American Bullfrog | |
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adult male | |
Scientific classification | |
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Species: | R. catesbeiana
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Binomial name | |
Rana catesbeiana Shaw, 1802
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Bullfrog range, red: natural range; dark green: countries or regions where it has been introduced | |
Synonyms | |
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The American bullfrog (Rana catesbeiana[2][3][4] or by some authorities as Lithobates catesbeianus)[1][5][6] often simply known as the bullfrog in Canada and the United States, is an aquatic frog, a member of the family Ranidae, or “true frogs”, native to much of North America.[7] This is a frog of larger, permanent water bodies, swamps, ponds, and lakes, where it is usually found along the water's edge.[8] On rainy nights, bullfrogs, along with many other amphibians, travel overland, and may be seen in numbers on country roads.
American bullfrogs live longer in warm weather. They have been widely introduced across North America (see range map). The original, naturally determined range did not include the far western regions where it is found today.[9]
Description
They grow on average to be about 3.6 to 6 in (9.1 to 15.2 cm) in body length (although there are records of some up to 8.0 in (20 cm)), legs add another 7 to 10 in (18 to 25 cm) to length. Bullfrogs go from 5 to 175 g (0.18 to 6.17 oz) on average in the first 8 months of life.[10] Large mature bullfrogs can weigh up to 500 g (1.1 lb), with exceptional ones attaining 800 g (1.8 lb).[11][12]
The bullfrog skull is highly fenestrated; females have eardrums (tympana) the same size as their eyes. The males' eardrums are larger.[13]
Ecology and behavior
Reproduction
Fertilization is external in ranid frogs. During 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 territories and call to females from them, attracting females into their 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.[9]
Mating Systems
The bullfrog breeding season typically lasts 2–3 months.[14][15][16][17] A study on bullfrogs in Michigan showed bullfrog males arriving at site in late May or early June. The males remained in the area into the month of the July, which is consistent with the finding that bullfrog mating seasons last approximately 2 to 3 months.[14] The territorial males that occupy sites, ranging from 3–6 meters apart. These males call to females.[14][15][16][17] At least three different types of calls have been noted in male Rana catesbeiana. Encounter calls precede combat.[18] The males make distinctive calls that are either territorial calls, which were made as threats to other males, or mating calls made to beckon females.[19]
The bullfrogs have a prolonged breeding season [14] with the males continuously engaging in sexual activity throughout the mating season. Males are present for longer times than females during the entire season, increasing their chances of multiple matings.[16][17] The sex ratio is typically skewed toward males.[19] Conversely, females have brief periods of sexual receptivity throughout the season. In one study, female sexual activity typically lasted for one night and mating did not occur until the females initiated the physical contact.[14] Males only clasp females that initiate physical contact.[14] This finding refutes previous claims that all frog males clasp onto any proximate female with no regard to the female’s consent.[18][20][21][22] Females have been observed to approached and initiate contact with the calling males. After mating with the selected males, the females deposited their eggs on the male territory.[19]
These male and female behaviors cause male to male competition to be high within the bullfrog population and sexual selection for the females to be an intense process.[14] Wells postulates that leks, territorial polygyny, and harems are the most likely classifications for the bullfrog mating system. Leks would be a valid description because males congregate in order to attract females, and the females arrive to the site for the purpose of copulation.[14][18] In a 1980 study on bullfrogs in New Jersey the mating system was classified as resource defense polygyny. The males defended territories within the group and demonstrated typical physical forms of defense [19].
Choruses
Male bullfrogs aggregate into groups called choruses. The male chorus behavior is analogous to the lek formation of birds, mammals, and other vertebrates. Choruses are dynamic, forming and remaining associated for a few days, breaking down temporarily, and then forming again in a new area with a different group of males.[18] Male movement has experimentally be noted to be dynamic.[19] In the Michigan study, the choruses were described as “centers of attraction” in which their larger numbers enhanced the males’ overall acoustical displays. This is more attractive to females and also attractive to other sexually active males. Choruses in this study were dynamic, constantly forming and breaking up. New choruses were formed in other areas of the site. Males moved around and were highly mobile within the choruses.[14]
A review of multiple studies on bullfrogs and other anurans notes that male behavior within the groups changes according to the population density of the leks. At higher population densities, leks are favored due to difficulty defending individual territories amongst a large population of males. This variance causes differences in how females choose their mates. When the male population density is low and males maintain clearer, more distinct territories, female choice is mostly determined by territory quality.[18] When male population density is higher, females depend on other cues to select their mates. These cues include the males’ positions within the chorus and differences in male display behaviors among other determinants.[16][18] Social dominance within the choruses is established through challenges, threats, and other physical displays. Older males tend to acquire more central locations while younger males were located on the periphery.[14]
Energetic Constraint Hypothesis and Chorus Tenure
Chorus tenure is the number of nights that a male participates in the breeding chorus.[23] One study distinguishes between chorus tenure and dominant tenure. Dominant tenure is the more strictly defined as the amount of time a male maintains a dominant status.[24] Chorus tenure is restricted due to increased risk of predation,[25] lost foraging opportunities,[26] and higher energy consumption.[27] Calling is postulated to be energetically costly to anurans in general.[28] Energy is also expended through locomotion and aggressive interactions of male bullfrogs within choruses.[24]
Aggressive Behavior and Defense Mechanisms among Males
Within choruses, in order to establish social dominance bullfrogs demonstrate typical forms of aggression, especially through visual displays. Posture is a key factor in establishing social position and threatening challengers.[18] Territorial males have inflated postures while non-territorial males only have their heads above the water. For dominant, also called territorial, males their elevated posture reveals yellow-colored throats.[16][18] When two dominant males encounter each other they will engage in a wrestling bout. The males have their venters clasped, each individual in an erect position rising to well above water level.[18] The New Jersey study noted that males would approach each other to within a few centimeters and then tilt back their heads, displaying their brilliantly-colored gular sac. The gular is dichromatic in bullfrogs, with dominant and fitter males displaying yellow gulars. The New Jersey study also reported that low posture with only the head exposed above the water surface was typical of subordinate, or non-territorial males, and females. High posture was demonstrated by territorial males, who floated on the surface of the water with their lungs inflated, displaying a yellow sac, the gular, on their throats.[19] Males optimize their reproductive fitness in a number of ways. Early arrival to the breeding site, prolonged breeding with continuous sexual activity throughout the season, ownership of a centrally located territory within the chorus, and successful movement between the dynamically changing choruses are all common ways that males maintain dominant, or territorial, status within the chorus. Older males have greater success in all of these areas than younger males did.[14] Some of the males display a more inferior role, termed by many researchers as the silent male status. These silent males sit near resident males, moving into the dominant males’ territories. The silent males will sit for many hours near the resident males demonstrating low postures. The silent males are not in the practice of intercepting matings. They are actually waiting for territories to be vacated.[16][18] This has also been called the alternate or satellite male strategy.[18]
Growth and development
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.[9] Maximum lifespan in the wild is estimated at 8 to 10 years, but one captive lived almost 16 years.[9]
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, and a bat, as well as the many invertebrates, such as insects, which are the usual food of ranid frogs.[29] 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.[29] 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.[29] 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.
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.[29] The tiny teeth of bullfrogs are useful only in grasping. Asphyxiation is the most likely cause of death of endothermic (warm-blooded) bullfrog prey.
Human use
While occasionally kept as pets, they will still bite even when tamed and can live up to 16 years or more in captivity. The American bullfrog provides a food source, especially in the Southern United States and in some areas of the Midwestern United States. In a few locations, they are commercially cultured in ponds, but the traditional way of hunting them is to paddle or pole silently by canoe or flatboat in streams or swamps at night; when the frog call is heard, a light is shone on the frog to temporarily inhibit it. The frog will not jump into deeper water as long as movement is slow and steady. When close enough, the frog is gigged and brought into the boat. In some states, breaking the skin while catching them is illegal, and either grasping gigs or hand capture are used. The only parts eaten are the rear legs, which resemble small chicken drumsticks and, sometimes, the backs, which are usually fried.
The American bullfrog is also used as a specimen for dissection in many schools across the world.
The American bullfrog is the state amphibian of Iowa, Missouri, and Oklahoma.
The American bullfrog has been introduced to many countries and regions in the world, such as South Korea, Western Europe,[30] Brazil, Colombia, and Australia, where it has become a nuisance to those countries' natural ecology because of its appetite. They are sold in China for eating.
The American bullfrog has been widely introduced to most western states, and is now very common there, especially in California, and poses a serious threat to native species, such as the California Red-legged Frog because bullfrogs are aggressive and will eat anything smaller than themselves. They are considered a factor of the red-legged frogs' decline.
Footnotes
- ^ a b Lithobates catesbeianus, IUCN
- ^ 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 Frogs (Rana. Molecular Phylogenetics and Evolution 34:299–314.
- ^ AmphibiaWeb: Information on amphibian biology and conservation.
- ^ American Bullfrog (Lithobates catesbeianus), Ontario Nature
- ^ Lithobates catesbeianus, USDA
- ^ Hillis & Wilcox (2005), Pauly et al. (2009)
- ^ Conant, 1958
- ^ a b c d Casper, G. S. and Hendricks, R. (2005) Amphibian Declines: The Conservation Status of United States Species, M. Lannoo, ed. University of California Press.
- ^ [1]
- ^ [2]
- ^ [3]
- ^ Frog World: American Bullfrog
- ^ a b c d e f g h i j k Emlen, Stephen T. “Lek organization and mating strategies in the bullfrog.” Behavioral Ecology and Sociobiology 1.3 (1976): 283-313.
- ^ a b Blair, Albert P. “Notes on anuran behavior, especially Rana catesbeiana.” Herpetologica 19.2 (1963): 151-151.
- ^ a b c d e f Emlen, Stephen T. “Territoriality in the bullfrog, Rana catesbeiana.” Copeia (1968): 240-243.
- ^ a b c Wiewandt, Thomas A. “Vocalization, aggressive behavior, and territoriality in the bullfrog, Rana catesbeiana.” Copeia (1969): 276-285.
- ^ a b c d e f g h i j k Wells, Kentwood D. “Territoriality and male mating success in the green frog (Rana clamitans).” Ecology (1977): 750-762.
- ^ a b c d e f Ryan, Michael J. (1980) The Reproductive Behavior of the Bullfrog(Rana catesbiana).Copeia. pp. 108-114.
- ^ Blair, W. Frank. “Call structure and species groups in US treefrogs (Hyla).” The Southwestern Naturalist (1958): 77-89.
- ^ Bogert, Charles Mitchill. “The influence of sound on the behavior of amphibians and reptiles.” (1960).
- ^ Littlejohn, M. J., and A. A. Martin. “Acoustic interaction between two species of leptodactylid frogs.” Animal Behaviour 17.4 (1969): 785-791.
- ^ Murphy, Christopher G. “Chorus tenure of male barking treefrogs, Hyla gratiosa.” Animal Behaviour; Animal Behaviour (1994).
- ^ a b Judge, Kevin A., and Ronald J. Brooks. “Chorus participation by male bullfrogs, Rana catesbeiana: a test of the energetic constraint hypothesis.” Animal Behaviour 62.5 (2001): 849-861.
- ^ Ryan, Michael J., Merlin D. Tuttle, and Lucinda K. Taft. “The costs and benefits of frog chorusing behavior.” Behavioral Ecology and Sociobiology 8.4 (1981): 273-278.
- ^ Woolbright, Lawrence L., and Margaret M. Stewart. “Foraging success of the tropical frog, Eleutherodactylus coqui: the cost of calling.” Copeia (1987): 69-75.
- ^ Prestwich, Kenneth N., Kristin E. Brugger, and Mary Topping. “Energy and communication in three species of hylid frogs: power input, power output and efficiency.” Journal of Experimental Biology 144.1 (1989): 53-80.
- ^ Pough, F. H., et al. “Behavioral energetics.” Environmental physiology of the amphibians (1992): 395-436.
- ^ a b c d Cardini 1974
- ^ Western Europe is not a country.
References
- AmphibiaWeb: Information on amphibian biology and conservation. [web application]. 2012. Berkeley, California: . AmphibiaWeb, available at http://amphibiaweb.org/. (Accessed: August 20, 2012).
- 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
- Conant, R. (1958) A Field Guide to Reptiles and Amphibians. Houghton Mifflin Co., Boston.
- 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.
- Template:IUCN2006 Database entry includes a range map and justification for why this species is of least concern.
- Pauly, Greg B., Hillis, David M. & Cannatella, David C. (2009): Taxonomic freedom and the role of official lists of species names. Herpetologica 65: 115–128. PDF fulltext