different types of calls have been noted in male bullfrogs under different circumstances. These distinctive calls include territorial calls made as threats to other males, advertisement calls made to attract females, and encounter calls which precede combat.
The bullfrogs have a prolonged breeding season, with the males continuously engaging in sexual activity throughout. Males are present at the breeding pond for longer periods than females during the entire season, increasing their chances of multiple matings. The sex ratio is typically skewed toward males. Conversely, females have brief periods of sexual receptivity during the season. In one study, female sexual activity typically lasted for a single night and mating did not occur unless the females initiated the physical contact. Males only clasp females after they have indicated their willingness to mate. This finding refutes previous claims that a male frog will clasp any proximate female with no regard to whether the female has consented. Once a male finds a receptive female he will clasp onto her and undergo amplexus--reproductive position--by utilization of the males forelimbs. The enlargement forelimbs is a sexually dimorphic trait seen in the male bullfrog. One study investigating male and female bullfrog forelimbs muscles found males had significantly stronger muscles that could undergo longer durations of activity before the onset of fatigue. The significance of forelimb sexual dimorphism allow males to remain in amplexus with the female for longer durations increasing their chance at reproductive success in the highly competitive mating environment.
After selecting a male, the female deposits eggs in his territory. During the mating grasp, or amplexus, the male rides on top of the female, grasping her just behind her fore limbs. The female chooses a site in shallow water among vegetation, and lays a batch of up to 20,000 eggs, and the male simultaneously releases sperm, resulting in external fertilization. The eggs form a thin, floating sheet which may cover an area of 5.4 to 10.8 sq ft. The embryos develop best at water temperatures between 75 and 86 ℉ and hatch in three to five days. If the water temperature rises above 90 ℉, developmental abnormalities occur, and if it falls below 59 ℉, normal development ceases. Newly hatched tadpoles show a preference for living in shallow water on fine gravel bottoms. This may reflect a lesser number of predators in these locations. As they grow, they tend to move into deeper water. The tadpoles initially have three pairs of external gills and several rows of labial teeth. They pump water through their gills by movements of the floor of their mouths, trapping bacteria, single-celled algae, protozoans, pollen grains, and other small particles on mucus in a filtration organ in their pharynges. As they grow, they begin to ingest larger particles and use their teeth for rasping. They have downward-facing mouths, deep bodies, and tails with broad dorsal and ventral fins.
Time to metamorphosis ranges from a few months in the southern part of the range to 3 years in the north, where the colder water slows development.
Status: Historically, American bullfrogs were common in the United States. Today many native bullfrog populations appear to be declining, with habitat loss and degradation, water pollution, pesticide contamination, and over-harvesting commonly invoked as causal factors.
Subspecies: None
Taxonomy: This species was placed in the genus Lithobates by Frost et al. (2006). However, Yuan et al. (2016, Systematic Biology, doi: 10.1093/sysbio/syw055) showed that this action created problems of paraphyly in other genera. Yuan et al. (2016) recognized subgenera within Rana for the major traditional species groups, with Lithobates used as the subgenus for the Rana palmipes group. AmphibiaWeb recommends the optional use of these subgenera to refer to these major species groups, with names written as Rana (Aquarana) catesbeiana, for example.
Disclaimer: ITIS taxonomy is based on the latest scientific consensus available, and is provided as a general reference source for interested parties. However, it is not a legal authority for statutory or regulatory purposes. While every effort has been made to provide the most reliable and up-to-date information available, ultimate legal requirements with respect to species are contained in provisions of treaties to which the United States is a party, wildlife statutes, regulations, and any applicable notices that have been published in the Federal Register. For further information on U.S. legal requirements with respect to protected taxa, please contact the U.S. Fish and Wildlife Service.
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