The American Bullfrog (Rana catesbeiana[1][2][3]) is an aquatic frog, a member of the family Ranidae, or "true frogs", native to much of North America.
Description
The bullfrog is the largest of the North American Ranidae; it can grow to a length of 20 cm, body weight up to 750 g.[4] 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 usually mostly green, the body brown with irregular dark blotches. The rear legs may be darkly banded.
The adult bullfrog skeleton is representative of tetrapod vertebrates, comprised of an axial skeleton (skull and vertebrae) and an appendicular skeleton (pectoral and pelvic girdles and associated forelimbs and hindlimbs, respectively). Ranids, however, have no 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 (cf.orbit (anatomy)), exceptionally so, 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
Bullfrog in typical aquatic habitat.
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 buccal (mouth) 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 (closing off the airway to the lungs) 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 [5] 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 just 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. The call, reminiscent of the roar of a bull hence the frog's common name, attracts the female into a territory where mating takes place. A female may produce up to 20,000 eggs in one clutch [6]
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 [7]
Maximum lifespan in the wild is estimated at 8-10 years, but at least one captive lived almost 16 years [8].
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 [9]. 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 [10]. 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 [11]. 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 [12].
The bullfrog, like many other ranid frogs, is an ambush predator. At a chosen site, the visually vigilant frog waits relatively motionless until moving prey appears and elicits its feeding behavior.[13]
Prey motion elicits feeding behavior[14]. 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.
Geographic Range
The bullfrog, native to North America, has been widely introduced and its current range (cf. range map) is greater than its original, naturally limited range.
Trivia
See also
External links
Footnotes
- ^ Hillis, D. M. 2007. Constraints in naming parts of the Tree of Life. Molecular Phylogenetics and Evolution 42:331-338.
- ^ Hillis, D. M., and T. P. Wilcox. 2005. Phylogeny of the New World True Frigs (Rana). Molecular Phylogenetics and Evolution 34:299-314.
- ^ Rana catesbeiana (TSN 173441). Integrated Taxonomic Information System. Retrieved on 6 February 2006.
- ^ Bullfrog Fact Sheet
- ^ Noble, G. K. (1931) The Biology of the Amphibia. McGraw-Hill.
- ^ Casper, G. S. and Hendricks, R. (2005) Amphibian Declines: The Conservation Status of United States Species, M. Lannoo, ed. University of California Press.
- ^ Casper, G. S. and Hendricks, R. (2005) Amphibian Declines: The Conservation Status of United States Species, M. Lannoo, ed. University of California Press.
- ^ Casper, G. S. and Hendricks, R. (2005) Amphibian Declines: The Conservation Status of United States Species, M. Lannoo, ed. University of California Press.
- ^ Cardini 1974
- ^ Cardini,1974
- ^ Cardini, 1974
- ^ Cardini, 1974
- ^ http://www.twingroves.district96.k12.il.us/Wetlands/Frogs/BullfrogFood
- ^ Cardini, 1974
References
- Santos-Barrera, G. et al. (2004). Rana catesbiana. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 12 May 2006. Database entry includes a range map and justification for why this species is of least concern.
- Hillis, D.M. & Wilcox, T.P. (2005): Phylogeny of the New World true frogs (Rana). Mol. Phylogenet. Evol. 34(2): 299–314. doi:10.1016/j.ympev.2004.10.007 PDF fulltext.
- Hillis, D. M. (2007) Constraints in naming parts of the Tree of Life. Mol. Phylogenet. Evol. 42: 331–338.
- Cardini, F. (1974) Specializations of the Feeding Response of the Bullfrog, Rana catesbeiana, for the Capture of Prey Submerged in Water. M.S. Thesis, U. of Massachusetts, Amherst, MA
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