The Carlisle Mosquito Online

Friday, February 1, 2008


Biodiversity Corner - Wee, sleekit, cow'rin', tim'rous beastie

Last week on January 25 was the 249th anniversary of Robert Burns's birthday. I have taken

Wee beastie, aka a deer mouse or maybe a white-footed mouse. (Photo by Tom Brownrigg)

the first line of his poem, "To a Mouse," as the title of this week's column. Titles in the Biodiversity Corner are usually prosaic and in that vein I could have used a title of "Rodent" or even Peromyscus, the genus name of this week's beastie. The trouble came when I tried to distinguish a deer mouse (Peromyscus maniculatus) from a white-footed mouse (Peromyscus leucopus).

Names.The name Peromyscus comes from the Greek pera, meaning a small pouch and myscus meaning a small mouse. Maniculatus (deer mouse) translates to "small-handed" and leucopus translates to "white-footed." The white-footed mouse is also known as wood gnome or woodland white-footed mouse. To complicate matters, both species are often referred to collectively as "deer mice."

When the trouble started. On December 22 and again on January 2, Tom Brownrigg saw this wee beastie eating seeds under one of his bird feeders. He and I discussed its identity, examined many books and arrived at nothing conclusive. The books all agree that the differences between the species are subtle.

Distinguishing characteristic. Bernd Heinrich, a professor of biology at the University of Vermont and author of several nature books, writes in Winter World that expert mammologists can distinguish white-footed mice from deer mice by testing for molecular variations in the salivary enzyme, amylase. The mice themselves can tell the difference — the two species don't interbreed and create hybrids. I am reminded of my younger nubile days when I encountered creatures I thought were Homo sapiens, and something I loosely refer to as "chemistry" told me there would be no possibility of interbreeding. We never even reached the stage of exchanging salivary enzymes — we just knew it was pointless.

Shared characteristics. Both species have a body about three to four inches long and a tail of two to five inches in length. They weigh anywhere from half an ounce to just over an ounce. (For comparison, my big Cecropia caterpillars weighed three quarters of an ounce.) Both mouse species have white feet and white bellies. The white-footed mouse is light brown to reddish brown on its back while the deer mouse may be grey or brown to deep reddish brown. The tail is bi-colored — dark on top and white below, with the line of demarcation being much sharper in the deer mouse. Also, the deer mouse has a tuft of whitish hair in front of its ears, which we couldn't detect in this specimen. (In the photo you can see the outline of a white question mark in the ear, entirely appropriate, but not mentioned in the field guides.) Both species eat fruits, seeds, berries and insects.

Getting through the winter. Neither species hibernates and both cache seeds for the winter. They are able to go into a torpid state during the day with a body temperature of around 20 degrees C. They huddle together in communal nests with the dominant mouse taking the warmest spot in the middle. Being noctural, the mice are active when the temperatures are probably the coldest. One of their adaptations is that they can add red blood cells with a richer hemoglobin content, which means they can increase their metabolic rate.

Eeek! - not so cute. Deer mice and white-footed mice are both reservoir species for Lyme bacteria. Also, the white-footed mouse is the only one of the pair known to carry the Hanta virus, in our region. It is also the less likely of the two to make its nests in buildings.

Sources. Mammals of North America — Temperate and Arctic Regions, Adrian Forsyth; National Geographic Society Wild Animals of North America; Guide to Animal Tracking and Behavior, Donald and Lillian Stokes; Winter World — The Ingenuity of Animal Survival, Bernd Heinrich.

2008 The Carlisle Mosquito