Colorado is alive with rats and mice, its meadows, mountains, forests and prairies virtually overrun with rodents. So why aren't we all shrieking and running from our homes? Why don't we see our food supplies devoured and our sewers running wild with nasty rats? Why don't we hire a Pied Piper to lead them all away? Because the majority of wild mice and rats inhabiting Colorado's many habitats go about their daily lives with few of us ever aware of their presence. They are native wildlife, essential components of Colorado's ecosystem since long before the arrival of European settlers.
It may surprise some to learn that a mouse is not just a mouse, and a rat not just a rat. Depending upon how you count and classify them, Colorado is home to 24 species of native mice and nine species of native rats. This includes jumping, harvest, deer and pocket mice; voles; kangaroo and wood rats and the muskrat. Non-native, or Old World, species are represented in Colorado by the Norway rat and house mouse, both of which came to North America in the holds of ships and the baggage of immigrants from Europe and Asia. The observable differences between our native New World animals and their introduced cousins lie with their tails and their coloring. Old World species are uniform in color and have naked tails, while New World mice are two-toned—dark on their backs with paler undersides. Their tails are furred instead of naked and may also be countershaded—dark on top, light below.
The deer mouse and its cousins within the group known as white-footed mice are probably the best known among Colorado's native mice. With soft brown fur, white belly, long, two-toned tail and large, doe-like eyes, the deer mouse is an appealing creature. White-footed mice, including the piñon mouse illustrated on the cover, have a varied diet that includes insects.
Another group, the voles, or meadow mice, are bob-tailed, small-eared, stocky-bodied rodents that spend their lives scurrying along runways chewed through the grass. In winter these galleries are just beneath the snow and sometimes voles can be heard scratching about inside their runways. Come spring, the meadow mouse's galleries materialize from the melting snow, little trenches snaking in all directions across the meadow. Vole tunnels are like micro game trails, used by many species to move about under relative cover.
Jumping mice are built like tiny kangaroos, with powerful hind legs and feet and long tails used for balance when hopping. Though only about three inches long, jumping mice can jump as far as seven feet when startled. Unlike many mice, Colorado's jumping mice hibernate during the winter. They prefer moist habitats and usually live among riparian vegetation. Jumping mice are mainly active at night but sometimes one can be seen during the day jumping through the grass.
Like pocket gophers, the pocket mice and kangaroo rats have external fur-lined cheek pouches used for carrying seeds. To empty them, the animal turns the pockets inside-out, then smooths the fur back into place with its paws. Pocket mice and kangaroo rats are nocturnal, live in dry habitats and can go for long periods without drinking free water, deriving the moisture they need from their food.
Unlike those beady-eyed, naked-tailed vermin that cause us all to give a collective cultural shudder—the house mouse and Norway rat—our native rats and mice are for the most part good guys. The brunt of responsibility for destruction of grain and food supplies and the carrying of disease must be borne by the foreign invaders. Native species do occasionally cause some problems. Cotton rats can damage alfalfa and other green crops. Voles sometimes destroy grain crops and kill fruit trees by chewing on the roots or girdling the trunks. Wood rats and deer mice may damage property and become nuisances by nesting in homes, buildings and mountain cabins. And the emergence of the hantavirus in the Southwest, which is borne by deer mice, demonstrates that native rodents can act as disease vectors.
| The mouse is a sober citizen who knows that grass grows in order that mice may store it as underground haystacks, and that snow falls in order that mice may build subways from stack to stack: supply, demand, and transport all neatly organized. — Aldo Leopold: A Sand County Almanac |
But native mice and rats are major players in Colorado's ecosystem and their important role far outweighs any occasional problems they cause. We may not often see them, but these gnawing natives are out there in tremendous numbers, going about their lives. Their very abundance combined with their role as herbivores, converting plant energy to animal tissue, means that in the food web, they are like tiny Atlases holding up the world. Think of them as a vast work force of seed-eaters and grass-chewers, occupying nearly every terrestrial habitat in the state, all busily processing an incredible biomass of vegetation and making it available for predators. Just about any animal that eats other creatures for a living eats mice—foxes, coyotes, bobcats, owls, hawks, eagles, snakes, skunks, herons. A study of barn owl pellets estimated each owl consumes an average of 2000 rats and mice a year, or about five to six rodents a night. If this vast food source suddenly disappeared, Colorado's predators would soon follow. As the snack of choice for so many hunters, how do mice and rats survive? By overwhelming the competition with numbers. A study of reproduction in voles estimated 200 to 300 adults in a one-acre plot of habitat. The female voles female voles, about half the total, began breeding at 30 days of age, and kept on breeding, producing a litter every three to four weeks. Considering an average life span of two years, each female-vole would produce about 24 litters and 150 young in her life. Each of her female offspring would also produce young. The resulting geometric progression would mean an astounding potential population explosion. Fortunately, natural controls keep mice populations in balance. Only one or two young from a litter survive to adulthood. Some rodent populations cycle, with explosive years of very high population followed by crashes or declines in numbers.
The important ecological role of mice and rats goes far beyond being a Big Mac for predators. In addition to unlocking the energy contained in plants, they distribute seeds. Caching by rodents is important to the germination of several tree and plant species. They control the spread of weeds. One study estimated white-footed mice each eat 260 seeds per day. Those that are carnivorous, like the grasshopper mouse, eat a great many insects. Burrowing and tunneling by rodents aerates the soil while the animals' feces and dead bodies fertilize the soil. Their activities help release and recycle other nutrients, such as the calcium locked in deer antlers, a favorite target of rodent gnawing.
So the next time you start to denigrate a mouse, or shriek at sight of a small scurrying form, remember that the rodent you are about to slight is a valuable and important component of Colorado's natural heritage.
Next: Rodents Gallery
(The information contained in this issue of Colorado's Wildlife Company was accurate at the time of original publication. Situations and circumstances described, staff positions, contact information, and dates of some events may have changed in the interim. Present knowledge and understanding of biological and behavioral facts and information may also be different, now, than presented here.)