American bullfrog

Appearance

The dorsal surface of the bullfrog has an olive-green basal color, either plain or with mottling and banding of grayish brown. The ventral 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 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 bigger 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 in in snout–to–vent length. They grow fast in the first eight months of life, typically increasing in weight from 5 to 175 g, and large, mature individuals can weigh up to 500 g. In some cases bullfrogs have been recorded as attaining 800 g and measuring up to 8 in from snout to vent. The American bullfrog is the largest species of true frog in North America.

Naming

The specific name, "catesbeiana" or "catesbeianus", is in honor of English naturalist Mark Catesby.In areas where the American bullfrog is introduced, the population can be controlled by various means. One project uses sterile triploïd bullfrogs. In Europe, the American bullfrog is included since 2016 in the list of Invasive Alien Species of Union concern. This implies that this species cannot be imported, bred, transported, commercialized, or intentionally released into the environment in the whole of the European Union.

Distribution

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. Bullfrogs have been found to feed on the young of the California endemic giant garter snake, a threatened species. In early 2023, the Utah Department of Natural Resources began tweeting tips on how to catch and cook bullfrogs in an effort to encourage residents to help control the growing population by catching the invasive frogs for food.

Other countries and regions into which the bullfrog has been introduced include the western provinces of Canada, Mexico, Cuba, the Dominican Republic, Jamaica, Italy, the Netherlands, Belgium, and France. It is also found in Argentina, Brazil, Uruguay, Venezuela, Colombia, China, South Korea and Japan. The reasons for introducing the bullfrog to these countries have included their intentional release, either to provide a source of food or as biological control agents, the escape of frogs from breeding establishments, and the escape or release of frogs kept as pets. Conservationists are concerned the bullfrog is relatively immune to the fungal infection chytridiomycosis and as it invades new territories, it may assist the spread of this lethal disease as an asymptomatic carrier to more susceptible native species of frog.

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 3 to 6 m 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.

These male and female behaviors cause male-to-male competition to be high within the bullfrog population and sexual selection for the females to be an intense process. Kentwood Wells postulated leks, territorial polygyny, and harems are the most likely classifications for the bullfrog mating system. Leks would be a valid description because males congregate to attract females, and the females arrive to the site for the purpose of copulation. In a 1980 study on bullfrogs in New Jersey, the mating system was classified as resource-defense polygyny. The males defended territories within the group and demonstrated typical physical forms of defense.

Food

Bullfrogs are voracious, opportunistic, ambush predators that prey on any small animal they can overpower and consume. Bullfrog stomachs have been found to contain rodents, small lizards and snakes, other frogs and toads, amphibians, crayfish, other crustaceans, 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, as well as the aquatic eggs of fish, frogs, insects, or salamanders. Bullfrogs are able to capture large, strong prey because of the powerful grip of their jaws after the initial ranid tongue strike. However, there is a correlation found with size of prey relative to body size of the bullfrog. Juveniles and adults typically go after prey that is relative to their own body size. 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. The comparative ability of bullfrogs to capture submerged prey, compared to that of the green frog, leopard frog, and wood frog was also demonstrated in laboratory experiments.

Prey motion elicits feeding behavior. First, if necessary, the frog performs a single, orienting bodily rotation ending with the frog aimed towards the prey, followed by approaching leaps, if necessary. Once within striking distance, the bullfrog begins its feeding strike, which consists of a ballistic lunge that ends with the mouth opening. At this stage, the fleshy, mucus-coated tongue is extended towards the prey, often engulfing it, while the jaws continue their forward travel to close just as the tongue is retracted. Large prey that do not fit entirely into the mouth are stuffed in with the hands. In laboratory observations, bullfrogs taking mice usually swam underwater with prey in mouth, apparently with the advantageous result of altering the mouse's defense from counter-attack to struggling for air. Asphyxiation is the most likely cause of death of warm-blooded prey.