Usually an evening forager, this brave deer mouse has decided to find a midday snack.
Photo: Seney Natural History Assn.
Deer Mice: Cuter in the Wild than in our Cupboards
By Kevin McKeon, Maine Master Naturalist
Maine has several species of mice, shrews, moles, voles, and lemmings scurrying around our woods, fields, and wetlands. These small mammals can sometimes test our sanity as their scurrying sounds echo through our walls and gnaw marks appear on our stored foodstuffs. They’re also known to carry disease-causing parasites and can damage our property. But they provide important ecological services to outdoor habitats. This column will take a look at the deer mouse that populates our woods and fields and leaves its marks on the snow cover for us to ponder during our winter walks.
The eastern deer mouse and the white-footed mouse are so closely related and similar that they are commonly grouped together and called deer mice. The most definitive difference is their throat hair: It’s whiter on the white-footed mouse and a tad more grayish on the eastern deer mouse. Lab testing of red blood cell and/or chromosome properties is often required to properly discern the species. As rodents, they have a single pair of continuously growing incisors in each of the upper and lower jaws which require much gnawing to keep them both sharp and at a manageable length.
Deer mice are very small mammals, 3 to 4 inches long, and spend their days holed up in their leafy, grassy nests, built and hidden in small tree cavities and burrows. Their big, dark eyes and prominent ears help nighttime foraging for whatever is seasonally available: Seeds, fruits, shrimp, crabs, spiders, caterpillars, leaves, and sometimes fungi. Their renowned reproductive habits are indeed needed for the species’ survival, as many other critters feed on them. Everything from coyotes and foxes to even a fly (the mouse bot fly) eats deer mice, including owls, bobcats, snakes, minks — even the smaller, short-tailed shrew, which uses a bit of paralyzing venom to aid with its kill. A family of barn owls will help to keep deer mice populations in harmony within their habitats, eating a dozen or so mice in one night. Ecologically, mice are considered a keystone prey species, in that the mice are vital for maintaining an ecosystem’s food chain; they’re also excellent disbursers of an area’s native seeds, key to a habitat’s flora health, diversity, and vitality.
Fresh snow shows evidence of deer mice foraging along a McKeon Reserve trail. The center line between footprints is its dragging tail. Photo: Kevin McKeon
Deer mice are found throughout central North America and into Mexico. Some higher-elevation colonies have developed evolutionary adaptations to their blood composition to better live in low-level oxygen areas: Their oxygen-carrying hemoglobin has evolved to hold and deliver more oxygen to their lungs; their hearts have improved pumping abilities, and muscles have more blood vessels for higher respiration efficiencies. These adaptations allow higher aerobic activity and efficient metabolic rates in low-oxygen, low-temperature atmospheres.
Males often mate with several partners, living among their mates’ nests, and will sometimes eat other’s young if they’re unattended, probably to induce mating in an effort to ensure their genes’ proliferation. In winter, families will often nest together in dark, hidden spots — and if given the chance, will use human-warmed areas like cellars, sheds, and of course, our insulated walls! They’ll squeeze through openings as small as ¼ inch to find these places. Whether in our walls or outside in the woods and fields, they do almost all their foraging at night. And they’re capable climbers, often foraging among dense trees. Otherwise, they’ll spend the daytime in their nests and burrows, hoping a weasel or fox doesn’t sniff them out!
Deer mice are lucky to live to their first birthday in the wild, but labs have kept some alive for eight years. Their home range is about one-third acre, and they’ll often aggressively defend their territory. But sometimes, they’ll warily interact with recognized neighbors. Unlike voles, deer mice don’t build or use burrowed runways, preferring to travel in trees, and on and under branches on the ground. During winter, however, they’ll tunnel through the subnivean zone for foraging. Nest-building is done under rocks, downed wood, brush piles, tree hollows, and even high up in tree hollows or loose bark. Nests have been found over 75 feet high up in trees.
In folklore, mice often symbolize the goodness that small or invisible things can do that larger beings cannot, an honor given for a focus on details and keen thinking. Aesop’s famous tale has Mouse disturbing Lion’s sleep, whereupon Lion threatens to eat Mouse. Mouse pleads to Lion, promising help in the future. Lion laughs at the thought of needing Mouse’s help, but frees Mouse anyway. Later, Mouse finds Lion trapped in Hunter’s net. Mouse chews through the captive ropes, freeing Lion and keeping his promise, showing the kindness is never wasted.
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