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Last month’s Natural Mystery became something of a theme at January Tracking Club. We saw many examples of this and related sign on the ice at Wargo, and had some great conversations. Congratulations to Collin Arnett for digesting this complex subject and giving us a terrific write up. Congratulations also to Erica Schroyer for correctly identifying this sign. Finally, an honorable mention to Ronnie Hartman who correctly identified the species that made this mound.
This is the house, lodge, or hut of a muskrat (Ondatra zibethicus). I’m going to have Erica start us off with a quick summary.
"Muskrat lodge, made of water based vegetation and mud. We can differentiate them from beaver lodge by their lack of chewed wood/logs; these are made with softer plant materials."
I’m going to turn most of the rest of this over to Collin for his excellent and detailed analysis:
"This construction appears to be made of softer plant debris. I can see what look like roots, reeds and possibly some thin-diameter woody material. This also looks like it’s made in shallow water versus just having that appearance and being on ice. I’d expect a construction of this size to be made by a mammal and suspect aquatic mammal given it’s location.
This gets me to beaver and muskrat as two common aquatic builders. Beavers typically made lodges of woodier material. While I don’t have scale, this seems to be a bit small for beaver as well. Scent mounds are another consideration, but a scent mound would typically be seen on land. The ones I’ve seen also had very fine vegetation like grass rather than higher caliber material.
Muskrats make many constructions. According to Elbroch, these include lodges built of softer material (such as reeds, cattails and phragmites) than beavers do. Lodges are excavated from within and can be multi-chambered in areas of “rapidly fluctuating water levels”, which may include this area of Fort Snelling.
Another construction is a push-up: an area of vegetation pushed through a hole in the ice to in part keep it open. This can be made of the same material as lodge construction and can get quite large. Feeding platforms can also be large piles of debris on which to feed but are not excavated, often built on floating mats to allow feeding out of water.
Yet another construction is a “feeding hut” which can look like a lodge but is less for warmth and young-rearing (the typical purpose of a lodge) and more for cover when feeding. These are excavated internally and apparently outnumber lodges in a muskrat’s territory by 2or 3 to 1.
This appears larger by a lot than most push-ups I’ve seen in person or in photos online based on using the diameter of the debris. There is also a soda can we can use as a rough scale, which suggests this hut may be close to 5 feet tall. I’d call this a lodge versus a feeding hut and feel like I’d need to investigate it more to say for sure. I imagine one can convert to the other with use/disuse and a little excavation."
Spot on. This is a muskrat house, made by mounding up mud and aquatic vegetation, then excavating from the bottom to create lodging chambers. Like Collin, I used the can (right side, half way up) as a scale to estimate the size of the house. My (equally crude) measurements are about 1 m (3.5 ft) tall and 2.5 m (8 ft) in diameter. This is much larger than a push-up. The form of the house is also more uniform than push-ups I have seen, which generally appear irregular and haphazard. As far as I know, beavers do not create push-ups.
As Erica and Collin note above, muskrat lodges are made with aquatic vegetation, rather than wood. This results in a much less durable structure. Unlike beaver lodges, which often persist for decades, muskrat houses are ephemeral, usually lasting less than a year.
Similar to beavers, muskrats create both houses and bank dens depending on local conditions. Muskrats appear to create houses wherever there is a suitable combination of shallow water and emergent aquatic plants. They appear to create bank burrows wherever there are suitable banks for burrowing.
Muskrats are one of the most prolific fur-bearers in North America, and were of great economic interest for generations. There is a wealth of literature on muskrat ecology and behavior, much of it from the first half of the 20th century. If you want to take a deep dive, and explore some older natural history literature in the process, consider either of these two monographs:
Johnson CE. 1925. The muskrat in New York: its natural history and economics. Roosevelt Wild Life Bulletin. 3(2):205–320.
This monograph includes floor plans of several muskrat houses, reproduced from field drawings. Once contained a snapping turtle nest. View online, or link to Google Books, where you can download the PDF: https://hdl.handle.net/2027/uc1.b3149193
Sather JH. 1958. Biology of the great plains muskrat in Nebraska. Wildlife Monographs.(2):1–35.
This monograph includes a brief description of how muskrat houses are constructed. Download here if you have university library access, otherwise try plugging this URL into Sci-Hub: https://www.jstor.org/stable/3830465
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