GREEN FROG
Rana clamitans melanota
 
INTRODUCTION

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.

 
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).

 Male green frog
tympanum is larger than the eye


 
SIZE
AGE / SEX SVL (SNOUT VENT LENGTH) (cm) SAMPLE SIZE

AVERAGE

RANGE

Std. deviation

ADULT FEMALE

7.7

6.4- 9.8

0.9

36

ADULT MALE

7.5

6.3- 9.5

0.7

44

JUVENILE

4.9

3.6- 6.5

0.6

77

METAMORPH

3.3

1.8- 4.5

0.6

871



AGE/ SEX MASS (g) SAMPLE SIZE

AVERAGE

RANGE

Std. deviation

ADULT FEMALE

51.2

28.5- 85.0

16.9

36

ADULT MALE

44.2

28.5- 58.3

8.7

44

JUVENILE

11.7

5.3- 26.1

4.7

77

METAMORPH

3.6

0.4- 7.0

1.2

871







 
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.
Regional
Abundant and presently secure through the region (Klemens 1993).
 
DISTRIBUTION
Rhode Island
Green frogs are common throughout the state found in a variety of habitats, including urban areas and the islands.
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).
 
HABITAT NON BREEDING

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).

 
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).  


 
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).

 
MOVEMENT CHRONOLOGY

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).

 

 
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.).
 
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).  
green frog egg mass

 
LARVAE

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.


green frog tadpole


 
METAMORPHS

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.


green frog tadpole metamorphing

 

 
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).
 
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).

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.
 
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).
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).
 
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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