May 2023 Natural Mystery Answered
Last month’s Natural Mystery showed the tracks of a species we have rarely identified in Ft. Snelling State Park. The only other time I have confirmed tracks of this species in the park was under this same bridge on a CyberTracker evaluation with Nate Harvey. Every one of us taking the evaluation got the question wrong. This time around, Kirsten Welge sorted out the mystery and successfully identified who left these diminutive footprints.
These are the tracks of a weasel, most likely an ermine (Mustela richardsonii), also known as the short-tailed weasel.
Kirsten’s answer is thorough, detailed, and walks us through her process. I’ll turn it over to her to explain how she sorted out this mystery:
Four small compressions are present, showing distinct bulbous impressions. Overall track shape is roughly round and even-sided. I measure the track below the 1 cm mark at 1.3 cm x 1.4 cm. There is no notable difference in size.
The compressions closest to the ruler at the 5 cm & 1 cm marks show three toes together. The two compressions further from the ruler under 4.5 cm & 2.5 cm show a 1-2-1 toe configuration. At first glance, I interpreted this as the classic rodent hind and front configuration, respectively.
Between the apparent rodent hind and size of 1/2" x 1/2", my initial candidates included deer mouse, house mouse, jumping mouse, and vole. Tracks of shrews and harvest mice are too small; tracks of flying squirrel & chipmunk are too large.
Looking closer at these tracks:
Tracks are canted slightly towards the centerline, not splayed outwards. This is uncharacteristic for jumping mouse... or any of the rodents I know of. The rats, mice, and vole track set illustrations and bounding track sets I recall showed fronts that point outwards from the animal's centerline. Huh.
Those three middle toes aren't straight across, but present in an arc. Toe five is set weirdly far back. Every rodent I recall except pocket mice shows three hind toes in a straight line. (However, pocket mice would show a massive size difference between front & hind tracks.) Perhaps it's just an odd presentation...
Toes appear bulbous and separate from metapodial pads. This is less likely for jumping mouse with its long spindly toes, or for vole. This is more in line with the two mouse species, however...
Metapodial pads are not clearly present. There's a general dent in the ground for the right front track palm and trailing edge that might come to a point. This is not the clear chunky dots I'd expect from a deer mouse, the daintier metapodial pads of house mouse, or the "waistcoat" of jumping mouse hinds at the back of spindly toes. I can't make any of my candidates fit this.
When no candidates fit the track, it's time to re-examine my candidate pool. Who *does* have:
Three toes in a splayed arc on the hinds, with a very dropped toe 5,
Round tracks about 1/2" x 1/2",
Bounds with a trail width about 5.1 cm (2"),
Shows some inward canting on the front tracks,
Furred or flat feet that don't show metapodial pads?
Looking further afield for candidates:
Frog/toad? I briefly considered frog/toad due to front turn-in, but ruled out immediately due to hind toe morphology & foot placement.
Rabbit? I can't match their symmetric four toed hinds or highly irregular five-toed front tracks to any of these options. Also, this track set is a mess compared to the neat bounds I expect from local leporids. I'd be shocked if this was a cottontail.
Mole? There's not enough claw or inward curvature from the front tracks.
Mustela species (least weasel or short-tailed weasel)?
Fronts may register T2-4 together, and may show a carpal pad. That's a reasonable match for the two fronts here.
Hinds may not register T1, and may present as a tiny symmetric canid-like track (like right hind under 1 cm).
Tracks would show furred negative space between bulbous toe pads and a narrow crescent metapodial pad.
Small mustelids tend to lope or bound, and "hind tracks may register behind, on top of, to the side of, or ahead of the fronts."
Track width of 1/2" is reasonable for either least or short-tailed weasel. Trail width of 2" places this outside least weasel range listed in Animal Tracks of the Midwest and squarely in short-tailed weasel range.
I'm calling this short-tailed weasel."
Exactly right. I like Kirsten’s thorough process of elimination here, and her willingness to cast a wider net when her observations weren’t matching her initial candidates. Sometimes we misidentify tracks simply because we never consider the right candidate. Congratulations again to Kirsten for sorting out this challenging mystery.