October 2022 Natural Mystery Answered

Our October Natural Mystery proved to be a challenge. We did not receive any correct answers, and I misidentified this skull myself when I was in the field. With the help of a friend and a copy of Elbroch's Animal Skulls, I was able to identify the original owner of these bones.

This is the skull of a woodchuck (Marmota monax).

Let's walk though the identification together. The first thing we can notice are the prominent incisors, the lack of canine teeth, and the large gap between the incisors and premolars. These features point us to either a lagomorph or a rodent. Lagomorph skulls have a porous appearance (known as fenestrae), which distinguishes them from rodent skulls like this one.

Most of our local rodents' heads are much smaller than this. The only four rodents in Minnesota that grow this large are beaver, muskrat, porcupine and woodchuck.

Porcupine are not found in the Metro area. Beaver (as well as porcupine) incisors have a characteristic rusty color not seen here. That leaves us with two options: our region's largest vole, and our region's largest squirrel. Both species have fairly flat skulls, stout zygomatic arches, a fairly large drop in front of the cheek teeth on the mandible. Looking at this in the field, perhaps distracted by the river flowing a short distance away, I thought these were muskrat bones. 

Reviewing Elbroch's illustrations of muskrat and woodchuck quickly set me straight. The sutures on the top of the skull and the profile of the mandible are a match for woodchuck. Elbroch notes that the sagittal crest forms a narrow U-shape at the rear of the braincase. The drop in front of the cheek teeth on the mandible is large, but not nearly as large or as steep as in muskrat. The rear of the mandible has a dramatically different shape as well.

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