AppearanceThey grow on average to be about 3.6 to 6 inch in body length, legs add another 7 to 10 inch to length. Bullfrogs go from 5 to 175 g on average in the first 8 months of life. Large mature bullfrogs can weigh up to 500 g, with exceptional ones attaining 800 g.
The bullfrog skull is highly fenestrated; females have eardrums the same size as their eyes. The males' eardrums are larger.
ReproductionFertilization 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.
FoodStomach 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, birds, and a bat, as well as the many invertebrates, such as insects, 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, 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. 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 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 emits its feeding strike, which consists of a ballistic 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 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 bullfrog prey.
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