Spot the wood frog

June 16, 2014 • 10:00 am

by Greg Mayer

Well, it’s not that hard to spot, but you can see how the wood frog (Rana sylvatica) is aptly named.

Wood frog, near Lake Superior, Minnesota, 6 June 2014.
Wood frog, near Lake Superior, Minnesota, 10 June 2014.

My Minnesota correspondent found this fellow along Caribou Trail (a road) and Jonvick Creek near Lutsen, Cook County, Minnesota, about a half mile from the north shore of Lake Superior, on 10 June 2014. The region is mixed spruce and maple forest; the frog was in a “mapley” area. The great herpetologist Robert C. Stebbins thought the species’ distribution tracked, for much of its range, pretty closely to the distribution of spruce.

The distribution of wood frogs is interesting for at least two reasons. First, they are the most northerly distributed of any North American amphibian (or reptile, for that matter), and extremes are always interesting. They can survive for weeks at temperatures below freezing, in part through elevated levels of blood glucose acting as an “anti-freeze”.

Range of the wood frog (Rana sylvatica), from USGS via Wikipedia.
Range of the wood frog (Rana sylvatica), from USGS via Wikipedia.

They’re not immune to freezing though—I once found, during an early spring field trip near Northampton, Massachusetts, a dead female who had laid her eggs in a small pond. She was perfectly intact, and I suspected she had frozen, as contact with ice crystals (from the pond) makes them more vulnerable to freezing.

Second, there are a number of outlying populations to the south of the main range (which, as shown above, crosses northern North America from the Bering Strait to the North Atlantic, descending into eastern North America along the Appalachians). In particular, note the outliers in Colorado and Wyoming. These are almost certainly relicts from cooler glacial times when the frog occurred further south in the Rockies; it moved northward as the glaciers retreated, leaving behind populations in some favorable southern localities. The isolated Colorado-Wyoming population was named as a distinct species (maslini), but currently it is not recognized, not even as a subspecies.

Wood frogs are are also famous for another “non-subspecies”: cantabrigensis, a short-legged form from the northwestern part of the range, versus the longer legged ones to the east. While the variation in leg size is real, there is a gradual cross-continental gradient (a cline, in technical terminology), with no break in leg size, and most systematists do not distinguish such clinal patterns of geographic variation with nomenclatural recognition. So cantabrigensis is not recognized either, and the wood frog has become a classic case of clinal variation.


Bagdonas, K.R. and D. Pettus. 1976. Genetic compatibility in wood frogs (Amphibia, Anura, Ranidae). Journal of Herpetology 10:105-112 (jstor)

Costanzo, J.P., M.C.F. do Amaral, A.J. Rosendale and R.E. Lee. 2013. Hibernation physiology, freezing adaptation and extreme freeze tolerance in a northern population of the wood frog. Journal of Experimental Biology 216:3461-3473. (pdf)

Dodd, C.K. 2013. Frogs of the United States and Canada. Johns Hopkins University Press, Baltimore (publisher) (Google books)

Porter, K.R. 1969. Evolutionary status of the Rocky Mountain population of wood frogs. Evolution 23:163-170. (jstor)

Stebbins, R.C. 2003. A Field Guide to Western Reptiles and Amphibians. 3rd ed. Houghton Mifflin, Boston. (publisher)

16 thoughts on “Spot the wood frog

  1. This year I saw a leopard frog in my carport that completely froze. I figured he was dead but he moved, found a puddle and froze up again.

  2. One of 3 frog species that herald spring in our little spring-fed pond each year. The cricket frogs arrive first with their thumb-on-a-comb song, next the peepers, next the Wood Frogs with their croaking that always reminds me of ducks. (What most people think of as duck sounds, anyway; really, only mallards quack the classic way.)

    One spring my young son collected from the pond a huge mass of frog eggs, which we blithely set out to raise ourselves. We soon had 250 ravenous tadpoles for which I blanched romaine leaves 3 or four times a day and also for which I had to change the tank water an equal number of times. Happily nearly 100% successfully metamorphosed (at which time we could be sure they were Wood Frogs) and were returned to the pond environs. I’m sure that fewer would have survived in the wild, so I was curious to see if I noticed an uptick in the Wood Frog population the following year. No–the pond seems to support about the same small number every year. With all the problems amphibians face these days I’m always relieved to hear them return each year.

    1. The pond next door to me has frogs show up in that same order (I can only tell by the sounds they make). I’m always amazed at the little spring peepers out when it is still quite cold.

      The last to arrive are green frogs and toads.

      1. That all sounds very familiar. 🙂

        Yes, the chorus frogs & peepers are my harbingers of spring and it is amazing that these little ectotherms (or whatever the preferred term is these days) are so lively & comfortable when many times there can still be snow on the ground or flurries still to come.

        The Gray Treefrogs are a favorite, too. They’ll hang out on the windows & slider at night where the house lights draw the bugs; and take residence on the deck some years, often spending the daylight hours sitting in what seems a terribly exposed location, sun-wise.

      1. Wow, glad you caught that! Chorus Frogs, of course, is what I meant to type. They & the peepers show up in numbers.

        Blanchard’s Cricket Frog is a species of special concern in MI; I wouldn’t be surprised if there are a few around here as our habitat is great for small amphibians, but I haven’t spent enough time looking/listening for them.

        (By the time the Green Frogs, Gray Treefrogs, and Toads start calling the mosquitoes & deerflies are also in full bloom, along with the heat & humidity. Spring herping is much more appealing. 😀 )

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