Last week I discovered a tiny mouse under a sawn section of log in a nearby garden. It was so small it fit easily in one of the plastic, snap-top tubes I carry everywhere to catch bugs for Kieran. I brought it home to show to him, then released it again in the same area, along the mixed salt- and freshwater estuary that connects Lake Merritt to San Francisco Bay.
Before I released it, I sat down with A Field Guide to Mammals and tried to identify it. But even with the mouse in hand and the field guide in my lap, it wasn’t at all easy! Was it Peromyscus maniculatus? Not likely, since its belly wasn’t white. How about Peromyscus californicus? No, the top of its tail wasn’t “blackish” and it wasn’t “large size.” Quite the opposite, in fact. Common Mus musculus? Nope. Its tail wasn’t “scaly.”
Perognathus? Who knows. Some of the “helpful” identifying characteristics in the book were not helpful at all with a living specimen! Like “grooves on upper incisors.” Well, I wasn’t going to be prying its tiny mouth open and peering in there with my magnifying glass!
In the end, I considered it might be Reithrodontomys, a harvest mouse. Given that it was found in the marshy area close to the estuary, I figured it could be R. raviventris, though I’m not sure it had a “fulvous” belly. The species lives around San Francisco Bay on salt marsh land, and is under pressure, I understand, as its habitat is being destroyed. How nice that much of the Measure DD work around Lake Merritt (about which I’ve written quite a bit) has helped to restore this type of habitat!