by Greg Mayer
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”.
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)
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)