The Mystery:
This picture was taken on the same early March trip to central Washington as last week's mystery.
The Questions:For all of these questions, you may want to click to enlarge the picture to get a good enough view.
1. What is the predominantly used gait?It is difficult to tell what gait is used where the tracks are all overlapping on the trails, but pretty much everywhere that individual sets of tracks are visible the animal is moving in a bounding pattern.
In the close up picture, the tracks toward the bottom are the front feet, toward the top are the hind. Notice how the back feet are in line with each other, they need not be exactly aligned but this alignment is part of the definition of a bound. The front feet can be more or less aligned.
A bound is a subcategory of a gallop though in my experience it is rarely discussed as such.
2. What species? (or as close as you can get)
The gait, size and foot morphology point to a small rodent. Most rodent species show four toes in their front tracks and five in their hind. It may often be difficult to see this exact count, but one thing that often is visible is how in the hind foot there are three aligned toes more or less pointing forward. The other two toes point of to the sides and are sometimes less visible. The four toes on the front foot are often more evenly spread apart.
One of the things I find most useful when trying to identify unfamiliar tracks is having a master list of species for that area that I can work down from. Most of the animals this size that come to mind are rodents, but there certainly are other types of animals in this size range including shrews and lizards. Shrews are actually even smaller than this though and I have a hard time imagining a lizard moving in a bound. In this region though there are quite a few potential rodent options. Broadly there are (potentially): pocket mice, kangaroo rats, pocket gophers, wood rats, voles, deer mice, harvest mice and grasshopper mice.
Wood rats, kangaroo rats and pocket gophers all have tracks noticably larger than these. This is not a habitat I would expect to see voles, they seem to hang out largely in grassy areas rather than dry sandy patches like this. Pocket mice (closely related to kangaroo rats) moving in a bound often have their front tracks overlapping rather than separated as is the case in most of the visible track patterns here, they also have a narrower trail width (I think the trail coming from the lower middle portion of the picture and exiting mid right may be that of a pocket mouse). Peterson's Mammals field guide says western harvest mice are usually found in dense vegetation close to water, so again this habitat doesn't quite fit. I've found no information on grasshopper mice tracks, so I can't rule them out. But I think the best fit is the deer mouse.
Deer mice are common in this habitat (and many others). The toes of their tracks are generally bulbous (compare with voles whose toes tend to show up more thin and finger like.)
I'm impressed by the amount of traffic evidenced here. I wonder if it was more than one mouse. I imagine that a single mouse could make such trails, but it was late winter so there could be breeding mice sharing a place or mice sharing a place for warmth at night.
3. Why are there sections in the middle of the heavily used paths that only have a couple sets of tracks in them?
I was pretty puzzled when I came upon these tracks. I imagined little mice full of bravado running up towards each other, but stopping in time to maintain a safe distance. They would taunt and threaten and run back. Occasionally one would boldly cross the gap to the other's side to show what a brave fellow he was.
This hypothetical posturing didn't seem very likely to me though.
If I recall correctly, I got the hypothesis that I think is correct from Dave Moskowitz who is the lead tracking instructor at Wilderness Awareness School (I was out as part of WAS's tracking intensive class when I took these pictures.)
If you imagined the major paths to continue straight across the gap then this picture would be easy to interpret: there was a mouse (or mice) going back and forth along a couple of major trails. If you assume that the major paths generally did not connect up (except in the couple of sets of tracks that span the gap) it would be much more difficult to put together. Why would a mouse be stopping suddenly in the same spot over and over?
So let's start by supposing that the trails generally do connect up and for some reason we aren't seeing most of the tracks in the gap section.
One possibility is that something moved through the gap section and wiped out most of those tracks and the mouse ran through afterwords a time or two leaving the gap tracks that are visible. This hypothesis seems to be supported by the fact that the main trail has a variety of agedness while the gap tracks seem to be fairly fresh. However the area seems quite smooth, so it is a little hard to imagine something went through there wiping tracks clean while leaving so little disturbance behind it.
Some people mentioned in the comments that it seemed like there might be water involved. There certainly is water not too far away - there are lots of little ponds spattered around, but overall it is a pretty dry place (as evidenced by the sand and vegetation), so I don't think there was running or standing water in the area pictured anytime close to when the picture was taken. This was however taken in early March and it tends to be pretty cold at night in the desert. So what I think happened is that the gap portions of the trails were frozen during the night. Presumably small scale topographical features allowed the gap areas to have more moisture which froze hard enough that the tracks did not register until it started to warm up. This would also help explain the crispness of the gap tracks as they would likely have been made as the sand was still damp. The gaps were not more heavily tracked up after the thaw because as the sun comes up the mouse becomes less active, so probably went back to its nest for the day
The last question is one I may have still been puzzling over if I hadn't been working on it with other people. I'll be interested to see what ideas you all come up with.
I'm adding the picture below in the hopes that it will be helpful. I'm not sure that it will be any more helpful than the picture above, but at least you will be able to see a closer view of the individual tracks.
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