|Natural range - red; |
Introduced range - dark green
The American bullfrog (Lithobates catesbeianus or Rana catesbeiana), often simply known as the bullfrog in Canada and the United States, is an amphibious frog, a member of the family Ranidae, or "true frogs". This frog has an olive green back and sides blotched with brownish markings and a whitish belly spotted with yellow or grey. The upper lip is often bright green and males have yellow throats. It inhabits large, permanent water bodies, such as swamps, ponds, and lakes, where it is usually found along the water's edge. The male bullfrog defends a territory during the breeding season. His call is reminiscent of the roar of a bull, which gives the frog its common name. This frog is native to southern and eastern parts of the United States and Canada, but has been widely introduced across other parts of North, Central and South America, Western Europe, and parts of Asia, and in some areas is regarded as an invasive species.
The bullfrog is harvested for use as food in North America and in several countries into which it has been introduced. It is also cultured in controlled environments, though this is a difficult and not always successful undertaking. Some international trade in frog legs occurs for human consumption. Bullfrogs are used in biology classes in schools for dissection and are sometimes kept as pets.
The dorsal (upper) surface of the bullfrog has an olive-green basal color, either plain or with a 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 end close to 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 in (9 to 15 cm) from snout to vent. 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) in length.
The bullfrog is native to eastern North America. Its natural range extends from the Atlantic Coast 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 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 into which the bullfrog has been introduced include Mexico, the western half of Canada, Cuba, Jamaica, Italy, the Netherlands, 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 to more susceptible native species of frog.
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 (9.8 to 19.7 ft) 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.
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.
Male bullfrogs aggregate into groups called choruses. The male chorus behavior is analogous to the lek formation of birds, mammals, and other vertebrates. Choruses are dynamic, forming and remaining associated for a few days, breaking down temporarily, and then forming again in a new area with a different group of males. Male movement has experimentally been noted to be dynamic. In the Michigan study, the choruses were described as "centers of attraction" in which their larger numbers enhanced the males' overall acoustical displays. This is more attractive to females and also attractive to other sexually active males. Choruses in this study were dynamic, constantly forming and breaking up. New choruses were formed in other areas of the site. Males moved around and were highly mobile within the choruses.
A review of multiple studies on bullfrogs and other anurans noted male behavior within the groups changes according to the population density of the leks. At higher population densities, leks are favored due to the difficulty in defending individual territories among a large population of males. This variance causes differences in how females choose their mates. When the male population density is low and males maintain clearer, more distinct territories, female choice is mostly determined by territory quality. When male population density is higher, females depend on other cues to select their mates. These cues include the males' positions within the chorus and differences in male display behaviors among other determinants.Social dominance within the choruses is established through challenges, threats, and other physical displays. Older males tend to acquire more central locations while younger males were restricted to the periphery.
Chorus tenure is the number of nights that a male participates in the breeding chorus. One study distinguishes between chorus tenure and dominant tenure. Dominant tenure is more strictly defined as the amount of time a male maintains a dominant status. Chorus tenure is restricted due to increased risk of predation, lost foraging opportunities, and higher energy consumption. Calling is postulated to be energetically costly to anurans in general. Energy is also expended through locomotion and aggressive interactions of male bullfrogs within the chorus.
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. Territorial males have inflated postures while nonterritorial males remain in the water with only their heads showing. For dominant (territorial) males, their elevated posture reveals their yellow-colored throats. 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. 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 nonterritorial 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. 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. 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.
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 0.5 to 1.0 m2 (5.4 to 10.8 sq ft). The embryos develop best at water temperatures between 24 and 30 °C (75 and 86 °F) and hatch in three to five days. If the water temperature rises above 32 °C (90 °F), developmental abnormalities occur, and if it falls below 15 °C (59 °F), 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. Maximum lifespan in the wild is estimated to be 8 to 10 years, but one frog lived for almost 16 years in captivity.
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 reptiles, amphibians, crayfish, birds, and bats, as well as the many invertebrates, such as 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 can 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. The comparative ability of bullfrogs to capture submerged prey, compared to that of the green frog, leopard frog, and wood frog (L. clamitans, L. pipiens, and L. sylvaticus, respectively) 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 (eyes closed as during all leaps) that ends with the mouth opening. At this stage, the fleshy, mucous-coated tongue is extended towards the prey, often engulfing it, while the jaws continue their forward travel to close (bite) 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.
Ballistic tongue projection of the related leopard frog is possible due to the presence of elastic structures that allow storage and subsequent release of elastic recoil energy. This accounts for the tongue projecting with higher power output than would develop by muscular action alone. Also, such mechanism relieves the tongue's musculature from physiological constraints such as limited peak power output - mechanical efficiency and thermal dependence by uncoupling the activation of the depressor mandibulae's contractile units from actual muscular movement. In other words, the kinematic parameters developed by contribution of the elastic structures differ from those developed by muscular projection, accounting for the difference in velocity, power output, and thermal dependence.
Bullfrogs are able to jump distances 10 times their body length due to the ability of activating muscular contractile units prior to extension of the plantaris muscle. In this process, sarcomeric contraction generates strain in elastic structures such as tendons, which in turn "catch" the energy generated by the strain. Such is subsequently released as the plantaris actively stretched during jumping, adding to the energy generated by muscular action alone, thus generating supramaximal powers that allow the bullfrog to jump great distances. Thus, the catch mechanism relies on elastic structures acting as muscle power amplifiers and their ability to absorb elastic strain energy and retain it for later explosive release during jumping.
The fact that the catch mechanism relies on elastic structures used as power amplifiers implies a series of conditions much convenient for the bullfrog. Sonomicrometry and electromyographic recordings of the plantaris muscle corroborate a relationship between presence of muscle-tendon units and decreasing, variable gearing. By supplying additional work output through the coupling of elastic strain energy, elastic structures relieve the musculature from the need of shortening at velocities that would otherwise hinder the amount of force generated, thus the magnitude of power output. This is also aided by a decreasing gearing, which makes it possible for the bullfrog to generate the desired velocity of movement by relieving the plantaris from strenuous shortening velocities, achieving the velocity desired by slower, forceful contractions. As in the case of its ballistic tongue projection, elastic structures in the plantaris muscle also confer it thermal independence. Jumping peak power output and mechanical efficiency of high-elevation frogs of the genera Hyla, Eleutherodactylus,Colostethus, and Atelopus, which achieve these at temperatures as low as 5 °C, are not statistically different from those achieved by tropical Bufo frogs at 35 °C.
Bullfrogs are an important item of prey to many birds (especially large herons), North American river otters (Lontra canadensis), predatory fish, and occasionally other amphibians. Predators of American bullfrogs once in their adult stages can range from 150 g (5.3 oz) belted kingfishers (Megaceryle alcyon) to 1,100 pound American alligators (Alligator mississippiensis). The eggs and larvae are unpalatable to many salamanders and fish, but the high levels of activity of the tadpoles may make them more noticeable to a predator not deterred by their unpleasant taste. Humans hunt bullfrogs as game and consume their legs. Adult frogs try to escape by splashing and leaping into deep water. A trapped individual may squawk or emit a piercing scream, which may surprise the attacker sufficiently for the frog to escape. An attack on one bullfrog is likely to alert others in the vicinity to danger and they will all retreat into the safety of deeper water. Bullfrogs may be at least partially resistant to the venom of copperhead (Agkistrodon contortrix) and cottonmouth (Agkistrodon piscivorus) snakes, though these species are known natural predators of bullfrogs as are northern water snakes (Nerodia sipedon).
The American bullfrog provides a food source, especially in the Southern and some areas of the Midwestern United States. The traditional way of hunting them is to paddle or pole silently by canoe or flatboat in ponds or swamps at night; when the frog's call is heard, a light is shone at the frog which temporarily inhibits its movement. The frog will not jump into deeper water as long as it is approached slowly and steadily. When close enough, the frog is gigged with a multiple-tined spear and brought into the boat. Bullfrogs can also be stalked on land, by again taking great care not to startle them. In some states, breaking the skin while catching them is illegal, and either grasping gigs or hand captures are used. The only parts normally eaten are the rear legs, which resemble small chicken drumsticks and can be cooked in similar ways.
Commercial bullfrog culture in near-natural enclosed ponds has been attempted, but is fraught with difficulties. Although pelleted feed is available, the frogs will not willingly consume artificial diets, and providing sufficient live prey is challenging. Disease also tends to be a problem even when great care is taken to provide sanitary conditions. Other challenges to be overcome may be predation, cannibalism, and low water quality. The frogs are large, have powerful leaps, and inevitably escape after which they may wreak havoc among the native frog population. Countries that export bullfrog legs include the Netherlands, Belgium, Mexico, Bangladesh, Japan, China, Taiwan, and Indonesia. Most of these frogs are caught from the wild, but some are captive-reared. The United States is a net importer of frog legs.