Alaska Southeaster Magazine

In the famous Christmas poem we hear every holiday season, the small, bright-eyed rodent with big ears and a long tail was probably a house mouse, commonly found throughout the United States and Europe in barns, agricultural fields, and sometimes homes. In Southeast Alaska, however, no verified specimens of the house mouse have ever been found. A tiny large-eared, long-tailed Christmas Eve visitor here would almost certainly be what is now known as a Keen's deer mouse (Peromyscus keeni).

"Keen's deer mouse" is a new title, a name now officially given to the small rodents that until 1993 were thought to represent several different species and subspecies of deer mice in Southeast. You've perhaps found the droppings of these creatures in a cabin or tent site, and you may have encountered them if you camped in an old cabin and woke to find something skittering over your sleeping bag, or - arghh! - running across your face. They're called "deer mice" because the coloring of their fur closely resembles that of the Sitka black-tailed deer. They are predominantly gray or grayish-brown, with white undersides and white feet. Even their long skinny tails are brown or gray on top and white underneath.

Deer mice are the most widespread and numerous small mammals in Southeast Alaska, but we seldom see them because they are active only at night. If we look carefully, though, we can often see evidence of their existence - small black droppings about the size of rice grains, pinched and twisted at one end; tiny tracks with toe-prints in the snow; chew marks on forest mushrooms or shed antlers; or balls of grass, paper, or mattress shreddings the animals use as nests.

Here a mouse, there a mouse

Deer mice are very prolific, and their numbers can increase quickly. Females can begin breeding in their first year, and each male may breed with several females. A female typically has four or five young in a litter, but may have as many as 10 or 11. Young mice grow quickly. They are weaned in three or four weeks and can probably live on their own once they're about a month old.

People who study small mammals in Southeast Alaska have long observed that deer mouse populations fluctuate substantially from year to year. Populations seem to peak every five or six years, then drop, then peak again in a kind of cycle.

Researchers Thomas Hanley and Jeffrey Barnard of the U.S. Forest Service Pacific Northwest Regional Laboratory in Juneau noted that food availability is a major factor in deer mouse population levels, since it influences female aggression, social tolerance and dispersion, and survival, growth, and maturation of juvenile mice. Their research on Chichagof Island showed that in 1993, for example, there was an abundant berry crop, and the breeding season lasted longer into the autumn than usual because of low population density (and low aggression in adult females). A highly-productive spruce-seed crop that year favored over-winter survival, and there was a major increase in mouse populations across a variety of habitats.

Though deer mice eat a variety of foods, they prefer fruits and seeds, whose abundance may vary substantially from year to year. Hanley and Barnard found mice on Chichagof Island ate mostly the fruits and seeds of understory plants (69% of overall stomach contents). The mice they studied preferred the fruits and seeds of salmonberry and stink currant. They also ate tree seeds and leaf material, but mice apparently do not digest the fibrous leaves and stems of plants well. Among tree seeds they preferred those of Sitka spruce, which are believed to be especially important in winter and early spring. The mice also ate blueberries, beetles, devil's club and elderberry seeds, and small amounts of fungi.

What good is a mouse?

If deer mice are so prolific, why isn't Southeast completely overrun with them? Partly because, as tidy packages of protein with few defenses except running away, deer mice are ideal prey for larger carnivorous animals. They are, in fact, a kind of "renewable resource." Since they are active primarily at night, their main predators are animals that hunt by night - owls, martens, and weasels.

In studying martens over several years on Chichagof Island, Alaska Department of Fish and Game researchers Rod Flynn and Tom Schumacher found that martens survived primarily by eating small mammals. "We found that martens prefer voles," Schumacher said, "but they eat mice because mice are more numerous than voles."

Deer mice and other small mammals are also an important food of most owls. In one study of some 400 owl pellets, about 80 % of the contents were remains of deer mice; the other 20% those of voles and shrews. Another study showed that deer mice comprise up to 80% of the diet of northern saw-whet owls. And when the major food groups of owls are summarized in Biology and Natural History of North American Owls, small mammals make up more than 90% of the diet of most species of owls that occur in Southeast Alaska.

Mice may be particularly important prey in another sense. Since they are active all winter, even above the surface of the snow, they may be especially valuable to owls and other predators at a time of year when not many food resources are available.

Mice may also play another important role in Southeast forests. Although they do not eat large quantities of fungi, they may, like the flying squirrels we described in "Nuts About Truffles" (Alaskan Southeaster, July 2000), help disperse the spores of truffles, the below-ground fruiting bodies of fungi that depend on small mammals to spread them around in the forest.

Deer mice undoubtedly eat a great many seeds that might otherwise germinate into plants, particularly Sitka spruce. But mice drop seeds when they are feeding, perhaps moving them to places with more potential for future germination and growth. If their seed caches are forgotten or for other reasons never retrieved, the seeds within them may sprout on favorable ground. Mouse droppings also provide fertilizer to help young seeds and plants grow.

Mice may also have less beneficial effects on ecosystems. Juneau naturalist Mary Willson has suggested that mice may eat songbird eggs; and in one case that may be unusual or not, they were found to be major predators on seabird eggs on Triangle Island, British Columbia. Researchers Louise Blight, John Ryder, and Douglas Bertram reported that "mouse depredation was likely responsible for the loss of more (rhinoceros auklet) eggs than all other causes combined, with mice commonly opening and eating eggs of nearly twice their mass. In one study plot, mice depredated up to 34% of eggs."

The researchers noted that predation on the seabird eggs may increase in years of low marine productivity, when adult birds must increase their foraging time and leave the eggs unprotected. As Alaskans wrestle with questions of whether our fish, especially small "forage" species, are being depleted faster than they are being replaced, the interactions of deer mice and seabirds offer one small example of how events far at sea can influence even what happens miles away on land.

Getting out and about

Deer mice have a number of physical features that seem especially useful for their nocturnal existence. They have large eyes with night vision; large ears, indicating a strong dependence on hearing; long whiskers they can use to feel their way around in the dark; and a keen sense of smell.

In winter, besides huddling together to conserve heat, they can drop into brief states of torpor, slowing their body functions to conserve energy in the cold.

Deer mice also have very strong navigational instincts. If a Southeast "Christmas mouse" were to get caught in Santa's sleigh as it lifted off the roof, the little rodent would probably have little trouble finding its way home after the adventure. In one study, a number of adult mice returned "regularly and swiftly" to a house after being released nearly three-quarters of a mile away. In another, one mouse that was displaced returned home from two miles away, and six of them returned from more than a mile away.

"Not a creature was stirring, not even a mouse." Not likely in Southeast Alaska. When Dasher and Dancer fly up to the rooftop, and the "little old driver" slides down the chimney, any self-respecting deer mouse will be out scouring the neighborhood, rustling quietly in the dark as it gathers the fixings for its own Christmas dinner.

Deer Mice and Hantavirus

In the continental United States and Canada, deer mice have become unwelcome in homes and buildings because they've been identified as the carriers of several strains of hantavirus, which causes respiratory illness in humans. Hantavirus was identified in 1993 in the southwestern United States, but it has probably been affecting humans for centuries. The disease is very rare, but nearly 40% of the people who contract it have died, so it is an ongoing concern of both U.S. and Canadian public health agencies.

Hantavirus is carried in the droppings, urine, and mucus of deer mice that have been infected, and it can be transmitted when virus particles are dispersed in the air, as, for example, when people create dust by sweeping out little-used buildings where mice have been living, and don't let the dust clear before they move in.

In April 2001, the U.S. Centers for Disease Control (CDC) reported there had been 283 cases of hantavirus in 31 states, particularly California, Arizona, New Mexico, and Washington; 107 of the cases (38%) were fatal. Canada had reported 33 cases as of May 2000; 13 of those victims died.

Six of the cases reported in Canada were in British Columbia but, according to the CDC web site, no cases of hantavirus have been reported in Alaska, and tests of deer mice in Alaska have failed to turn up any sign of the virus. We found the CDC web site had excellent suggestions for avoiding exposure to hantavirus if you travel to areas where the virus is found.