Bullfrog (Rana catesbeiana)
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.
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.
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.
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.
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.
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.