Head to Head: A Case of Two Confusing Frogs

Where the Green Frog (Lithobates clamitans) and the Mink Frog (Lithobates septentrionalis) co-occur, confusion over species identity often exists when unable to hear them call. This post highlights a few visual features to help with species identification in the field. Below are a few useful features to examine when differentiating between green frogs and mink frogs.

Left: Green frog (Lithobates clamitans). Right: Mink Frog (Lithobates septentrionalis). Photo: Christopher E. Smith

Left: Green frog (Lithobates clamitans). Right: Mink Frog (Lithobates septentrionalis). Photo: Christopher E. Smith

Figure 1. Adult green frog (Lithobates clamitans) with prominent dorsolateral folds. Wikimedia Commons

Figure 1. Adult green frog (Lithobates clamitans) with prominent dorsolateral folds. Wikimedia Commons

Dorsolateral Folds: These ridges are raised skin that run down the sides of a frog's back (Figure 1). In the green frogs these ridges are prominent, whereas in mink frogs these ridges are typically absent or faintly present.

Smell: As bizarre as it sounds, smell can also be useful in this case. Mink frogs smell distinctly like onions when handled. Green frogs... well, do not.

Body Shape: Though perhaps a bit more subjective, mink frogs have a slightly more compressed-appearing head and slightly more rounded snout when compared to a green frog.

Mink frog (Lithobates septentrionalis) from northern Minnesota. Photo: Christopher E. Smith.

Mink frog (Lithobates septentrionalis) from northern Minnesota. Photo: Christopher E. Smith.

Figure 2. Green frog (Lithobates clamitans) with striping on high leg. Photo courtesy of Mark Nenadov, Wikimedia Commons.

Figure 2. Green frog (Lithobates clamitans) with striping on high leg. Photo courtesy of Mark Nenadov, Wikimedia Commons.

Hind Leg Markings: In addition to the presence / absence of a dorsolateral fold, the pattern of marking on the hind leg can be very helpful. Green frogs have striping that often spans the crus and thigh (Figure 2). Mink frogs have blotches on the hind leg.

Get it? Got it? Not so fast... Interestingly, in some areas individuals possessing intermediate appearance occur (Figure 3). Are they green frogs? Mink frogs? Or something else entirety? I've often wondered if the two species hybridize, but to date I am not aware of molecular evidence to support a hybrid hypothesis. Perhaps someday someone will illuminate the mystery behind these intermediates. Until then... happy herping and document your finds on HerpMapper.org.

Figure 3. Frog displaying intermediate appearance. Prominent dorsolateral folds and pointed snout point toward green frog, hind leg pattern most similar to a mink frog. Photo courtesy of Simon Pierre Barrette, Wikimedia Commons.

Figure 3. Frog displaying intermediate appearance. Prominent dorsolateral folds and pointed snout point toward green frog, hind leg pattern most similar to a mink frog. Photo courtesy of Simon Pierre Barrette, Wikimedia Commons.

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