The Bullfrog (true frog family)

There are typical frogs with adults being truly amphibious. They tend to live at the edge of water bodies and enter the water to catch prey, flee danger, and spawn. The bullfrog’s (Rana catesbeiana) natural range includes the eastern and central United States and southeastern Canada.

However, it has been introduced in many areas in the western United States and other parts of North America. It is continuing to expand its range, apparently at the expense of several native species in many locations.

Size of Bullfrog

The bullfrog is the largest North American ranid. The adults usually range between 9 and 15 cm in length from snout-to-vent length (SVL) and exceptional individuals can reach one-half kilogram or more in weight. The males are usually smaller than the females. Frogs exhibit indeterminate growth, and bullfrogs continue to increase in size for at least 6 years after metamorphosis.

Habitat

The Adult bullfrogs live at the edges of ponds, lakes, and slow-moving streams large enough to avoid crowding and with sufficient vegetation to provide easily accessible cover. Small streams are used when a better habitat is lacking. Bullfrogs require permanent bodies of water because the tadpoles generally require 1 or more years to develop prior to metamorphosis. Small frogs favor areas of very shallow water where short grasses or other vegetation or debris offer cover. Larger bullfrogs seem to avoid such areas. Tadpoles tend to congregate around green plants.

Food habits

The adult R. catesbeianaare indiscriminate and aggressive predators, feeding at the edge of the water and among water weeds on any available small animals, including insects, crayfish, other frogs, tadpoles, minnows, snails, young turtles, and occasionally small birds, small mammals, and young snakes.

Bullfrogs often focus on locally abundant foods (e.g., cicadas, meadow voles). Crustaceans and insects probably make up the bulk of the diet in most areas. Moreover, Bullfrog tadpoles consume primarily aquatic plant material and some invertebrates, but also scavenge dead fish and eat live or dead tadpoles and eggs.

Temperature regulation and daily activities

Bullfrogs forage by day. They thermoregulate behaviorally by positioning themselves relative to the sun and by entering or leaving the water. the body temperatures measured in bullfrogs during their normal daily activities averaged 30C and ranged from 26 to 33C. At night, their body temperatures were found to range between 14.4 and 24.9C.

Tadpoles also select relatively warm areas, 24 to 30C. Despite this narrow range of temperatures in which bullfrogs normally maintain themselves, they are not immobilized by moderately lower temperatures. The metabolic rate of bullfrogs increases with increasing body temperature. Between 15 and 25C.

Hibernation

Most bullfrogs hibernate in mud and leave underwater beginning in the fall, but some bullfrogs in the southern states may be active year-round. They emerge sometime in the spring, usually when air temperatures are about 19 to 24C and water temperatures are at least 13 to 14C. Bullfrogs emerge from hibernation later than other ranid species.

Breeding activities and social organization

Bullfrogs spawn at night close to shorelines in areas sheltered by shrubs. The timing and duration of the breeding season vary depending on the location. In the southern states, the breeding season extends from spring to fall, whereas, in the northern states, it is restricted to late spring and summer.

Males tend to be territorial during the breeding season, defending their calling posts and oviposition sites (i.e., submerged vegetation nearshore). However, the female visits to the pond tend to be brief and sporadic. Some males mate with several females whereas others, usually younger and smaller males, may not breed at all in a given year. Females attach their eggs, contained in floating films of jelly, to submerged vegetation. Adults are otherwise rather solitary occupying their own part of a stream or pond.

Tadpole and Metamorphosis

Eggs hatch in 3 to 5 days. Temperatures above 32C have been shown to cause abnormalities in tadpoles and above 35.9C to kill embryos. Tadpole growth rates increase with increasing oxygen levels, food availability, and water temperature. Tadpole gill ventilation at 20C can generate a branchial water flow of almost 0.3 ml/g-min. The metamorphosis from a tadpole to a frog can occur as early as 4 to 6 months in the southern parts of its range; however, most tadpoles metamorphose from 1 to 3 years after hatching, depending on latitude and temperature.

Range

The species’ home range includes its foraging areas and refugees in and around aquatic environments. Home range size decreases with increasing bullfrog density, and males tend to use larger home ranges than females. Bullfrogs tend to stay in the same pools throughout the summer months if the water level is stable.

During the breeding season, adult males establish territories that they defend against conspecific males. During the non-breeding season, found no evidence of the territorial defense. Males often do not return to the same pond the following spring.

During the breeding season, each breeding male may defend a few meters of shoreline. The densities of females and non-breeding males vary with time of day and season and are difficult to estimate. Tadpoles can be present locally in extremely high densities.

Sexual maturity is attained in 1 to 3 years after metamorphosis depending on latitude. Only females that are at least 2 years past metamorphosis mate during the early breeding season. The males and females 1-year past metamorphosis may breed during the later breeding periods.

Also, some older females have been observed to mate and lay a second clutch during the later breeding period estimating the minimum breeding length for females to be 123 to 125 mm SVL. Mortality of tadpoles is high, and adult frogs are unlikely to live beyond 5 to 8 years post metamorphosis.

In some areas, snapping turtles may be responsible for a large component of adult bullfrog mortality. The pig frog (Rana grylio) is smaller than the bullfrog (8 to 14 cm). It is found in South Carolina – South Florida and South Texas. The remaining ranid species are more similar in size to the green (or bronze) frog.

Read More – The Snapping Turtles (Chelydra serpentina)

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