June 2024 Natural Mystery Answered
Congratulations to Kirsten Welge and new Track & Sign Professional Maria Wesserle for correctly identifying the tracks in last month's Natural Mystery. Despite the deep mud obscuring some details, both Maria and Kirsten narrowed the track maker down to the genus. Though we can take a good guess based on location, it is likely that we cannot positively identify the species based on the photo alone.
These are the tracks of an Old World rat (Rattus sp.), most likely a brown rat (Rattus norvegicus).
Kirsten gets us started with some detailed observations:
Several tracks are visible, all about the same age, which can be sorted into two groups:
One group registers four toes, with the outer toes at nearly a 180* angle, and inner two toes pointing forward. Size on these tracks is 5/8" L x 7/8" W, including claws of 1/16".
The other group registers five toes. Again, outer toes are widely spread, forming a 180* angle; the inner three toes point forward. Size of this track is 7/8" L x 15/16" W, including claws of 1/16".
The track to the left of the ruler shows three circular dots that appear to completely fill the palm space, as well as several bulbous tips on toes that connect back to the palm. Most of the registers show bulbous toe tips that do not connect back to the palm.
Maria points out that the tracks with five toes show "the classic 1-3-1 toe structure of a rodent" while the "other tracks with four toes further confirms rodent."
Using the size of the tracks, Kirsten creates a candidate list of three groups of rodents: chipmunk, flying squirrel, and Old World rat. Maria adds red squirrel to her list. Both then turn to the track morphology to arrive at an ID.
Maria points out "the significant size difference between the front and hind tracks, with the hinds being much larger," and she and Kirsten note that the wide splay of the tracks strongly suggests an Old World rat, rather than a squirrel. I'll note briefly here that our one native local "rat," the muskrat, shows an even larger size different between the front and hind feet, while they splay of its front toes are similar to that of a squirrel.
Maria also points to the symmetrical gait shown in the banner image and notes that "rats commonly walk or trot, while chipmunks and red squirrels mostly bound and flying squirrels typically hop with a boxy gait."
Narrowing these tracks down to species may be tricky. Kirsten takes a crack at it based on the morphology, writing: "The bulbous appearance of the toe tips and palm tubercles leans me towards a black rat (Rattus rattus)." I agree, and might have leaned that way myself based only on the photo. However, as far as I know, there are no black rats in Minnesota. In the United States, Black rats are most common in the Southeast and along the West Coast. There are some isolated populations of black rats around Chicago, St. Louis, and Kansas City--but outside of these three metropolitan areas, they seem to be essentially absent from the Midwest.
The basic morphology of brown rat and black rat feet is quite similar. Black rats do have more bulbous pads, and some tracks are easy to identify to one species or the other. But distinguishing a group of mostly messy tracks in soft mud may not be possible. Given the range information we have about these species, I would not feel comfortable calling these images evidence of black rats arriving in Minnesota. These are most likely brown rat tracks.
Finally, there was a typo on the form for submitting answers that both Maria and Kirsten picked up on. Maria ignored it, while Kirsten took it on as an additional challenge. Although the question was simply "Who?", the answer form asked for "Who? & Which foot?" There was no mention of which track this referred to, so Kirsten answered for the track closest to the ruler, writing:
"The track to the left of the ruler appears to be a left front: Front, due to morphology of four toes. Left, given the larger gap between toes by the 2-2.5 cm marks compared to the gap between the toes by the 3-3.5 cm marks. As shown on our human hands, the gaps between digits from greatest to least are between T1-2, T4-5, T2-3, and T3-4. After double-checking rat tracks in Elbroch, that appears to hold true for rat tracks! (Images p 596) (Note: The vestigial T1 often shows, though I do not see it in these tracks.)"
Exactly right, that is a left front track. Like most mammal tracks showing four toes, the largest gap is between toes 4 & 5. We see this feature in most rodent front tracks, canid and felid tracks (generally easier to see on front tracks), and lagomorph hind tracks.
You can see more photos of these tracks, plus the section of trail featured in the banner image, here on iNaturalist.
Congratulations again to Maria and Kirsten for identifying these rarely-seen-in-Minnesota tracks. And thanks to everyone who submitted an answer to this mystery.
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