Northern green frog (Rana [Lithobates] clamitans)

Close up of the topside of an American green frog sitting in shallow water
This non-native frog in Washington has a fold extending from the eye to  around the eardrum, plus it has parallel ridges along its back.  (Alan Schmierer - Creative Commons Public Domain)

AIS Aquatic invasive species

Classification: Regulated
Invasive species family: Ranidae
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Category: Amphibians
Family: Ranidae - True frogs
Common names: American green frog, Green frog

The northern green frog is native to the eastern United States. The frog has been introduced to Washington in a few places. NatureServe identifies the status of the northern green frog as an “exotic” (non-native) in Washington. Documented occurrences are in Whatcom, Stevens and King counties. Populations introduced to Washington were probably for harvest of frog legs, but little has been published regarding the species in the state.

Description and Range

Physical description

The northern green frog is a large, heavy-bodied frog with a distinct fold of skin (supratympanic fold) that extends from the eye, around the posterior (upper) edge of the external ear (tympanum) to the shoulder. Two distinct parallel ridges of skin (dorsolateral folds) are present on the dorsal (upper) sides of the body. The tympanum has a light-yellow area in the center.

Adults attain sizes up to 4 inches snout to vent length. The dorsal color is tan, brown or olive-brown with little to no black speckling. Dark spots may be present on the dorsal surface. Dark bands are present on the legs. Adults have green pigmentation on the sides of the head. Ventrally (underside), the body and legs are white to cream colored. Females may have some dark chest mottling. Juveniles resemble adults. Mature males have a tympanum much larger than the size of the eye, whereas females have a tympanum similar in size to the eye. Mature males also have a bright yellow throat and gray swollen pads (nuptial pads) on the thumbs.

No native Washington frog or toad has the supratympanic fold around the outer ear area. The exotic but common American bullfrog can be easily distinguished from the northern green frog by the lack of dorsolateral folds. This trait is present and distinct in American green frogs. Also, full-sized American bullfrogs are much larger than the northern green frog.

Larvae

Northern green frog tadpoles are easy to distinguish from all native Washington tadpoles. The tadpole (aquatic larval stage) has an oval body with dorsal eyes. The dorsal (topside) fin terminates on the body and the tail tip is pointed. Large tadpoles (greater than one inch total length) are olive green dorsally with dark spots over the dorsal surface. The ventral surface (underside) is opaque and cream colored with a coppery sheen. The throat area has dark green mottling. Small tadpoles (less than one inch total length) are darker. Tadpoles may attain a length of 3.5 inches total length before metamorphosis.

American bullfrogs tadpoles are similar but have distinct black flecks (not spots) over the dorsal surface and the belly has no metallic pigmentation. Bullfrogs also lack the dark green pigmentation on the throat.

Eggs 

A female lays 1000 to 5000 eggs in a thin film at the water surface usually attached to vegetation. The eggs typically cover an area less than 12 inches in diameter. Three gelatinous layers (envelope) surround the egg (ovum).

The non-native American bullfrog is the only species one will encounter in Washington with an egg mass similar to the northern green frog. The American bullfrog is widespread in the lowlands of Washington; their egg mass differs in being larger than 12 inches in diameter, being more than one egg layer thick, and individual eggs have only one gelatinous layer (envelope) surrounding the egg (ovum).

Voice 

Throughout the summer, Northern green frog males produce vocalizations. The species has a low-pitched, single-note advertisement call that sounds like a low-note banjo string being plucked “Clung!” This call may be given as a single note or repeated several times in a row. Alarmed green frogs, basking on water edges, produce a loud squawk as they dive into the water.

For more details about northern green frog, see the Washington Herp Atlas.

Ecology and life history

In their native range they occupy water bodies including wetlands, ponds, lakes, sloughs, creeks, and rivers. Adults are primarily “shore frogs,” spending most of their time on the edges of the water body they occupy. Juveniles will disperse into terrestrial habitats during rainy periods. Breeding habitats include a variety of permanent water bodies.

Northern green frogs likely overwinter in permanent water bodies near sources of well-oxygenated water.

Life history traits are probably similar to the American bullfrog. Calling likely commences in May or June. Egg laying likely takes place June to August. Tadpoles transform their first year or may overwinter and transform their second year.

Males hold territories scattered around the edges of permanent water bodies. Males aggressively defend their territories by wrestling invading males out of the territory or under water. Females approach males at their calling stations, apparently selecting males for mating based on habitat conditions at the calling site.

Egg development is rapid, taking anywhere from a few days to a week depending on water temperatures. Tadpoles grow quickly and may metamorphose their first year or may over winter and metamorphose their second summer. The tadpoles can grow to large sizes, 3.5 inches total length or more, before metamorphosis.

Geographic range

In Washington, northern green frogs have been documented in the Puget Trough Ecoregion and Canadian Rocky Mountain Ecoregion. More specifically, populations have been documented at Toad Lake, Whatcom County; Lake Washington, King County; and Lake Gillette, Stevens County.

This map from the Washington Herp Atlas illustrates the known distribution of northern green frog in Washington as of 2016. Populations are present just north of the Canadian-American border. It is possible the species has expanded its range into northern Whatcom County.

Green frog state distribution map prior to 2006, showing detection only in Stevens County
Washington Herp Atlas (2017)

For a map of range-wide distribution and conservation status of this species, check out NatureServe Explorer and the International Union for Conservation of Nature Redlist

Regulations

Licenses and permits

It is unlawful to take from the wild or import into Washington, hold, possess, offer for sale, sell, or release all frog species into the wild without the proper license to do so (WAC 220-450-030).

Rules and seasons

The northern green frog is classified as a “regulated” aquatic animal species under Washington law RCW 77.135.030.

It is against the law to release non-native species into state waters through intentional or unintentional means. 

Conservation

NatureServe identifies the status of the northern green frog as “exotic” (non-native) in Washington. These frogs are smaller than American bullfrogs and, therefore, may not be as problematic to native amphibians when it comes to predation. Competitive interactions and introduction of disease are potential issues.

Living with wildlife

Frogs, along with salamanders and newts, are members of the animal group called amphibians. Frogs start their lives as totally aquatic larvae with gills and a pronounced tail fin; this is familiar to many people as the tadpole stage. Over time, legs develop, the tail and gills are absorbed, and the frog transforms into a terrestrial, air-breathing animal.

Learn more about living with frogs, including how to attract them or prevent conflicts.

Resources

References

Dvornich, K. M., K. R. McAllister and K. B. Aubry. 1997. Amphibians and Reptiles of Washington State: Location Data and Predicted Distributions, Volume 2 in Washington State Gap Analysis - Final Report, (K.M. Cassidy, C.E. Grue, M.R. Smith and K.M. Dvornich, eds.), Washington Cooperative Fish and Wildlife Research Unit, University of Washington, Seattle, 146 pp.

Harding, J.A. and J.A. Holman. 1992. Michigan Frogs, Toads, and Salamanders: A Field Guide and Pocket Reference. Cooperative Extension Service, Michigan State University. 144 pp.

Hecnar, S.J.  and R.T. M’Closkey. 1996. Changes in the composition of a ranid frog community following Bullfrog extinction. The American Midland Naturalist 137: 145-150.

Nussbaum, R. A., E. D. Brodie, Jr., and R.M. Storm. 1983. Amphibians and Reptiles of the Pacific Northwest. University of Idaho Press, Moscow, Idaho. 332 pp.

Stebbins. R. C. 2003. A Field Guide to Western Reptiles and Amphibians, 3rd Edition. The Peterson Field Guide Series. Houghton Mifflin Company, Boston. 533 pp.

WDFW publications 

WDFW educational resources

Other resources