The American bullfrog (Lithobates catesbeianus), often simply known as the bullfrog in Canada and the United States, is a large true frog native to eastern North America. It typically inhabits large permanent water bodies such as swamps, ponds, and lakes.
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 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 (9 to 15 cm) in snout–to–vent length. They grow fast in the first eight months of life, typically increasing in weight from 5 to 175 g (0.18 to 6.17 oz), and large, mature individuals can weigh up to 500 g (1.1 lb). In some cases bullfrogs have been recorded as attaining 800 g (1.8 lb) and measuring up to 8 in (20 cm) from snout to vent. The American bullfrog is the largest species of frog in North America.
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.[16] 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.
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.[18] 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.
The bullfrog breeding season typically lasts two to three months.[23][24] 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 (9.8 to 19.7 ft) apart and call loudly.[25][26] 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,[23] 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.[25][26] The sex ratio is typically skewed toward males.[28] 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.[23][28] 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.[27][29][30][31] 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.[32] 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.[23] 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.[23][27] 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.
To establish social dominance within choruses, bullfrogs demonstrate various forms of aggression, especially through visual displays. Posture is a key factor in establishing social position and threatening challengers.[27] Territorial males have inflated postures while non-territorial males remain in the water with only their heads showing. For dominant (territorial) males, their elevated posture reveals their yellow-colored throats.[25][27] When two dominant males encounter each other, they engage in a wrestling bout. The males have their venters clasped, each individual in an erect position rising to well above water level.[27] The New Jersey study noted the males would approach each other to within a few centimeters and then tilt back their heads, displaying their brilliantly colored gular sacs. The gular is dichromatic in bullfrogs, with dominant and fitter males displaying yellow gulars. The New Jersey study also reported low posture with only the head exposed above the water surface was typical of subordinate, or non-territorial males, and females. High posture was demonstrated by territorial males, which floated on the surface of the water with their lungs inflated, displaying their yellow gulars.[28] Males optimize their reproductive fitness in a number of ways. Early arrival at the breeding site, prolonged breeding with continuous sexual activity throughout the season, ownership of a centrally located territory within the chorus, and successful movement between the dynamically changing choruses are all common ways for males to maintain dominant, or territorial, status within the chorus. Older males have greater success in all of these areas than younger males.[23] Some of the males display a more inferior role, termed by many researchers as the silent male status. These silent males adopt a submissive posture, sit near resident males and make no attempt to displace them. The silent males do not attempt to intercept females but are waiting for the territories to become vacant. This has also been called the alternate or satellite male strategy.
