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).

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."

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.

• 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).
  

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).

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).

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.

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.