January 2019 Natural Mystery Answered

Last month's natural mystery generated lots of responses. Everyone who wrote in identified these as squirrel tracks, and they are indeed the tracks of a squirrel. But we had a few different guesses about which squirrel left these prints. Congratulations to Kirsten Welge, Anne Marie Meegan and R Scott Semmens who correctly identified the species. These are the tracks of a southern flying squirrel.

Flying squirrels are both the most nocturnal members of the squirrel family, and the most arboreal. They can be quite illusive, even where they are common, and it is rare to see their tracks except in snow. The size and gait are two of the best clues, but there are a few morphological details that can help us distinguish these tracks as well.

Kirsten Welge starts us off with these observations. Some point to rodent or squirrel generally, while others are more specific to flying squirrels and southern flying squirrel:

"The front tracks show four toes, symmetrically arranged. Hind tracks show five toes, arranged in a 1-3-1 pattern. The metatarsals register as a thin, curved line.

The gait looks like a hop, with hinds behind the fronts.

Habitat is also a match – particularly with plenty of woodpecker nearby to create holes in punky wood for nesting and latrine sites."

Anne Marie Meegan also noted the classic rodent characteristics in the number and arrangement of the toes, and added: "the boxy shape and fronts leading led me to conclude southern flying squirrel." She also notes: "I remember reading something by David Moskowitz about toe 5 (hind, outside toe) in northern being almost as long as the middle 3." The toe arrangement Moskowitz describes is characteristic of all three North American flying squirrels. Here is his blog post on the subject.

Finlaly, R Scott Semmens noted the habitat and the "boxy" hopping gait, then added: "trail width of around 5 cm [is a match for southern flying squirrel, while] the hind foot length is over 2.54cm ruling out a brave chipmunk."

Semmens' comment about the length of the hind print caught my eye and got me thinking a bit more deeply about track measurements. As part of my recent research on southern flying squirrel tracks, I was comparing the size of this nocturnal glider to that of the eastern chipmunk. On average, eastern chipmunks are larger in every dimension—including the length of their hind feet. Published ranges for the hind foot of southern flying squirrel (including the heel) range from 21-33mm, while those for eastern chipmunk range form 31-38mm.

Yet as Semmens notes, the published ranges for eastern chipmunk hind tracks tops out smaller than those for southern flying squirrel. Elbroch lists the length of eastern chipmunk hind tracks as ½-⅞” and the length of southern flying squirrel hind tracks as ½-1⅜”. Paul Rezendes offers us a clue as to how the larger animal might be leaving smaller tracks. In Tracking & the Art of Seeing, Rezendes gives measurements similar to Elbroch's for eastern chipmunk tracks, and then adds:

“The full length of the hind foot often does not show in mud, which explains the longer length of the front track compared to the hind. In snow, the hind foot will register its full length, and the hind track will be longer than the front track.”

It seems that the difference here is in the typical presentation of the tracks. Chipmunks tracks are rarely seen in the snow. These fossorial squirrels tend to stay underground most of the winter. Flying squirrel tracks, on the other hand, are rarely seen except in the snow. As a result, some of what we think of as differences between chipmunk and flying squirrel tracks may actually be difference between squirrel tracks in mud and squirrel tracks in snow. We could expect a chipmunk hind track in the snow to be a little longer than that of a southern flying squirrel.

Curiously, although it appears that Elbroch's measurements for chipmunk hind tracks do not to include the heel, both of his photos for this species in Mammal Tracks & Sign show the heel registering. Although neither photo includes a scale, using his other measurement ranges as a guide, the hind tracks appear to range up to 1½” long—considerably more than the ⅞” he lists as the maximum length.

I found this to be a good lesson in both the value and the limitations of track measurements published in the literature.

References:

Dolan, P. G., & Carter, D. C. (1977). Glaucomys volans. Mammalian Species, (78), 1.

Elbroch, M. (2003). Mammal Tracks & Sign: A Guide to North American Species (1st ed). Mechanicsburg, PA: Stackpole Books.

Rezendes, P. (1999). Tracking & the Art of Seeing : How to Read Animal Tracks & Sign (2nd ed.). New York: HarperCollins.

Schwartz, C. W., Schwartz, E. R., Fantz, D. K., & Jackson, V. L. (2016). The Wild Mammals of Missouri (Third revised edition). Columbia: University of Missouri Press : Missouri Department of Conservation.