male bullfrog
AMERICAN BULLFROG
Rana catesbeiana
 
INTRODUCTION

The bullfrog is Rhode Island's largest anuran. In contrast to other frogs in Rhode Island, bullfrogs spend most of their life in the water where they hibernate, breed, and hunt. During the winter months, they hibernate in the muddy substrate of ponds and lakes. Once spring arrives, they emerge and spend the warm months of the year lurking in shallow waters at the edge of wetlands where they depend on aquatic vegetation for cover. Bullfrogs usually linger in these vegetated shoals, although they occasionally venture into deeper parts of lakes and ponds. Unlike the similar green frog (Rana clamitans), bullfrogs also primarily hunt from the water. They have a voracious appetite and as adults, they are carnivorous and will eat anything that moves. Thus, the size of adult's prey is limited only by the gape of their mouth. In contrast, their tadpoles are largely vegetarian.

Due to their large size, bullfrogs are a favorite for those people whom consider frog legs to be a delicacy. This has lead to their introduction throughout North America and other continents, a practice which has had devastating impacts on native fauna.

There is some evidence that some local populations of bullfrogs may be declining (DeGraaf and Yamasaki 2001), but for the most part their populations appear to be stable in the region (Klemens 1993).

 
IDENTIFICATION

  • The only large, greenish frog in Rhode Island without dorsal-lateral folds. The green frog has prominent dorsal-lateral folds (photo comparison).
  • The bullfrog is the largest and most aquatic North American anuran.
  • Color is variable, but they typically have a green or greenish brown dorsum and a whitish venter with grayish mottling (Dickerson 1906; Conant and Collins 1991). Individuals can change color tone depending on environment (Dickerson 1906; Albright 1999).
  • Feet are strongly webbed.
  • The vocal sac is under the throat and extends backwards over the arm; therefore, when the bullfrog croaks not only does the throat swell but also the space between the ear and arm (Dickerson 1906). Both males and females croak.
  • Voice: Bullfrogs do not sing in chorus. Dickerson (1906) described the bullfrog mating call with 4 different phrases- "Be drowned," "Better go round," "Jug o' rum," and "More rum"; all said in a deep hoarse voice. Wiewandt (1969) described three distinct calls; the "mating call"- 3 to 6
    bass croaks, the 'gronk'- a single note of the mating call, and the 'bonk'-
    a "distinctly abrupt, forceful vocalization."

bullfrog
Bullfrogs lack dorsal-lateral folds (green frogs have prominent dorsal-lateral folds.)

On both the bullfrog and the green frog the tympanum (eardrum) is larger than the eye.


Sexual dimorphism
  • Adult males have tympana that are larger in diameter than the eyes, whereas the female's tympana are approximately the same size as the eyes.
  • Adult males have a bright yellow throat and chest. The female's throat is white.
  • Breeding males have strong, swollen thumbs (used to grasp females during amplexus).
  • Males tend to have a narrower head than females (Wright and Wright 1949).
 
SIZE
AGE / SEX
SVL (SNOUT VENT LENGTH) (cm)
SAMPLE SIZE

AVERAGE

RANGE

Std. Deviation

ADULT FEMALE

10.7

8.0- 14.9

1.6

46

ADULT MALE

9.2

7.4- 11.2

1.5

12

JUVENILE

5.7

3.3- 8.2

1.2

95

METAMORPH

3.7

2.5- 5.2

0.9

36



AGE/ SEX
MASS (g)
SAMPLE SIZE

AVERAGE

RANGE

Std. Deviation

ADULT FEMALE

115.8

54.5- 200.0

43.8

46

ADULT MALE

76.5

42.2- 124.4

76.5

12

JUVENILE

19.6

5.7- 57.0

11.8

95

METAMORPH

5.0

1.4- 9.9

3.3

36







measurement data

 

 
RELATIVE ABUNDANCE
Rhode Island
Bullfrogs are widespread and locally common in our area (Raithel pers. comm.).
We found bullfrog tadpoles in 7 (23%) out of 30 permanent ponds we sampled during the 2000 field season (Paton and Egan 2001).
Regional
Bullfrogs are common throughout the region, but populations may be declining in parts of New England (Degraaf and Yamasaki 2001). Klemens (1993) classified bullfrogs as common in Connecticut; he characterized them as widespread and flourishing in places where other frogs have disappeared.{Relative Abundance General}
 
DISTRIBUTION
Rhode Island
Bullfrogs are found throughout mainland Rhode Island (Raithel pers. comm).
Regional
Bullfrogs range throughout the region except for northeastern Vermont, northern New Hampshire, and northern Maine (Klemens 1993).
General
Bullfrogs range from Nova Scotia west to Wisconsin, south through the Great Plains to eastern Colorado and Texas (Degraaf and Yamasaki 2001). They occur all along the eastern seaboard from southern Florida to northern Maine (Klemens 1993). They have been introduced throughout the western United States, British Columbia and Mexico, as well as other countries including Cuba, and Jamaica (Conant and Collins 1991).
 
HABITAT NON BREEDING
Bullfrogs overwinter in permanent waters including marshes (Emlen 1968), lakes, and ponds (Willis et al. 1956). They typically hibernate under mud, leaves, and other debris (Wright 1914).
 
HABITAT BREEDING

Bullfrogs are the most aquatic frog found in Rhode Island. They may breed in the same water body that they overwinter or move to a different wetland (Howard 1978). They are found in a wide array of freshwater habitats, but primarily in permanent water (Klemens 1993). Klemens (1993) collected bullfrogs in lakes, ponds, reservoirs, rivers, stock ponds, marshes, woodland pools, floodplain swamps, bogs, and a variety of manmade ponds and impoundments. Klemens (1993) also found that juvenile bullfrogs move among waterbodies more frequently than adults. We documented a similar pattern in Rhode Island (P. Paton, unpubl. data).

Because bullfrogs tadpoles take more than 1 year to develop, they require semi-permanent or permanent ponds for breeding (Degraaf and Yamasaki 2001). Bullfrogs observed in vernal ponds are not breeding, but feeding and rehydrating as they pass through the landscape. It is more likely that these individuals are juveniles (Klemens 1993), although we commonly have seen adult bullfrog spend the summer in temporary vernal ponds that do not provide suitable breeding habitat (P. Paton, unpubl. data).

 
HYDROLOGY
Bullfrogs seem to prefer large ponds and lakes where the shore is shaded by hydrophilic trees and shrubs and the water has numerous aquatic plants for refuge (Dickerson 1906). They breed in semi-permanent or permanent ponds (Paton and Egan 2001).

hydrology data
 
MOVEMENT CHRONOLOGY

In Rhode Island, adult bullfrogs first start immigrating to breeding ponds at the end of March. Paton and Crouch (In press) started capturing adults on average around 25 March, with peak immigration by mid-April, and some individuals moving as late as mid-June. Even though bullfrogs are present in wetlands as early as late March, they do not initiate active choruses for at least a month- until early May in Rhode Island (Crouch 1999). During the summer, movement among age cohorts and genders of bullfrogs varies considerably. Raney (1940), reported that some individuals move as little as 100 feet (30 meters); whereas others may move up to 3,000 feet (915 meters). Raney (1940) found no obvious correlation in these movements.

Metamorph departure from breeding ponds is late compared to other species. In southern Rhode Island, the first emigration movement was detected in early August, and the latest movement witnessed was at the end of October; the data showed that bullfrogs have a very protracted emigration period (Paton and Crouch In press).

 
REPRODUCTION

Bullfrogs are the last anuran to breed in Rhode Island, typically beginning in late May and extending into July (Dickerson 1906). Bullfrogs do not have a synchronized, rapid breeding period, such as wood frog (Rana sylvatica) or spotted salamander (Ambystoma maculatum). Rather, they have a prolonged breeding period similar to green frogs and may breed twice in one season (Emlen 1968; Howard 1978). In Rhode Island, we have detected bullfrogs calling at breeding ponds from early May through early August (Crouch 1999); therefore, they also have a protracted calling period compared to other species.

Having emerged from hibernation, bullfrogs either remain in the pond where they overwintered, or move to another suitable breeding site. For about a month, they remain silent (Albright 1999) waiting for optimum conditions to begin courtship. In Maine, optimum conditions are when air temperatures and water temperatures reach 20-22 °C (68-72° F) (Albright 1999). The majority of mating rituals (sustained choruses, male to male encounters, and copulation) take place at night (Emlen 1976), though in the onset of the breeding season, it is not uncommon to hear choruses during the day (Howard 1978). In Michigan, Howard (1978) reported prolonged choruses starting around 9:00 PM and continuing until dawn and copulation occurring during the early morning hours. Interestingly, Howard (1978) found that as the season progressed calling began later and later, frequently not beginning until midnight. This may explain why Bridges and Dorcas (2000) only detected bullfrogs calling after midnight when they monitored a pond with an automated data recording system during June.

At breeding sites, males establish territories by spacing themselves at regular intervals along the water's edge and remain in these zones for up to 3 weeks (Emlen 1968). They are aggressive and will engage in physical encounters with conspecific males (Emlen 1968; Wiewandt 1969). Females lured to the breeding site by the calls of the males, initiate copulation by making contact with a carefully selected male (Emlen 1968). Females generally deposit eggs within the males territory. Howard (1978) observed 73 pairs of bullfrogs in amplexus and only three pairs deposited their eggs away from the original copulation site.

 
EGG MASS
Eggs are deposited on aquatic vegetation in shallow water as a thin gelatinous mat similar to the green frog. The 12,000 to 20,000 bicolored (black and white) (Wright and Wright 1949) eggs float on the surface at first, but typically sink to the bottom before they have fully developed (Kenney and Burne 2000). The development of the thousands of eggs is temperature dependent but hatching usually occurs within 5 to 20 days (Oliver 1955).
 
LARVAE

The length of the larval period is unknown in Rhode Island. Bullfrog larvae are known to overwinter for at least one winter (Collins 1979), but more typically they require 2 to 3 years to metamorphose (Wright and Wright 1949; Degraaf and Rudis 1983). At any given breeding pond in Rhode Island, several sizes of bullfrogs tadpoles can be observed, suggesting that it may take over 1 year for bullfrog tadpoles to undergo metamorphosis in southern New England (C. Heinz and S. Egan, pers. observ.).

Small tadpoles are easily confused with green frog tadpoles, but as they grow they become distinctly different. Bullfrogs typically have a green to greenish brown dorsum and a yellowish venter. The transition between the dorsum and venter is gradual, not abrupt as in the green frog. The back, tail musculature, and dorsal fin of the bullfrog is peppered with fine black spots. The green frog usually has a more mottled appearance.

bullfrog tadpole
Bullfrog tadpole
 
METAMORPHS
Newly metamorphosed bullfrogs average approximately 3.8 cm (range 2.5-5.2) (Paton unpubl. data). Size, however, is not necessarily a good way to assess the age of a bullfrog, because the size at transformation is generally dependent on the size attained as a tadpole (Dickerson 1906). Thus, larger tadpoles result in bigger metamorphs. Because studies show that bullfrogs can remain as tadpoles for 1 to 3 years, a wide range of sizes are possible. Irregardless of size, recently metamorphosed bullfrogs are easily distinguished from young green frogs by the absence of dorsal-lateral folds.
 
JUVENILES
During warmer months, juveniles tends to move across the landscape more frquently than adults. They ulitize numerous types wetlands, including vernal ponds, for food and cover.

Sexual maturity: Bullfrogs are not sexually mature until 4-5 years old (Albright 1999; Degraaf and Yamasaki 2001).
 
FOOD
Adults
Adult bullfrogs are aggressive and will eat virtually anything that moves (Conant and Collins 1991). Their diet regularly includes: snakes, fish, small turtles, mice, young waterfowl, crayfish, diving beetles, dragonfly larvae, spiders and other invertebrates (Albright 1999). Green and Pauley (1987) reported finding bullfrogs with spring peepers (Pseudacris c. crucifer) and American toads (Bufo americanus) in amplexus in their stomachs. Stewart and Sandison (1972) found frog remains in every bullfrog stomachs they examined and noted that bullfrogs were frequently observed chasing and consuming green frogs (Rana clamitans). Bullfrogs are also cannibalistic, eating both their own tadpoles and other adults (Albright 1999).
Larvae
Tadpoles are largely vegetarians, however they will feed on dead animal matter (Albright 1999).
 
PREDATION
Adults
Adults are prey to a large number of animals including wading birds, raccoons, minks, snakes, and humans (Albright 1999).
Larvae
Tadpoles are prey to aquatic invertebrates such as predacious diving beetles and dragonfly larvae, as well as, to adult bullfrogs (Albright 1999).
 
CONSERVATION CONCERNS

Bullfrogs are common and presently secure throughout the state. Their populations are increasing in some areas due to habitat alternations (creation of permanent ponds and stream impoundments) and degradation of wetlands because they are proficient at colonizing newly created habitats (Klemens 1993).

Bullfrogs have been introduced to many areas, most notably the western United States (Conant and Collins 1991). Once established in a new area, bullfrogs can readily extirpate native anurans (Klemens 1993; Conant and Collins 1991) and other species such as garter snakes and small turtles (Conant and Collins 1991).

Bullfrogs are the primary source of frogs legs in the United States; therefore, some areas they are considered economically important (Klemens 1993; Albright 1999). To protect bullfrogs from being overharvested, many states have listed bullfrogs as a game species (Degraaf and Rudis 1983; Albright 1999). Harvesting bullfrogs is regulated in Rhode Island.

green frog
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pickerel frog