opossum – Linda J. Spielman

Zig-Zags

In past posts I’ve used the term zig-zag to describe certain track patterns. In this article I’d like to delve more deeply into how zig-zags arise and what they can tell us about the animals that make them. When we humans walk in a relaxed, natural manner we place our feet in a zig-zag pattern because each foot falls to its own side of the line made by our moving center of gravity, the center line of the trail. It’s easy to verify this: Just walk naturally in snow or mud or on a dry surface with wet feet and then look at your tracks. The same logic applies to birds, so we often see patterns like the one in the next photo, made by a turkey walking from left to right. Each print angles inward, which helps to distinguish right from left. The sequence, starting at the left, is right, left, right, left, right.

Two legged zig-zags are pretty straightforward, but four-footed animals also create zig-zags, and it’s not as easy to understand how a four-footed animal can do that. Watching animals helps, but it’s hard to follow foot placement when animals are moving in real time. Fortunately for us twenty-first century trackers, there’s a tool that can bridge the gap–the internet. So let’s take a look at a video of a horse. If you click on this link: Bing Videos, then click on horses walking youtube and start the video, you’ll see a horse walking in slow motion. Notice that as each front foot leaves the ground the rear foot on the same side comes down in the spot just vacated by the front foot. The video doesn’t show the pattern on the ground, but it’s easy to see how the horse leaves a series of double impressions, each one a front track overlaid by a rear track. And since the feet on each side fall to their own side of the center line, the overall pattern is a zig-zag. The trail in the next photo, made by a deer walking from bottom to top, is a good example of a zig-zag made by a four-footed animal.

But all zig-zags aren’t the same. The physical characteristics of animals vary, and this affects the kinds of patterns they leave when they walk. There are also different types of walks, with differing relative placement of the front and rear tracks. In the photo above the walk is an almost perfect direct register gait, meaning that the rear feet fell almost exactly on top of the corresponding front tracks. The next photo shows tracks made by a woodchuck walking from lower left to upper right (and just below the second impression, tracks of a squirrel bounding toward the bottom). The trail is more variable but the tracks are mostly in indirect register, meaning that the rear tracks fell partly but not completely on top of the corresponding front tracks. Starting at the lower left the track sequence for the woodchuck is: right rear on right front, left rear on left front, right rear, right front, left rear on left front. Even in this more irregular trail the zig-zag is apparent.

The width of the zig-zag, known among trackers as trail width, varies from one species of animal to another. To measure trail width, find a relatively straight part of the trail and imagine or draw out two parallel straight lines that just touch the outsides of the alternate sets of tracks. Then measure the perpendicular distance between the lines. This is diagrammed in the next photo of the indirect register track pattern made by a walking opossum heading toward the upper right.

In the next photo you see a trail made by a gray fox walking from right to left. The trail has a different look from the opossum and woodchuck trails, both because of its narrower width and also because the fox’s step lengths are longer. But the zig-zag is still apparent. Trail widths, combined with step length, can be helpful in identification, since chunky animals like woodchucks and possums make wider trails and take shorter steps than slimmer, longer-legged animals do. And trail widths are especially important when you’re considering animals with similar step lengths. For example, trail widths for a walking coyote are generally between 4 and 5 inches while trail widths for deer moving at a walk range from 5 to 10 inches. Even when the tracks are degraded or obscured by collapsing snow it’s usually possible to differentiate between a coyote trail and a deer trail.

Animals find it harder to move in deep snow, but when they’re walking their trails still show the zig-zag pattern. In the photo below a red fox walked from bottom to top leaving a zig-zag arrangement of deep holes in the snow.

All of the gaits discussed above (and the one the horse was doing in the video) fit into what I call the regular walk–also called the diagonal walk in the tracking literature. But that’s not the only kind of walk animals can do. A common variant is the overstep walk. To see a dog doing the overstep walk click on this link: Bing Videos and then click on dog gaits youtube and start the video. The recording shows a dog walking at actual speed followed by the same sequence in slow motion. If you keep your eye on the spot just vacated by a front foot you’ll see the corresponding rear foot come down a little past it. (This video also does a nice job with the amble, equivalent to the pace-walk of the raccoon, and the trot.)

The interesting thing about the overstep walk is that the pattern of tracks on the ground also makes a zig-zag, but the points of the zig-zag consist of sets of two prints, front and rear from the same side, rather than the impressions of two superimposed tracks. In the next photo you see an overstep pattern made by a house cat moving from lower right to upper left. Because a cat’s front tracks are wider and shorter than the rear ones we can see that in each set the front track is behind the rear. The sequence, starting at the lower right, is: right front, right rear, left front, left rear, right front, right rear. Among animals that are habitual walkers, overstep walks are common.

Another variation you’ll come across is the understep walk. The next photo shows the trail of an opossum doing an understep walk, heading from the lower left to the upper right. Again, the prints are arranged in sets of two, each set the front and rear from the same side. In each pair the hand-like hind track, with its thumb pointing inward, lies behind the front track with its more evenly spread toes.

We sometimes find zig-zag walking patterns in the trails of animals that aren’t habitual walkers. Fishers move mostly in bounds or lopes, but they walk when extra caution is needed or when the footing isn’t secure. The trail in the photo below was made by a fisher walking, mostly in direct register, from lower right to upper left.

Walking trails are less common for minks than for fishers, and for minks it seems to be mostly about the animal’s dislike of unstable surfaces. In the next photo a mink walked from right to left through mud (looking pretty dry in the photo but probably much wetter and slipperier when the tracks were made), leaving sets of paired tracks. But which walk is this, overstep or understep? We can tell because the middle toe in the mink’s hind print usually angles a little to the outside. So the sequence, starting at the right, is: left rear, left front, right rear, right front, left rear, left front, right rear, right front, and this is an understep walk.

White-footed mice are even less likely to walk than minks, but the next image attests to the fact that they do it on rare occasions. A white-footed mouse walked from bottom to top, leaving sets of paired tracks. The four-toed front prints lie behind the five-toed rear prints in each set, so the mouse was doing an overstep walk. The trail both before and after the walking part was on drier footing with normal mouse bounding patterns, so it was the wet mud that made the mouse shift to a walk.

Many animals get around mostly at a walk, and zig-zags abound in the tracking world. The details of the patterns can tell us a lot about the nature of the track maker. But the sight of a zig-zag for an animal whose default gait is not the walk is an even more compelling call to investigate. In addition to their help in species identification, zig-zags can tell us how animals interact with each other and with their surroundings. In this post we’ve only made a start. There are other kinds of zig-zags, and even patterns that look like zig-zags but aren’t. I’ll keep these topics for a future article. In the meantime, follow the zig-zags wherever they lead.

Possum Puzzles

The opossum is a humble animal, slow moving, shy, and generally of a placid disposition. But opossums can present surprising challenges to the tracker, not the least of which is getting a handle on the tracks themselves. To understand opossum tracks it may be helpful to see the animal’s actual feet, so let’s take a look. The photo below shows the underside of the left rear foot of an opossum–it resembles a human hand with a large, widely angled thumb and four additional, finger-like toes. If you hold up your left hand with the palm facing you, you’ll see the resemblance. Try to imagine your hands as the rear feet of the animal.

In the next photo you see the opossum’s left front foot–very different from the rear. The five toes of the front foot are somewhat finger-like and similar to each other in shape, and the middle pads are quite bulbous. Both front and rear feet are adapted for climbing but are less ideal–especially the rear feet–for moving on the ground. This, combined with the animal’s heavy body and relatively short legs, means opossums are not very agile.

Now let’s look at opossum tracks. In the photo below the right front track lies on the left and the right rear track lies behind it on the right, both tracks oriented toward the left. The spreading toe indentations of the front track radiate from a compact grouping of middle pad impressions. In the rear track the thumb points to the side (downward in the photo), and the other four toes are closer together and angled to the opposite side (upward in the photo).

Because the opossum rarely moves faster than a walk (or sometimes a trot), front and rear prints are often partly superimposed, and that’s another source of confusion. (The animal whose tracks are pictured above was drinking at a puddle, so it left some nicely separated prints.) In the photo below you see a left rear and a left front track, oriented toward the right. The two tracks are so close together it’s hard to tell where one ends and the other begins. If you look at the right side of the frame you’ll see five similar toe marks radiating outward from four closely set middle pad impressions. That’s the left front track. The hollow made by the thumb of the left rear track sits just behind the front middle pads, and above it you can see the middle pad and toe indentations of the left rear track.

The indirect register walk is the opossum’s preferred gait, so we often see sequences of front and hind prints like the ones shown above. In the photo below an opossum walked from the lower left to the upper right, leaving the zig-zag pattern typical of the walk. Each angle of the zig-zag is composed of front and rear prints from one side, and in each of these couplets the hind print lies just behind the front print. The sequence of tracks is right rear, right front, left rear, left front, right rear, right front, left rear, left front.

When tracks are less distinct, possum trails can be downright perplexing. The next photo shows another walking opossum trail, again proceeding from lower left to upper right. The rear feet fell farther behind the front feet at each step, but the zig-zag pattern can still be seen. A few of the prints are recognizable as possum tracks, and the rest are just weird looking.

If an opossum needs to move a little faster it shifts into a trot, leaving a trail like the one shown in the next photo (oriented from lower left to upper right). It’s harder to sort out front and rear tracks in this trail because the snow was dry and the faster gait created more disturbance. But if you look closely you’ll see that the rear tracks are consistently just behind the front tracks. The sequence of prints is right rear, right front, left rear, left front, right rear, right front, left rear, left front.

We know it’s a trot because the trail is straighter than the walking trails shown in the previous photos, and the distances between the sets of tracks are slightly greater. There must have been a slight hitch in the gait of the animal that made this trail, because the claws of one of the right feet (it’s hard to tell whether it was the front or the hind) seemed to brush the snow each time it moved forward to the next landing spot.

You may have noticed that none of the possum trails I’ve shown so far have tail drag marks. Opossums don’t drag their tails as often as people may think, but it does sometimes occur. Here’s a photo of a possum trail (oriented from upper left to lower right) with a nice tail drag mark. Don’t worry if the direction of travel isn’t obvious–it’s hard to tell from the photo because of the angle. A fox left a galloping trail on the left side of the frame, moving from bottom to top.

Much of the opossum’s winter diet comes from scavenging on carcasses, and the animals don’t generally move very far away while a food source lasts. So if you come across a possum trail it’s worth following–you may find a feeding site, or even a den like the one shown in the next photo. I had to climb through and around lots of tangles and thickets, but I eventually found the den the opossum was using while it fed on a deer carcass not far away.

Opossum tracks and signs give us a window into the lives of the animals. But I’m fond of them for an additional reason: the tracks are just so quirky. In fact, the consistent peculiarity of possum tracks is one of the clues to their identity. So be alert for weirdness, and when you find it, consider the opossum.

Mud, Glorious Mud!

Unlike many people I know, I’m always sad to see the snow disappear for good. But as soon as I think of what comes next–mud season–I get excited all over again. The transition between the seasons is highlighted in the photo below. A gray fox had stepped in some mud and then left it’s muddy footprints on the snow as it walked from left to right. Each deposit of mud is made by first the front and then the rear feet from the same side, and the zig-zag pattern of the walk shows nicely.

Once the snow is gone, ordinary mud takes its place as a medium for recording tracks. In the next photo an opossum stepped in some mud at the edge of a puddle, leaving a collection of both complete and partially obscured prints oriented toward the left. At the upper left there’s a right front track with a right rear track just behind it. Farther to the right you can see part of another right front track. In the lower right corner there’s a nearly complete left front print and just the suggestion of a left rear behind it. The front prints show the five widely spread toes that are characteristic of the opossum. In the front print at the upper left the segments of the semicircular middle pad are especially clear. The rear track in the upper center shows the strikingly different form of the opossum’s rear foot: a thumb-like inner toe pointing inward and four additional toes close together and pointing outward.

One of the nice things about mud is that it can record the presence of animals that we don’t encounter during the cold season. A spotted salamander (or maybe two of them) walked through the mud in the next photo. These amphibians hibernate in winter and come out in early spring, so mud season is a good time to look for their tacks. There are two trips: one going from the left side toward the upper right and the other proceeding from left to right along the lower part of the frame. Each trail consists of a central drag mark made by the tail and a sequence of front and rear prints on each side. The patterns of the prints tell us that the animal(s) were moving at an understep walk, with each rear foot touching down just behind the front foot from the same side. In the lower trail you can see the difference between the smaller, four-toed front prints and the larger, five-toed hind ones.

The tracks in the preceding photos are pretty obvious, but it’s not always easy to spot tracks in mud. In the photo below there’s a patch of shiny mud in the center of the shot, and on the right side of that patch there are some tire tread marks. If you look on the left side of the same shiny mud toward the top you’ll see a red fox track. The animal was moving from top to bottom, and because there weren’t many muddy spots the print in the photo was the only one I could find.

The close-up below shows the same track, but in this view it’s oriented in the opposite direction, toward the top. The central mound typical of canine tracks can be seen, and the marks made by the hair on the underside of the foot show clearly. There’s even a partial impression of the bar in the middle pad.

Woodchucks, like salamanders, spend the winter below ground and often emerge just as mud season is beginning. The next photo shows the left rear track of a woodchuck at the upper left and a left front track at the lower right. The five clawed toes of the rear print show clearly–the middle three toes set close together and the inner and outer toes angled toward the sides. Behind the toes you can see the four segments that make up the middle pad. In the front track the four toes with their substantial claws can be seen. The subdivided middle pad of the front foot lies behind the toes, and the heel pads show as two depressions behind the middle pad. The front print has a curvature toward the inside, a trait typical of the woodchuck.

It takes a medium with a fine texture to show details of the tracks of very small animals, and what better medium than mud? In the photo below you see the tracks of a white-footed mouse bounding from lower left to upper right. The tracks are arranged in the typical rodent bounding pattern–two rear prints (in the upper right quadrant) that are widely set and almost even with each other. Behind the rear tracks, the front prints are set more narrowly and, in this case, slightly staggered rather than even with each other. Track details show beautifully, especially in the left rear (the topmost track) and the left front (farthest to the left). If we compare these tracks with the woodchuck tracks above we see the rodent family resemblance, especially in the rear prints. The symmetrical mouse front tracks are more typical of other small rodents than the curved front prints of the woodchuck.

You can’t ask for a better rendition of detail than the porcupine tracks in the next photo. Porcupines have unique foot anatomy: their tough, undivided soles have a pebble-like texture that gives the animals good grip when climbing. The photo shows a left front print and, just behind it and overlapping slightly, a left rear print. The tracks are heading toward the left, and the texture of the soles shows beautifully. Because the leading edge of the rear foot touches the trailing edge of the front track, the two tracks seem to be joined together. You may be able to pick out the claw marks of the hind print along the leading edge of the sole of the front print. The four claws of the front foot made indentations at the very left, and if you look closely there are marks made by the three outer phalanges of the front foot behind the claw marks.

Once the snow melts and the weather warms, mud may not last long. Puddles may dry up and wet areas may fill in with plant growth. But mud can also appear in new places, and abundant rainfall can bring on new mud seasons long after the early one is over. As a matter of fact, I found the porcupine tracks in the photo above in the month of July. So keep an eye on the conditions of the puddles in your neighborhood, and don’t be surprised if you come across some beautiful mud when you least expect it.