Bullfrog

Lithobates catesbeianus

Description:

Bullfrogs grow on average to be about 3.6 to 6 inches in body length (although there are records of some up to 8.0 in). 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. Females have eardrums (tympana) the same size as their eyes. Males' eardrums are larger. 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. 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. Eggs hatch in three to five days. Time to metamorphosis ranges from a few months in the southern to three years in the northern parts of the geographic range. Maximum lifespan in the wild is estimated at eight to 10 years, but one captive lived almost 16 years. 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, and a bat, as well as the many invertebrates, such as insects, which are the usual food of ranid frogs. These studies 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, and 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. 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 emits its feeding strike, which consists of a ballistic lunge (eyes closed as during all leaps) 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.