Amphibians
Mink Frog
Lithobates septentrionalis · also called north frog
Pick up a mink frog and you will know it. The small green frog of the northern lakes gives off a sharp smell of rotten onions and musk, the same note a weasel uses in defence, and that smell is where it gets its name.
Overview
Lithobates septentrionalis is a small, highly aquatic green frog, 5 to 7 cm long, of the cold lakes, bogs, and slow streams of northeastern North America. It floats among lily pads and rarely comes onto land, hunting beetles, dragonflies, and other invertebrates from the water.
The smell and the sound
Handled or alarmed, it releases a skin secretion that smells strongly of onions, garlic, or mink. The male’s call is a fast, hollow wooden knocking, like a stick tapped rapidly against a log, carrying across a still lake at night.
A cold-water frog
It is a northern species, breeding late in summer, and its tadpoles usually overwinter, taking one to two years to transform. It looks much like the related green frog but usually lacks the long back ridges, has rounder blotches, and, of course, smells.
Status
Least Concern and common through the cold northern forests, though a warming climate is a concern along its southern edge.
Mink Frog: common questions
Why does the mink frog smell so bad?
Like many frogs, it produces defensive compounds in the granular glands of its skin, and in the mink frog those compounds are unusually pungent. The smell is released when the frog is handled, seized, or stressed, and it is thought to make the frog unpleasant to a predator that has it in its mouth, encouraging the animal to spit the frog out. The comparison to a mink is apt, since minks and other members of the weasel family also use a strong musky secretion for defence, and both smells hit the same sharp, oniony, slightly rotten note. Washing your hands after handling one takes a while.
How do I tell a mink frog from a green frog?
They are very similar, live in the same northern ponds, and hybridise where they meet, so it can be genuinely hard. The best clues are these. A green frog has a pair of prominent ridges of skin running from behind each eye down the length of the back, while the mink frog has these ridges absent or only weakly developed on the front part of the body. The mink frog's dark markings tend to be more rounded blotches and spots, and less like the green frog's finer reticulation. And the mink frog usually smells strongly of onions when handled, which the green frog does not. The calls are different too, a hollow banjo-like note for the green frog and a fast wooden knocking for the mink frog.
Where does the mink frog live, and why only in the north?
It is found across the cold-summer belt of northeastern North America, from the Maritimes and New England west through the Great Lakes into Minnesota and Manitoba, and it does not occur further south. It is a cold-water specialist, tied to permanent, clean, well-oxygenated ponds, bog edges, and slow streams, and it is active and breeds at water temperatures that would be marginal for many frogs. Its tadpoles usually spend at least one winter in the water. That reliance on cold, permanent water is why its range stops where it does, and why a warming climate is a concern for the species along its southern boundary.
Is the mink frog rare or threatened?
No. Across its range it is common, sometimes abundant, in suitable habitat, and it is listed as Least Concern. Because it lives in the cold, boggy, lake-rich country of the northern forests, much of its habitat is in lightly developed regions and it faces less pressure than frogs of farmland and cities. The main long-term concern is climate warming, which could push the species northward and shrink its range from the south, and, as for all amphibians, the spread of chytrid fungus and other diseases.