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.
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.
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.