bullfrog

bullfrog, common name of the largest North American frog, Rana catesbeiana. Native to the E United States, this species has been successfully introduced in the West and in other parts of the world. The body length is 4 to 8 in. (10-20 cm), and the legs may be up to 10 in. (25 cm) long. An aquatic form with fully webbed toes, the bullfrog can close its nostrils and lie at the bottom of a pond for some time. Males have a loud, booming call. Bullfrog tadpoles require two or three years to become adults. The bullfrog is the only frog whose legs are marketed in quantity for food in the United States. Several other large frogs of the genus Rana are called bullfrogs in other regions. Bullfrogs are classified in the phylum Chordata, subphylum Vertebrata, class Amphibia, order Anura, family Ranidae.

Solitary aquatic frog (Rana catesbeiana) named for its loud call. The largest U.S. frog, native to the eastern states, the bullfrog has been introduced into the western states and into other countries. The bullfrog is green or brown with a white to yellowish belly and dark-barred legs. Its body is about 8 in. (20 cm) long, and its hindlegs may be 10 in. (25 cm) long. Large adults weigh 1 lb (0.5 kg) or more. Bullfrogs usually live in or near a body of still water. They are used for food and as laboratory animals. The name is sometimes applied to other large frogs.

Learn more about bullfrog with a free trial on Britannica.com.

The American Bullfrog (Rana catesbeiana) is an aquatic frog, a member of the family Ranidae, or "true frogs", native to much of North America. This is a frog of larger, permanent water bodies, swamps, ponds, lakes, where it is usually found along the water's edge . On warm rainy nights, bullfrogs along with many other amphibians, go overland and may be seen in numbers on country roads.

Bullfrogs have been widely introduced across North America (see range map). The original, naturally determined range did not include far western regions where it is found today.

Description

The bullfrog is the largest of the North American ranids. Reaching a body length of , bullfrogs weigh up to . Females are typically larger than males. Males have a tympanum twice the diameter of the eye, and a yellow throat. Female tympanum diameter is the same as that of the eye. Bullfrogs are generally varying shades of green or brown. The head is usually mostly green. The body varies from olive to brown, and may be darkly blotched or spotted. The hindlimbs may be darkly banded.

Similar species include the green frog (Rana clamitans) and pig frog (Rana grylio). The bullfrog may be distinguished from the green frog by the bullfrog's lack of dorsolateral folds, prominent, seam-like skin folds that course down the sides of the back of the green frog. The pig frog has a more pointed snout and its fourth toe (counting medially to laterally) is webbed whereas that of the bullfrog extends un-webbed beyond the other toes.

The adult bullfrog skeleton is representative of tetrapod vertebrates, comprised of an axial skeleton (skull and vertebrae) and an appendicular skeleton (pectoral girdle and forelimbs, pelvic girdle and hindlimbs). Ranids, however, lack ribs. The pronounced pair of dorsal humps in the back of ranid frogs are the ends of the pelvic ilia, homologues of the human hips.

The bullfrog skull is highly fenestrated. The orbits open ventrally through the roof of the mouth to accommodate eye retraction during locomotion and swallowing. The skull bears a continuous row of tiny teeth on the maxilla and premaxilla and a pair of small vomerine teeth on the palate. The mandible is toothless.

The bullfrog nervous system consists of a brain, spinal cord, and peripheral nerves including cranial nerves, spinal nerves, and sympathetic nerves serving organs such as the heart, gastrointestinal tract, kidneys, gonads.

Ecology and behavior

Respiration

Ranid frogs absorb oxygen and eliminate carbon dioxide through their moist skin, the lining of the mouth, and the lungs. When in the air, as opposed to underwater, frogs continuously elevate and lower the floor of the mouth, which serves to ventilate the mouth, or buccal cavity, and exchange gases through the richly vascularized lining of the mouth. Periodically, the regular rhythmic pumping of the floor of the mouth is interrupted by a deeper lowering of the throat at the extreme of which the glottis opens and the throat muscles contract vigorously to force air from the mouth into the lungs—the nostrils are closed off. This lung ventilation may be performed several times after which the shallow buccal ventilation resumes. Lacking ribs, frogs must supply the pressure to force air into their lungs, whereas mammals can enlarge the cavity surrounded by the rigid rib cage and allow the atmosphere to supply the pressure.

Reproduction

Fertilization is external in ranid frogs. In the mating grasp, or amplexus, the male rides on top of the female, grasping her with his forelimbs posterior to her forelimbs. The female bullfrog deposits her eggs in the water and the male simultaneously releases sperm.

Breeding begins in late spring or early summer. Males defend and call from territories, attracting females into a territory to mate. The call is reminiscent of the roar of a bull, hence the frog's common name. A female may produce up to 20,000 eggs in one clutch.

Growth and development

Eggs hatch in 3–5 days. Time to metamorphosis ranges from a few months in the southern to 3 years in the northern parts of the geographic range.

Maximum lifespan in the wild is estimated at 8–10 years, but one captive lived almost 16 years.

Feeding

Stomach content studies going back to 1913 suggest the bullfrog preys on any animal it can overpower and stuff down its throat. Bullfrog stomachs have been found to contain rodents, small turtles, snakes, frogs including bullfrogs, birds, even a bat, as well as the many invertebrates which are the usual food of ranid frogs . These studies furthermore reveal the bullfrog's diet to be unique among North American Rana in the inclusion of large percentages of aquatic animals, e.g., fish, tadpoles, Planorbid snails, Dytiscid beetles. The specialized ability of bullfrogs to capture submerged and large strong prey comprises a pronounced biting motor pattern that follows up on the initial and typical ranid tongue strike . Adaptation to target image displacement due to light refraction at the water-air interface consists of the bullfrog's application of tongue surface comparatively posterior to the perceived location of the prey target . The comparative ability of bullfrogs to capture submerged prey, compared to that of the green frog, leopard frog, and wood frog (R. clamitans, R. pipiens, R. sylvatica, respectively) was also demonstrated in laboratory experiments.

The bullfrog, like many other ranid frogs, is an ambush predator. At a chosen site, the visually vigilant frog waits relatively motionless until prey appears and elicits its feeding behavior.

Prey motion elicits feeding behavior. First, if necessary, the frog performs a single orienting bodily rotation ending with the frog aimed towards the prey. This is followed by approaching leaps, if necessary. Once within striking distance, the bullfrog emits its feeding strike, which consists of a ballistic (eyes closed as during all leaps) lunge that ends with the mouth opening, extension of the fleshy and mucous-coated tongue upon the prey, often engulfing it, while the jaws continue their forward travel to close (bite) in close proximity to the prey's original location just as the tongue is retracted back into the mouth, prey attached. Large prey that do not travel entirely into the mouth are literally stuffed in with the forearms. In laboratory observations, bullfrogs taking mice usually dove underwater with prey in mouth, apparently with the advantageous result of altering the mouse's defense from counterattack to struggling for air. The tiny teeth of bullfrogs are useful only in grasping. Asphyxiation is the most likely cause of death of endothermic (warm-blooded) bullfrog prey.

External links

• Herps of Texas: Rana catesbeiana
• British Columbia Frog Watch Program: Bull Frog Fact Sheet
• Sonoran Desert Conservation Plan, Exotic Species: Bull Frog

Footnotes

References