Tag: rodents

Sorting Out the Small Rodents

Rodents are considered one of the most successful groups of mammals, so it’s not surprising that the northeast hosts many different kinds. They range in size from the tiny woodland vole (weighing an ounce or less) to the beaver (50 pounds or more). The small ones dominate, both in abundance and in their potential to confuse. Most of these little creatures are active in winter, so it’s a perfect time to get a handle on their distinguishing features.

The photo below shows a set of prints made by a red squirrel bounding from left to right. The five-toed rear tracks are nearly even with each other and set widely, their three middle toes parallel and their inner and outer toes diverging. The four-toed front tracks are set more narrowly and staggered, and their four toes are slightly splayed. Behind the toe impressions, the middle pads of both front and rear feet (analogous to the bumps over the knuckle joints in your palm and the ball of your foot) appear as clear indentations. The heel pads of the front tracks (like the heel of your hand) show in both right and left front prints, and the heel area of the right rear track (analogous to the heel of your foot) is a smooth elongation behind the middle pads.

Here’s a chipmunk group of four, with the direction of travel this time toward the top. The left front and rear tracks are partly superimposed, but the similarity to the tracks in the first photo is plain to see. This is what I call the rodent foot plan, and once you absorb it you’ll recognize it in other small rodents, including squirrels, chipmunks, mice, and voles.

But there are some variations which–if available–can be important in pinpointing an identification. The photo below came from the bounding trail of a southern flying squirrel, an animal similar to a chipmunk in body size (although lighter in weight). Compare the middle pads in the right rear tracks (the farthest to the right in each photo): in the chipmunk they’re well separated and form a sharp curve. The middle pads of the flying squirrel are closer together and form a gentle crescent.

If your reaction to that is, ‘you’ve got to be kidding!’ you’re not far off base. It’s a real difference, but snow conditions are rarely perfect enough to see that kind of detail. So how often can we be sure which small rodent made the tracks we’re seeing? Quite often, it turns out, because we have two additional diagnostic tools: trail width and habitual movement patterns. The tracks in the photo below, a white-footed mouse bounding toward the upper right and a gray squirrel bounding toward the lower right, are similar arrangements but are vastly different in size. In this case it’s easy to know which is which, but for less obvious differences, such as red squirrel versus gray squirrel, measurement of the overall width of the pattern, known as the trail width, can really help.

To measure the trail width of a bounding animal, imagine or mark lines parallel to the direction of travel which touch the outermost parts of the two rear tracks. Below you’ll see the same photo with lines delimiting the trail widths. Next, measure the distance between the two lines. The nice thing about this is that the trail widths of our most common small rodents fall into a simple size progression. In inches, trail widths for white-footed and deer mice measure 1 1/4-1 3/4; chipmunks, 1/1/2-2 3/4; red squirrels, 3-4 1/2; and gray squirrels, 4-6. At 1 3/4-3 inches the trail width for southern flying squirrels is similar to that of chipmunks, and northern flying squirrels, at 2 3/4-4 1/4 inches, overlap on the low side with red squirrels. Although trail width can be determined for any gait, the bounding gait so common in small rodents is especially suited to this measurement.

Habitual movement patterns are another useful tool for identifying small rodents. In the next photo a gray squirrel bounded at a good clip from bottom to top, leaving groups of four prints separated by relatively long distances. In each group of four the landing tracks of the smaller front feet are behind the take-off tracks of the larger rear feet. Bounding trails of red squirrels and chipmunks are similar in overall proportions. It’s not that these animals always make long leaps. If they’re moving slowly the distances between the groups of four can be smaller, and the hind feet may not pass as far ahead of the front feet. Compare the arrangement of the gray squirrel prints in the previous photo with that of the slower moving red squirrel in the opening illustration. The point is that the habitual travelling movement of these animals creates trails with characteristic four-track groupings and relatively large spaces between groups.

Compare the pattern above to the next photo, the trail of a southern flying squirrel, bounding from lower right to upper left. In this trail the larger rear prints are behind the smaller front ones, and the distance between the groups of four is smaller. In the trails of northern flying squirrels the rear tracks are often ahead of the front, but both species of flying squirrels have sacrificed strength for lightness and aerodynamic design and are unable to match the long leaps of their non-gliding relatives.

Snow depth can affect the foot placement of bounding rodents. To the white-footed mouse that made the tracks in the photo below the snow was fairly deep, so the groups of four are reduced to sets of two, each of the paired impressions made by sequential impacts of front and rear feet from the same side. All of the rodents I’ve been discussing do this when deep snow makes it more energy efficient. But even in these reduced patterns trail width can still be measured, as long as we make sure we’re looking at the actual tracks and not the larger openings around them. And like squirrels and chipmunks, mice make shorter leaps when moving less energetically. An example of mouse trails with consistently shorter leaps can be seen in the opening photo of last month’s article.

Meadow voles are chunkier and have shorter legs than white-footed mice, so they can’t make long leaps, but their trails are roughly as wide as those of mice. It’s not always easy to tell whether a bounding trail with short leaps was made by a vole or a mouse, but if the trail goes on long enough differences usually show up. A vole’s foot placement is rarely as even and foursquare as that of a mouse, and voles tend to make frequent shifts in gaits. It’s not unusual for an individual vole trail to vary among lopes, bounds, trots, overstep walks, and scurrying gaits that are difficult to categorize. In the next photo there’s a partly roofed vole tunnel meandering between the lower right and the top center. A vole traveled from the left side of the frame toward the tunnel in a bounding gait, with typical short leaps and uneven foot placement. The thin line in the center of the trail was made by the tail.

If you’ve made it this far in this treatise, you may feel like your brain is reeling. Believe it or not, I had to leave out many details, and I haven’t even addressed the issue of distinguishing small rodents from other small mammals. The important thing is to get started. Every time you work through a small rodent puzzle you’ll learn more. So be patient and persistent, and enjoy the eureka! moments when a few puzzle pieces fit together to form part of the larger picture.

A Perfect Storm of Jumping Mice

I guess I was just in the right place at the right time. I was in a part of the western Adirondacks where the soils are sandy–they’re called glacial outwash soils, and they’re a gift from our glacial past. After a day with several heavy downpours the weather cleared, and the next morning I went out to look for tracks. I headed to one of my favorite spots, a sandy truck trail that meanders through a mosaic of wet meadows, marshes, and shrubby uplands. The low spots in the road had filled with water during the rain, but the sandy soil had allowed the pooled water to drain away, leaving perfect tracking mud.

And there were tracks aplenty, mostly small rodents. But not just the usual small rodents–many tracks had the distinctive features of a very special animal. In the photo below (direction of travel toward the top) the five-toed hind tracks are in the upper section, nearly even with each other and set widely apart. Below them are the four-toed front tracks, one leading the other and set more narrowly.

These are the tracks of a small rodent called the meadow jumping mouse. (It’s cousin, the woodland jumping mouse, is restricted to boreal forests and would not have been found where I was tracking that day.) The meadow jumping mouse is pictured in the photo below by Martha Beck (from her blog, Martha’s Blog). This beautiful little creature has a very long tail, large ears and eyes, long back legs, and really interesting rear feet.

Here’s another example of the tracks I found that day. The direction of travel is toward the right, and the front prints (on the left side) are distorted by the impact of landing from a long jump. Nevertheless you can see that the rear tracks (on the right side) are much larger than the front,

especially in this example in which the entire lengths of the rear heels touched down. These long heels are unique to the jumping mouse, as are the long, slender toes. The three central toes are often slightly curved, and they spread more than the corresponding toes of most other small rodents. Another special feature lies in the elongated middle pad area of the rear prints. The inner and outer toes attach considerably behind the area where the three central toes come together.

In the photo below, another one from that amazing day, the elongated middle pad area shows nicely in the left rear print (farthest to the left). The right rear track is on the extreme right, the two front tracks lie between the two hind tracks, and the direction of travel is toward the top. Some interesting details of the front prints can be seen in the right front print (the lowest of the group): it’s canted toward the outside, with the innermost toe pointing up and a little to the left, the toe next to it pointing almost directly upward, the third toe pointing toward the right, and the outermost toe pointing downward. The middle pad area of this track is in the center, and the paired heel pads show at the lower left edge of the print.

Once you’re aware of the critical details, jumping mouse tracks look very different from the tracks of other small rodents. The meadow vole tracks in the photo below, also from that wonderful day on the sand road, are in the same relative positions as the jumping mouse tracks in the previous photo. But the front and rear prints of the meadow vole are similar in size and there is no elongation of toes, heel, or middle pad. The inner and outer toes of the hind tracks show as small ovals on either side of the middle pad area, and the front tracks are only slightly angled.

Although their feet are larger, jumping mice are actually much smaller than meadow voles, but they’re similar in size to another common small rodent, the white-footed mouse. But white-footed mouse tracks are also very different from jumping mouse tracks. The white-footed mouse tracks in the photo below (direction of travel toward the top with hind tracks above and front tracks below) are tiny compared to jumping mouse tracks, and the toes in both front and rear prints are small ovals with no connections to the middle pad. The three central toes of the rear print are close together and parallel with each other, and the middle pads are a distinct series of bumps.

My walk along the sand road that day was a real revelation. Although the drying mud puddles may have been more attractive to jumping mice than to other small rodents, I still have to believe that the abundance of jumping mouse tracks indicated high population numbers. Those scattered, moist habitats were just the kinds of places favored by meadow jumping mice, and the muddy low spots were perfectly situated to capture the animals’ movements. Since jumping mice spend the winter in extended hibernation we don’t have the luxury of seeing their tracks in snow. But a perfect storm of favorable influences created conditions rivaling the best snow tracking, revealing jumping mouse tracks like I’ve never seen them before.

9/26 – I just got a comment from Janet Pesaturo about the range of woodland jumping mice, which is broader than I realized. They’re found in mixed softwood/hardwood forests in temperate zones as well as in boreal forests. Thanks, Janet.

Feet Tell the Story: Family Resemblances Among Small Rodents

The smaller the creature, the tinier the feet–and the less often we’re able to see the kind of detail that we’re accustomed to seeing in the tracks of larger creatures. So I was delighted a few weeks ago to find these beautifully revealing chipmunk tracks. The one that first caught my eye was the right rear print that lies off by itself in the lower right part of the photo. The much larger rear print of a gray squirrel lies above it, and at least two other chipmunk tracks are visible among the unrelated disturbances in the upper part of the photo. The chipmunk’s right front print sits in the left part of the frame midway between top and bottom, and its left rear print can be seen above the squirrel track. The left front print isn’t obvious but a few small depressions suggest that it lies above the right front in the upper left quadrant of the photo. The chipmunk was moving toward the right.

The two right prints of the chipmunk show excellent detail, so I’ve focused in on them in the photo to the left. The toes and claw marks are visible, four of each in the front track (at the upper left) and five in the rear track (at the lower right). Behind the toes you can see the grouped depressions that make up the middle pads of both the front and rear tracks. For such a small creature those tracks are exquisite.

Why do I get so excited about such stuff? The finely formed details of animal tracks contain such energy and elegance that I just love to look at them. But beyond that, track details can reveal an animal’s affinities, in this case the affinities between chipmunks and other small rodents. The gray squirrel tracks in the next photo (moving toward the top of the frame, rear prints above and front prints below) help to illustrate the important features shared by this group. In the rear prints the central three toes lie close together and point forward, while the inner and outer ones sit farther back and are angled to the sides. The four toes of the front prints are spread more or less evenly. The middle pads of both front and rear feet are made up of four depressions, arranged in a crescent in the rear and a more triangular shape in the front. In the front print the heel pads, located just behind the middle pads, show as small paired depressions.

There are lots of rodents, and some have foot structures that depart from the characteristics I just described. But our most common small rodents–including one even smaller than a chipmunk–are surprisingly consistent in showing this suite of features. It took perfect mud to register the details in these white-footed mouse tracks (heading toward the top of the photo), but the family resemblance comes through clearly. The numbers and arrangements of the toes are the same, and the middle pads of both front and rear prints are similar to those of the chipmunk and the squirrel. You can even see the heel pads, albeit slightly smeared, in the front tracks!

Family resemblances can extend to the gait level as well, and they certainly do here. Widely placed rear prints and more narrowly placed front prints, positioned behind the rear ones, represent a typical pattern for bounding or jumping small rodents. Of course this pattern changes when different maneuvers are required, and even at a steady bound the four tracks aren’t usually as perfectly placed as the ones in the snow photo of the gray squirrel. But both gait patterns and track details are useful clues to the identities of our most common small rodents.