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Canadian Herpetological Society Société d'Herpétologie du Canada |
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Mink Frog
Lithobates septentrionalis
Family: Ranidae
Until recently, the Mink Frog was in the genus Rana, but it is now in the genus Lithobates.
COSEWIC status:- Not Assessed
- No Status
- Least Concern
Description
The Mink Frog is a medium sized frog that can reach 8 cm in length. Individuals are olive to brown in colour and usually have dark spots or mottling on the back, sides and hind legs. The belly is whitish, often grading to pale yellow at the sides. This species has slightly upturned eyes and the dorsolateral folds (folds of skin running down each side of the back) may be prominent, partial or absent. The webbing on the hind feet is extensive and reaches the last joint of the longest (4th) toe. Males are generally smaller than females and the tympanum (eardrums) is noticeably larger than the eyes, while the females’ tympanum is about the same size as the eyes. Larvae (tadpoles) have long tails with a large fin, lack front legs (newly hatched tadpoles are legless), and are dark brown to green with small, dark spots and straw yellow bellies. The larvae can reach 10 cm in length before transforming. The call of this species consists of a rapid series of three or more croaks, which sound like the tapping of a metal hammer on wood. A large chorus of mink frogs sounds like popcorn popping.
Similar Species
The Mink Frog can be confused with the Green Frog and the American Bullfrog, and these three species exhibit substantial variation in colour pattern. The Green Frog and Bullfrog have dark crossbands on the hind legs, whereas the Mink Frog has spots or blotches on the hind legs. The Mink Frog has more extensive webbing on the hind feet which reaches the last joint of the longest (4th) toe. The American Bullfrog lacks dorsolateral folds and there is a noticeable fold of skin that runs above and behind the tympanum. Unlike these other two species, Mink Frogs release a musky odour when disturbed.
Distribution
Click for larger image
The Mink Frog is found in eastern Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia, and Labrador. In the United States it is limited to the northeastern states, from Minnesota to Maine and no further south than central New York. The southern limit of the Mink Frog is farther north than any other frog in North America and, unlike most North American frogs, the majority of this species’ distribution is in Canada.
Habitat
Mink Frogs are highly aquatic and are rarely found on land. They inhabit large, permanent bodies of water with abundant vegetation and soft bottoms, such as wetlands, lakes, ponds and slow-moving rivers. Permanent aquatic habitats are required for breeding and hibernation. The larvae and adults hibernate underwater, and the adults bury into the mud at the bottom of the waterbody.
Biology
In Canada, Mink Frogs hibernate during the winter and are active from late April or May until September or October, depending on latitude. Mink Frogs breed from June to August in Canada. Males attract mates by calling, and choruses increase in intensity through the night and peak before dawn. During breeding, the male grasps the female (amplexus) and fertilization occurs externally in aquatic habitats as the female lays eggs. The female lays 500–2000 eggs in a globular mass that can be up to 15 cm long and 7.5 cm wide. The egg mass is usually attached to submerged vegetation that is more than 1 m below the surface. The eggs are brown to black on top with a lighter underside and are surrounded by two clear jelly envelopes. The eggs hatch in less than 2 weeks, and larvae will undergo metamorphosis after 1–2 years. Mink Frogs reach maturity 2–3 years after metamorphosis, and individuals can live up to 5–6 years after metamorphosis. Mink frogs are active during the day and night but feed primarily during the day. They forage among aquatic plants in open water for small invertebrates such as dragonflies, spiders, beetles, water striders, slugs, snails, butterflies and other insects. Males may not always establish and defend territories, but they will attack other males that get too close during the breeding season.
Threats
Mink Frogs occur throughout large expanses of northeastern Canada that are relatively undeveloped, and threats to this species are minimal throughout most of its range. Since this species is highly aquatic, the destruction of its wetland and shoreline habitat can result in localized population declines or extirpation. Pollution, such as herbicides, agricultural effluent and road salt, can be detrimental to frog populations by causing direct mortality as well as developmental deformities. This species is likely less at risk of being killed on roads than many other frog species due to its highly aquatic nature, though road mortality may still occur when roads are in close proximity to this species’ aquatic habitat. Pathogens such as chytrid fungus and Ranavirus can cause mass mortality of frog populations. Climate change may also pose future threats to Canada’s frog populations. Mink Frog populations are believed to be stable, but there has not been much research on this species in Canada.