Mink frog (Lithobates septentrionalis)
|Also known as:||north frog, northern frog, Rocky Mountain frog|
|Synonyms:||Rana septentrionalis, Rana sinuata|
|Size||Length: 4.5 - 7.6 cm (2) (3)|
Tadpole length: up to 10 cm (2) (3)
The mink frog is classified as Least Concern (LC) on the IUCN Red List (1).
The mink frog (Lithobates septentrionalis) is a medium-sized North American frog, named for the distinctive mink-like or rotten onion-like odour of its skin. The skin of this species is smooth and is green or olive-brown on the upperparts, mottled or spotted with variable amounts of darker spots or irregular blotches (2) (3) (4) (5). The upper surfaces of its hind legs bear spots or lengthwise bars or stripes (3) (5). The underparts of the mink frog are whitish to yellow (2) (3) (4), and the lips and side of the head are green (2) (4).
In the male mink frog, the tympanum is larger than the eye, whereas in the female it is the same size or smaller (3) (4). The male may also have a bright yellow rather than white or pale yellow throat (3). Female mink frogs are usually larger than the males (4) (6), and tend to be more darkly pigmented, with larger females in particular sometimes having a dark reticulate (criss-crossing) pattern across the back (7). In northern parts of its range, the mink frog tends to grow larger than in the south (6) (8).
The tadpole of this species is brownish, olive or green, with dark spots, mottling on the sides, a yellowish belly and a reddish tail, which may also have dark mottling (2) (3) (4). The tadpole’s tail is long and has a pointed tip (3).
Although quite similar in appearance to the green frog (Lithobates clamitans), with which its range overlaps, the mink frog can be distinguished by its distinctive skin odour and by the webbing on its hind foot, which extends to the tip of the fifth toe. In the green frog, the webbing does not reach this far, and the green frog also has dark cross-bands rather than spots or lengthwise stripes on its hind legs (3) (4) (5).
The male mink frog calls with a series of deep, soft croaks that have been described as ‘tok… tok tok, tok tok’ or ‘cut-cut-cut-cut-cut’, likened to the sound of distant hammering (3) (4) (5).
The northernmost limits of the mink frog’s range are unknown (2), but it is known to occur from Labrador and the Maritime Provinces to Manitoba in Canada, and south into the northern United States, from Minnesota and northern Wisconsin in the west to northern New York and New Hampshire in the east (1) (2) (3) (4) (5) (9).
The southern limit of the mink frog’s range is at the highest latitude of any other North American frog or toad, leading to its alternative names of ‘northern frog’ or ‘north frog’ (9).
The mink frog is a highly aquatic amphibian which rarely ventures onto land, except on humid nights or during rain (3) (4) (9). This species inhabits a range of cool, permanent waters, including rivers, lakes, ponds, pools, ditches, bogs and streams (1) (3) (4) (9). It tends to avoid areas with rapid currents and wave activity, usually preferring quiet waters with abundant floating or emergent vegetation (3) (9), particularly at the borders of ponds or lakes, or near the mouths of the streams that empty into them (5).
Adult mink frogs are often seen sitting on water lilies or other emergent vegetation, such as mats of sphagnum moss (1) (4) (9).
Despite inhabiting northern areas, the mink frog cannot survive freezing temperatures, and so escapes the cold by spending the winter hibernating underwater in the bottom mud (1) (3) (4) (9). It usually enters hibernation by late September, and remains inactive until May (3).
After emerging from hibernation, the male mink frog begins calling from the water to attract a mate, either alone or in a group chorus (3) (4). The breeding season runs from late May or June to early August (3) (4) (5) (9), and most breeding activity occurs at night (10). The female mink frog lays a globular mass of 500 to 4,000 brownish eggs, which are attached to submerged plant stems. The eggs may be laid at depths of a metre or more (3) (4) (9), helping to protect them if the surface water freezes, and the egg mass may sometimes sink to the bottom before the eggs hatch (3) (9).
The length of time needed for the mink frog’s eggs to hatch is not well known (9), but once hatched the tadpoles normally take a year to fully develop, with metamorphosis occurring the following July or August (9) (10) (11), when the tadpoles are about 2.5 to 4.2 centimetres long (9) (11). However, some mink frog tadpoles take two years to metamorphose, and may grow to a larger size before transforming into the adult (3) (4) (9) (11). Northern populations in particular may have a longer tadpole stage and metamorphose at a larger size (6) (9).
The adult mink frog reaches sexual maturity around one to two years after metamorphosis, with the female tending to mature later and at a slightly larger size than the male (3) (9) (10). Individuals in northern populations mature later than those in the south (6) (9). Most mink frogs only live for between one and four years after metamorphosis, but some survive for up to five or six years (9).
The tadpole of this species feeds on algae (3) (9) (12), but the adult mink frog is an opportunistic predator and eats a range of insects and other small animals, including dragonflies, beetles, aphids, spiders, snails and even small fish (3) (4) (9) (12) (13). The diet may vary depending on the available prey, and most food is taken at the water surface, with the mink frog typically sitting on floating vegetation and waiting for prey to come close (3) (9) (12). Although the mink frog has been found to eat some plant material, this is thought to be ingested accidentally when prey is captured (9) (12) (13).
The mink frog and its tadpoles may be predated by American bullfrogs (Lithobates catesbeianus) and other amphibians, as well as by raccoons, herons, snakes and fish. However, its musky skin secretions are likely to discourage some predators (3) (4) (9). When threatened, the mink frog usually dives into the water and hides in mud or vegetation (3) (9).
An abundant and widespread species, the mink frog is not currently considered to be globally threatened. Its populations are generally thought to be stable, and much of its range is still relatively unaffected by human activities (1).
However, the mink frog may face a number of localised threats, including pesticides and an increase in UV-B radiation due to ozone depletion in the atmosphere. These, together with other chemical contaminants, have been suggested as possible causes of deformities seen in mink frogs in some areas (9) (14) (15). Some populations may also face threats from habitat loss and degradation due to shoreline development, pollution, the use of motorised watercrafts, and invasive plant and animal species (15).
In addition, the mink frog is likely to be impacted by climate change, particularly in light of its preference for cool waters which allow proper egg development (9) (15). Climate change may also allow competitors such as green frogs (Lithobates clamitans) and American bullfrogs (Lithobates catesbeianus) to move further north into the mink frog’s range (15).
The mink frog occurs in many protected areas across its range (1). Although no specific conservation measures are currently aimed at this abundant species (1), it may benefit from better protection of shoreline habitats and from efforts to control invasive species, as well as from long-term monitoring of its population trends (15). Further research is also needed into competition between the mink frog and the American bullfrog and green frog (15).
In addition, the deformities seen in some mink frog populations should be investigated, particularly given their implications for the health of humans and wildlife also potentially exposed to chemical pollutants (14).
Find out more about the mink frog and about amphibian conservation:
IUCN/SSC Amphibian Specialist Group:
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- Algae: simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
- Emergent: aquatic plants whose stems and leaves extend beyond the water’s surface.
- Hibernation: a winter survival strategy in which an animal’s metabolic rate slows down and a state of deep sleep is attained. While hibernating, animals survive on stored reserves of fat that they have accumulated in summer.
- Metamorphosis: an abrupt physical change from the larval to the adult form.
- Tympanum: also known as the eardrum. A thin membrane that transmits sounds from the air to the middle ear.
IUCN Red List (August, 2011)
AmphibiaWeb (August, 2011)
- Harding, J.H. (1997) Amphibians and Reptiles of the Great Lakes Region. University of Michigan Press, Michigan.
- Oldfield, B. and Moriarty, J.J. (1994) Amphibians and Reptiles Native to Minnesota. University of Minnesota Press, Minneapolis, Minnesota.
- Conant, R. and Collins, J.T. (1998) A Field Guide to Reptiles and Amphibians of Eastern and Central North America. Third Edition, Expanded. Houghton Mifflin, Company, New York.
- Leclair, R. and Laurin, G. (1996) Growth and body size in populations of mink frogs Rana septentrionalis from two latitudes. Ecography, 19: 296-304.
- Kramek, W.C. and Stewart, M.M. (1980) Ontogenetic and sexual differences in the pattern of Rana septentrionalis. Journal of Herpetology, 14(4): 369-375.
- Schueler, F.W. (1975) Geographic variation in the size of Rana septentrionalis in Quebec, Ontario, and Manitoba. Journal of Herpetology, 9(2): 177-185.
- Lannoo, M. (Ed.) (2005) Amphibian Declines: The Conservation Status of United States Species. University of California Press, Berkeley, California.
- Hedeen, S.E. (1972) Postmetamorphic growth and reproduction of the mink frog, Rana septentrionalis Baird. Copeia, 1972(1): 169-175.
- Hedeen, S.E. (1971) Growth of the tadpoles of the mink frog, Rana septentrionalis. Herpetologica, 27(2): 160-165.
- Hedeen, S.E. (1972) Food and feeding behavior of the mink frog, Rana septentrionalis Baird, in Minnesota. American Midland Naturalist, 88(2): 291-300.
- Kramek, W.C. (1972) Food of the frog Rana septentrionalis in New York. Copeia, 1972(2): 390-392.
- Gardiner, D.M. and Hoppe, D.M. (1999) Environmentally induced limb malformations in mink frogs (Rana septentrionalis). Journal of Experimental Zoology, 284: 207-216.
Wisconsin Department of Natural Resources (2005) Wisconsin’s Strategy for Wildlife Species of Greatest Conservation Need. Wisconsin Department of Natural Resources, Madison, Wisconsin. Available at: