July 2020 Natural Mystery Answered

A few years ago, I ran across a large collection of barred owl pellets in Ft. Snelling State Park. In dissecting them, I found nearly 20 small mammal skulls. Eighty percent of those skulls were from this species. By some combination of them being so common, their habitat being so exposed, and perhaps them being so tasty, they appear to be an extremely common prey animal for barred owls. Congratulations to Mike Holtz and one anonymous contributor for correctly identifying these common, tiny skulls to species. And an honorable mention to Joe Conrad who identified the genus based on this species' analog in the UK.

These are the skulls of meadow voles, Microtus pennsylvanicus.

Joe Conrad and Mike Holtz gets us started with our analysis. Joe began by thinking about the context--that these skulls came from an owl pellet:

"Field Voles [ed: Microtus agrestis, a Eurasian species] are abundant and common prey for owls. I presume that there is a similar vole in North America. So I searched for vole skulls. The features look quite similar to the Field Vole. Kind of striking lower jaw shape."

Mike began by comparing measurements, then looking at teeth and mandible shape:

"I started with a chipmunk, just to pick a random starting point. The sizes for chipmunk skulls and mandibles seemed higher than what's in the picture. Dropping down in size from there, the teeth on the mandible don't match for shrews. Looking at mice vs voles, the sizes matched pretty close for both, but the shape of the mandible and the teeth seemed to line up nicely with meadow vole."

Now let's talk about those teeth. Our ridiculously knowledgeable anonymous naturalist explains:

"Voles are herbivores, so they have miniature cow's teeth built to withstand all of the coarse plant material they grind up. Like a cow, the maxilla (upper jaw) is wider than the mandible (lower jaw), allowing the animal to chew with a significant amount of lateral movement. (It's better to grind your fibrous food than pound it.) You can see zig-zag ridges of dentin showing on the occlusal surfaces, pointing almost perfectly at right angles to the midline of the skull (allowing upper and lower molars to slide sideways over each other during chewing). The sides of the teeth are also deeply and regularly corrugated, and an individual molar appears block-shaped from gumline to chewing surface. These are characteristics of teeth that continue to erupt over the animal's lifetime. The continual growth enables the animal's upper and lower teeth to maintain contact even as they wear away over time.

Contrast this against Mus spp. [ed: Old World mice, such as the common house mouse], which have molars that resemble a human's. More discrete molars (when viewed from the side), often with a visible 'neck' at the bone-tooth interface, and more irregular "mountains" on the chewing surface. They are seed eaters more than plant eaters, so they do not have a diet that requires constantly-erupting molars.

It isn't a member of the shrew family either; these tiny little predators have molars that resemble tiny dog or cat molars, with sharp shearing peaks."

So it's a vole skull. But how can we distinguish which species of vole? There are four species of voles and lemmings found in east-central Minnesota: the Meadow Vole (Microtus pennsylvanicus), the Prairie Vole (Microtus ochrogaster), the Southern Red-Backed Vole (Myodes gapperi, formerly classified as Clethrionomys gapperi), and the Southern Bog Lemming (Synaptomys cooperi).

As our anonymous naturalist notes: "Based on range distribution, a vole in the Twin Cities area is most likely to be M. pennsylvanicus, the meadow vole." And since we are looking at a minimum of three skulls that all came from a single owl pellet, this is likely an extremely abundant species. The Meadow Vole is the most common and abundant vole in this part of the state. But there are some details on the skulls that can help us confirm this guess.

Myodes species such as the Southern Red-Backed Vole have narrower and more rounded zygomatic arches (cheek bones) than Microtus species. Also, the angular process (the lower-most rearward extension on the lower jaw) is shorter and more blunt. Synaptomys species such as the Southern Bog Lemming have more robust zygomatic arches that extend much wider than the brain case. Their angular process is also shorter and more blunt than in Microtus. The differences between Meadow Vole and Prairie Vole skulls are subtle. Perhaps the most distinguishing feature is the structure of the cheek teeth. According to Elizabeth & Charles Schwartz in The Wild Mammals of Missouri, Meadow Voles have five "islands of dentine" surrounded by enamel on the second molar of their upper jaw, while Prairie Voles show four.

Congratulations again to Mike, Joe and our anonymous contributor.

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