Mink Fogs: The Frog of the North

Ever since my first introduction to the Northeast Kingdom (NEK) of Vermont back in high school I was hooked. Boreal forests, sphagnum wetlands, carnivorous plants, bubbling brooks, and moose-infested swamps all drew me back to the NEK time after time. It’s an area where the air even smells nice in a gas station parking lot! Birders flock to the NEK to catch a glimpse of many northern species found almost nowhere else in the United States, but it is the Mink Frog (Lithobates septentrionalis) in particular that I used as an excuse to justify my frequent trips to the kingdom.

Early on in college, I was already well-invested in herpetology, but it was the Mink Frog that really got me excited about filling gaps in the distribution maps. Not many people in the NEK were submitting their observations to the Vermont Herp Atlas at the time and far fewer would recognize a Mink Frog if they saw one, so it was relatively easy to drive up there for a day, pick a town with no Mink Frog reports, zero in on the appropriate habitat (beaver ponds, shrub swamps, and slow-moving brooks), and if successful in the hunt, extend the known range of the species. Plus, Mink Frogs are really cool.

In northern New England we have three “green-faced” frog species. They are “true frogs” (meaning they are in the Ranidae family), and they all look pretty similar. They are all greenish or brownish in color, can have dark mottling or spotting on their backs, have dark markings on their legs, a green upper lip, pale bellies, and mature males have bright yellow throats.  American Bullfrogs (L. catesbeianus) are the largest of the three species, have a rounded snout, and usually have distinct bands across their legs. Bullfrogs also lack dorso-lateral ridges, which are folds of skin running down each side of some frogs’ backs that are very useful for identification (dorso-lateral ridges are as useful for identifying true frog species as knowing whether a tree has alternate or opposite branching). Bullfrogs do have a fold of skin that wraps around their tympanum (ear drum), but the fold doesn’t go down the back. Green Frogs (L. clamitans) look much the same as Bullfrogs, but they are a little smaller, have a pointier snout, and have very distinct dorso-lateral ridges. Mink Frogs, however, are trickier.

The almond joy of the frog world (“sometimes you feel like a nut…”), Mink Frogs can have dorso-lateral ridges, but quite often they don’t. Their hind legs lack the banding pattern found on the other two species, though, and are instead patterned with roundish spots. More unique, if you are lucky enough to catch one, is their odor. Handling a Mink Frog causes them to release secretions that smell an awful lot like rancid onions, hence their reputation among northern herpetologists as the “scratch and sniff” frog. Mink Frogs are also smaller than Green and Bullfrogs, but that is hardly helpful for identification since all frogs start small. One frustrating (and also anecdotal) difference is the Mink Frog’s behavior when startled. When you scare a Green or Bullfrog they squeak, jump into the water, swim out a little way, then often turn around and come back to shore so nosy herpetologists can get a better look. Maybe it’s just me, but Mink Frogs fail to make that critical turn and instead just swim to the other side of the wetland, making that closer look near impossible.

Trudging through shrubs in the tranquil margins of boreal wetlands is one way to find Mink Frogs, but much easier in the summer is just driving around at night and listening. While Green Frogs can make a vocalization that sounds somewhat similar to a Mink Frog’s call, there is no mistaking a chorus. A lone Mink Frog makes a kind of knocking or clucking sound reminiscent of hammers gently striking wood, but a full blown chorus sounds more like the hooves of a dozen horses on a cobblestone street (the cobblestone analogy is common in field guides and might need to be updated…). I recently moved to a town in the NEK and searched tirelessly for Mink Frogs in the ponds and beaver meadows near my home without success, so imagine my surprise upon arriving home late one evening only to discover the sound of a strong Mink Frog chorus resonating from a small wetland a stone’s throw away from my house!

Common in eastern Canada, the range of Mink Frogs barely extends into the United States, dipping into the northern reaches of New England, New York, and the western Great Lakes region. In Vermont, Mink Frogs are really the only herp species not present anywhere to our south, so when herper friends and colleagues visit they are almost always anxious to find their first Mink Frog. Honestly, I still share that excitement. Perhaps it’s their association with habitats I love exploring in my favorite part of the world, but part of it could be that Mink Frogs are truly a northern species, and they are the only reptile or amphibian in my home state you can say that about. With the climate warming it is possible the species will one day recede out of the country, but for the time being I am elated to know all I need to do to find one step off my porch and walk downhill.