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 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 and 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 leaves 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.
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.
The sexual maturity
is attained in about 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 to lay a second clutch during the later
breeding period estimated 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)
and is found in South Carolina to 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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