The weather has been unusually cold and snowy for early November, and there have been days when conditions were perfect for seeing detail in the tracks of small animals–an icy base with about half an inch of new, soft snow on top. On one of those days I went to a location where I had seen flying squirrel trails in past years, and I found some 
beautifully detailed prints. In the photo on the right (direction of travel to the right), the right rear track is at the bottom of the frame, and the right front is just above it. The other two prints are at the top, the left rear just behind the left front. The toes and middle pads show up nicely in both front and rear tracks. The heels of the rear prints made impressions, and the paired heel pads of the right front track can also be seen.
Now compare the shot above with these chipmunk tracks, photographed on the same day and 
arranged almost identically except that the left front track is just below the left rear. There’s a similar amount of detail, with toes and middle pads clearly visible in both front and rear feet and the paired heel pads showing in both front feet. I had hoped that if I found really detailed tracks I would see features that would separate chipmunks and southern flying squirrels, but to my eyes there are no appreciable differences between the tracks in the two photos. The dimensions are similar as well: both sets of prints have a trail width (the distance from the right edge of the right rear print to the left edge of the left rear print) of 2 inches, and the length of the front track is 9/16 inch for the flying squirrel and 5/8 inch for the chipmunk, not significantly different. So how did I know that the tracks in the first photo were made by a southern flying squirrel, while those in the second belonged to a chipmunk?
The answer came from the differing trail patterns. 
Southern flying squirrels have flaps of skin (patagia) that connect the front and rear legs all the way out to the ankles, so they move differently from chipmunks (and also from tree squirrels, for that matter). The front tracks of a bounding southern flying squirrel are set almost as wide as the rear, and they are usually in front of, or occasionally between, the rear tracks. Because of the skin flaps, flying squirrels are not as fleet-footed on the ground as other small rodents, so their leaps are shorter. Compare the southern flying squirrel bounding trail in the photo above (traveling from bottom to top) with the next photo of a trail made by a chipmunk (traveling from top to bottom). 
In its normal traveling bound the chipmunk consistently places its rear feet ahead of its front, and its leaps can be much longer than those of the flying squirrel. Of course chipmunks do sometimes make short leaps, and they do sometimes place their front feet between (as in the second photo of the blog) or ahead of the rear. That kind of pattern in a chipmunk trail is an indication of a break in the rhythm, while it falls withing the normal bounding pattern for a southern flying squirrel. (By the way, neither of the bounding photos came from the day I took the close-up shots, but they illustrate the trail patterns I saw that day.)
More snow changes everything. All bounding animals switch to what I call a double-register bound when their feet sink deeply into the snow. The trail pattern consists of sets of two impressions more or less side-by-side, created when the rear feet come down in the holes just made by the front feet. 
For an animal the size of a flying squirrel even a few inches of soft snow can be enough to change its gait pattern from its normal bound to a double-register bound like the one in the photo at the right (direction of travel from lower right to upper left). The relative positions of front and hind prints no longer apply, but trail width can still be measured, and this trail had a trail width of 2 1/8 inches, squarely in the range for the southern flying squirrel. A chipmunk trail would have had a similar trail width, but the trail pictured above was made during a long stretch of cold weather. Chipmunks wait out winter’s coldest periods in a state of torpor in their underground refuges, while flying squirrels come out regularly even in frigid temperatures.