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GREEN
FROG
Rana
clamitans melanota
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| INTRODUCTION |
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The green frog (Rana clamitans), is a common anuran of
the eastern United States. They, along with the bullfrog (Rana
catesbeiana) are one of two aquatic frogs in Rhode Island
because they are never far from the water, except during hibernation.
In fact, they spend the majority of their time on the shores of
lakes and ponds and the banks of rivers waiting for prey to cross
their path. When approached, green frogs will typically leap into
the safety of the water while letting out a loud cry. Hence, the
old nickname 'the screaming frog'.
Green frog are commonly confused with bullfrogs during all stages of
their life. As small tadpoles, the two species are difficult to impossible
to separate but older tadpoles can be distinguished on close inspection.
As adults, green frogs can be readily indentified from bullfrogs by
the dorsal-lateral ridge whereas bullfrogs lack this ridge.
In Rhode Island, green frogs are widespread and common. They occupy
a wide variety of habitats and appear to be less affected by development
and degraded habitat than other amphibian species. In fact, some studies
suggest that green frog populations have actually increased over the
years and thus may benefit from manmade habitat alterations.
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| IDENTIFICATION |
- A large aquatic frog commonly confused with the bullfrog
(Rana catesbeiana).
- Dorsal-lateral
folds are prominent, extending the length of the body.
Bullfrogs do not have these folds.
- The dorsum of the
green frog is generally greenish-brown with darker spots
or mottling.
- The venter is creamy
white- sometimes with dark spots and mottling, especially
under the legs.
- The legs have striped bars.
- Mature males have yellow throats.
- On the male, the tympanum
(eardrum) is larger than the eye.
- Some adult females are only slightly larger than the
males (see SIZE below).
- In larger tadpoles, the mottled tail with diffuse black
spots and whitish vent distinguishes green frog from the
similar bullfrog, which has distinct, small black spots
on the tail and a yellowish vent.
- Voice: The call is explosive, prolonged, and
low-pitched (Dickerson
1906); producing a twang similar to the sound of plucking
the bass string of a banjo, usually given as a single
note, but sometimes repeated several times. Green frogs
rarely engage in a chorus (Green
and Pauley 1987).
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Male green frog
tympanum is larger than the eye
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| SIZE |
| AGE / SEX
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SVL (SNOUT
VENT LENGTH) (cm) |
SAMPLE SIZE |
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AVERAGE
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RANGE |
Std. deviation
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ADULT FEMALE
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7.7
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6.4- 9.8 |
0.9
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36
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ADULT MALE
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7.5
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6.3- 9.5 |
0.7
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44
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JUVENILE
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4.9
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3.6- 6.5 |
0.6
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77
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METAMORPH
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3.3
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1.8- 4.5 |
0.6
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871
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| AGE/ SEX
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MASS (g)
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SAMPLE SIZE |
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AVERAGE
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RANGE |
Std. deviation
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ADULT FEMALE
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51.2
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28.5- 85.0 |
16.9
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36
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ADULT MALE
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44.2
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28.5- 58.3 |
8.7
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44
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JUVENILE
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11.7
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5.3- 26.1 |
4.7
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77
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METAMORPH
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3.6
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0.4- 7.0 |
1.2
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871
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| RELATIVE ABUNDANCE |
Rhode Island
During surveys
we conducted at breeding ponds west of the Narragansett Bay, green
frogs were detected in approximately 50% (60 out of 119) of the
ponds. In urban areas green frogs were detected in 61% of the ponds,
suggesting that compared to the other Rhode Island amphibians they
are least affected by urbanization (Paton
and Egan 2001). This reflects suggestions by Vogt
(1981) that green frog are generalists who will take advantage
of any type of aquatic habitat not utilized by other species.
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Regional
Abundant
and presently secure through the region (Klemens
1993). |
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| DISTRIBUTION |
Rhode Island
Green frogs are common throughout
the state found in a variety of habitats, including urban areas and the
islands.
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Regional
Green frogs are
common throughout the region (Degraaf
and Yamasaki 2001) widely distributed on the mainland as well
as coastal islands from sea level to high elevations (1700-2000
feet / 520-610 meters) (Klemens 1993). |
General
Green frogs are widely
distributed in Canada from Nova Scotia west through Quebec and southern
Ontario (Degraaf and Yamasaki 2001). Their range covers the eastern half
of the United States, occurring as far west as eastern Minnesota, Iowa,
Missouri, Oklahoma, and Texas (Klemens 1993). They range as far southeast
as northern Florida. They are absent from central Illinois (Degraaf and
Yamasaki 2001; Klemens 1993).
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| HABITAT NON BREEDING |
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Green frogs are seldom observed far from water for they need
water to avoid desiccation and predation (Martof
1953a). They will utilize almost any type of freshwater habitat
including, rivers, streams, lakes, ponds, artificial impoundments,
springs and vernal pools. Green frogs have a relatively small
home range (the area an individual normally travels in its search
for food) and are very territorial (Stockwell
1999). In fact, during a two-year study in Michigan, Martof
(1953a) found that half of his recaptured animals were within
1 meter of original capture and five-sixths of them showed movement
of less then 10 meters.
During the colder months, green frogs hibernate underground in
moist soils or underwater (Martof 1953a). Lamoureux
and Madison (1999) tracked 11 radio-implanted green frog adults
and found that every individual overwintered in unfrozen stream
and seep beds- where water flow was observed throughout the winter.
Unlike other species of amphibians that go into hibernation for
the winter, there are reports of green frogs becoming sporadically
active during mild winter weather. (Dickerson 1906; Pope
1947).
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| HABITAT BREEDING |
| Green frogs breed in semi-permanent or permanent
freshwater habitats of various sizes and vegetation structure.
Typical breeding sites include lakes, ponds, margins of rivers,
streams, bogs, swamps, and vernal ponds with long hydroperiods
(Degraaf and Rudis 1983). |
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| HYDROLOGY |
Research conducted in Rhode
Island during the 2000 season, showed that green frog tadpoles were
detected in ponds of various hydroperiods, but were more commonly
observed using permanent ponds (Paton and Egan 2001).
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| MOVEMENT CHRONOLOGY |
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Adult green frogs emerge from hibernation and begin to arrive
at breeding sites in Rhode Island from early April to mid June;
however, it is not until mid to late May that the majority of
the animals arrive (Paton et al. 2000; Paton
and Crouch In press).
Metamorphs start to leave the breeding ponds in early July and
by mid September 95% have emigrated (Paton et al. 2000; Paton
and Crouch In press).
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| REPRODUCTION |
| In late spring to early
summer, green frogs migrate from their primary wetland habitats
to breeding sites (Martof 1953a). Males arrive first, occupy most
areas of the ponds except the non-vegetated deep zones, and begin
calling to establish (and defend) their territory and attract the
females (Klemens 1993; Martof 1953a). Females arrive later-usually
staying within their home range until they are ready to spawn-and
do not spend much time at breeding site (Martof 1953a). According
to Wells (1977) females are highly
selective and choose a male only after surveying all calling territories
for a few days. Mating occurs within the male's territory and is
usually complete within one day (Wells 1977). Breeding period and
egg deposition varies with locality (Degraaf and Rudis 1983), but
in Rhode Island typically peaks in June (C. Raithel, pers. comm.). |
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| EGG MASS |
| Each female green
frog deposits a clutch of 1,000 to 4,000 bicolored-black and
white-eggs (Wright and Wright
1949). Many females lay a second complete clutch within
three to four weeks after laying the first (Wells 1976). The
egg mass is laid on the surface of the water in a flat plane
usually less than a square foot across (15-30 cm). Individuals
eggs are 1/16 inch (1.5mm) and encased in an inner elliptic
shaped envelope (Wright and Wright 1949). Hatching time is
temperature dependent, but typically occurs in 3 to 6 days
(Wright and Wright 1949). |
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green frog egg mass
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| LARVAE |
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Green frog tadpoles are olive green above and iridescent
creamy-white below. The dorsum and ventral color change
is abrupt, unlike the similar bullfrog tadpole, which has
a more gradual transition. The dark mottled tail is elongated
with a low dorsal arc and a sharp tip (Altig
1970). In larger tadpoles, the diagnostic dorsal-lateral
ridge is sometimes visible long before metamorphosis is
complete (Dickerson 1906). Differences in the size, color,
and markings of the transforming tadpoles varies greatly
(Dickerson 1906).
Metamorphosis can occur within the same breeding season
or some tadpoles overwinter for one year depending on when
eggs were laid (Martof 1956).
In Michigan, Martof (1956) found that eggs deposited before
about June 25 were capable of developing in one season;
whereas, eggs deposited after about July 10 remained as
tadpoles until the following year.
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green frog tadpole
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| METAMORPHS |
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Recently transformed metamorphs in Rhode Island average
3.28 cm snout-vent length (see SIZE) (Paton, unpubl. data).
In a study in Ann Arbor, Michigan, Martof (1956) found very
similar results; the range and size of 286 frogs measured
was 2.8-3.6 cm with the mean at 3.26 cm. Furthermore, Martof
reported that tadpoles that overwintered transformed at
the same size as those who metamorphosed the same season
they hatched.
Metamorphs are olive-green to greenish-brown and look similar
to adults; however, as metamorphs, green frogs are difficult
to sex because the sexual dimorphism of tympanums present
in adults is not developed yet.
Martof (1956) reported that metamorphs generally dispersed
from the breeding sites soon after their tails stubs were
absorbed.
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green frog tadpole metamorphing |
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| JUVENILES |
| Sexual maturity: Males
are sexually active 1 year after metamorphosis and females mature
in either 2 or 3 years depending on their size (Martof 1956). |
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| FOOD |
Adults
Green frog feed as frequently
during the day as they do at night. They are opportunistic feeders,
who normally sit patiently in the water or close to shore and wait
for prey (Hamilton 1948). Stomach
analysis revealed that green frog eat a variety of aquatic and terrestrial
invertebrates such as beetles, spiders, flies, grasshoppers, caterpillars,
adult and larvae butterflies and moths, as well as small frogs,
molluscs, crustaceans (mostly crayfish) and their own cast skin
(Hamilton 1948). Dickerson (1906) also reports on green frogs feeding
on fairy shrimp, whirligig bugs, water striders, and backswimmers.
Terrestrial beetles are the most important food source (Stewart
and Sandison 1972).
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Larvae
A study by Jennsen
(1967) showed that tadpoles had no apparent food preferences,
but merely consumed whatever entomostracans and algae forms were
present. Farlowe (1928) found that the intestinal content of green
frog tadpoles reflected the floristic composition of the pond in
which it lived. Jennsen (1967) also documented that green
frog tadpoles fast during the period when external limbs appear
and the tail is absorbed. |
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| PREDATION |
Adults
American black ducks
(Anas rubripes), mallards (Anas platyrhynchos), herons
(Ardeidae), bitterns (Ardeidae), rails (Rallidae),
northern harriers (Circus cyaneus), and crows (Corvus
spp.) prey upon both tadpoles and adults (Stockwell 1999).
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Larvae
Tadpoles are eaten
by a variety of predators including bullfrogs, dragonfly, predacious diving
beetle, and giant water bug larvae and adults, fishing spiders, and backswimmers
(Stockwell 1999). |
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| CONSERVATION CONCERNS |
Green Frogs are secure
within their range. They are apparently less affected by
degraded landscapes and development than other species in this area (Klemens
1993, Paton and Egan 2001). They occupy a wide variety of habitats, most
which are protected under current wetland regulations; therefore, the
concerns that face obligate vernal pool species does not readily pertain
to them. |
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