December 2021 Natural Mystery Answered
Last month’s Natural Mystery got several of you cracking open Mark Elbroch’s Animal Skulls and brushing up on your cranial osteology. It looks like that was a good strategy. Congratulations to Alan Holzer, Bill Kass, Kirsten Welge, and Mike Holtz for correctly identfying this skull, solving the mystery of what died in Amy's garage, and sharing their process with the rest of us.
This is the skull of a House Mouse, Mus musculus.
Alan gets us started with these observations:
"This is a rodent for sure, based on the orange incisors and lack of canine teeth, and a very small one at that. I started with Mark Elbroch's book Animal Skulls, and looked at the Minnesota species that are in the 2cm range. Voles, mice, and jumping mice are the options."
Let’s go through those options, beginning with vole:
Mike notes that “vole skulls would usually be a bit bigger, and the shape would be different.” Kirsten describes that shape by noting that “vole skulls look very blocky.” Bill expands this further, pointing out “voles have more robust zygomatic arches (cheek bones) and noticeably shorter rostrum (noses).” Alan crossed voles off the list because “they are generally larger, and their molars are very different, looking like a line of zigzags.”
So how about other mice? Mike notes that “finding this in a garage also helps put house mouse at the top of the list.” But he and others also note that the skulls of harvest mice (Reithrodontomys spp.), deer mice (Peromyscus spp.), and jumping mice (Zapodidae) are similar. Deer mouse skulls are generally larger than (over 23mm, rather than the 21mm we see here), but animals have a habit of starting out small and getting larger over time. So let’s look deeper.
Mike notes that the zygomatic arches in Deer Mice (Peromyscus maniculatus) are pinched inward, while those of White-footed Mice (Peromyscus leucopus) are roughly parallel—both contrasting with the slightly tapering arches seen in this skull.
Jumping mice also show this slight taper to the zygomatic arches, but, as Kirsten notes, arches then meet the rostrum at roughly a 90 degree angle, while the angle we see on this skull is closer to 45 degrees.
Bill and Alan also note a few features that are unique among Minnesota mice. Bill notes that the molars on this skull “are largest closest to the incisors get progressively smaller farther from the incisors.” Alan points out that “the hard palate goes back farther than the edge of the last molars, and there is a little notch in the top incisors where they meet the bottom incisors.”
All these features together narrow the list down to a single species—the house mouse.
Congratulations again to Alan, Bill, Kirsten and Mike for solving this mystery and sharing their process with the rest of us. If you are interested in picking up a copy of Elbroch’s Animal Skulls for your own library, you can find it here on Bookshop.org.