I’ve heard several people say “I hate mice!” I think they really mean that they, understandably, don’t want to share their houses with mice. But it’s not reasonable or fair to hate all mice in general, blaming them all for the activities of the ubiquitous, now-globally-distributed house mouse (Mus musculus), which is native to the Old World, and is here without invitation. That one makes its living by scrounging our leftovers, granaries and other storehouses; it is chiefly associated with human habitations and facilities. In contrast, most of our native mice lead innocent little lives in field and forest, just trying to stay alive; in town, only a few opportunistic individuals pester us at home.
I was reminded to think about our native mice when I saw mousy footprints in the snow on the spreading roots of a big spruce tree, and I took the maker for a deermouse. After much dithering, taxonomists have decided that the deermouse of Southeast Alaska, coastal British Columbia and Washington is different from the closely related and widespread species (Peromyscus maniculatus) that ranges across the rest of North America. The local species is now known as the northwestern deermouse or Keen’s deermouse (Peromyscus keeni). It’s a pretty little thing, with big black eyes, large ears, a bicolored tail and soft fur, generally weighing about an ounce (give or take a bit — the size varies with location).
Researchers suspect that Keen’s deermouse may have survived the last major glaciation, thousands of years ago, in coastal refugia, including the coastal islands. Within its small geographic range, populations on different islands now differ genetically, having diverged because they are isolated from each other and their living conditions differ.
Keen’s deermouse occupies a variety of habitats, from the beach fringe to the alpine zone, but apparently likes the forest best. It climbs, jumps and swims well. Deermice are active year-round, especially at night. The widespread P. maniculatus is said to put on fat in preparation for winter and to make small caches of seeds, in tree holes, or in the soil; however, according to some reports, P. keeni may not do so.
It’s omnivorous, eating seeds, fruits, insects and spiders, worms, snails, green leaves, fungi, and sometimes bird eggs. A field study of deermice on Chichagof Island revealed that seeds and fruits of understory plants comprised most of the diet, and other foods were used much less often. Not surprisingly, they exhibit preferences among different kinds of seeds and fruits. For example, given a choice among five kinds of fruits, they strongly preferred salmonberries, with blueberries, elderberries, stink currents and devil’s club fruits in descending order of preference. Interestingly, the seeds themselves, cleaned of fruit pulp, fell out in a different order: devil’s club seeds were preferred over salmonberry seeds and elderberry seeds, with stink current seeds at the bottom of the list. The contrast makes me wonder if, in the wild, they would peel off the devil’s club fruit pulp in order to get the good seed inside. On the outer islands, where seabirds nest in colonies, Keen’s deermouse is on record as consuming eggs and probably young chicks of the smaller seabirds, as well as scavenging bird carcasses.
Deermice breed in spring and early summer. Their nests are built of plant materials, placed in treeholes, burrows, logs, stumps or sometimes old bird nests. Litter size varies but is typically four to six young, which are born blind, deaf and furless. The mating system has apparently not been studied; in other Peromyscus species, the mating system varies from monogamy with male parental care to promiscuous with no male parental care. Most deermice mature and breed for the first time when a year old (the spring after their birth). But those born very early in the season may mature and breed later in that same year, especially if food resources are abundant. They potentially can live for several years, but predators and starvation take their tolls.
Our other native mice belong to a different taxonomic family, and so are related only rather distantly to deermice. These are called jumping mice, related to the kangaroo mice of the southwest. There are two species in Southeast: the meadow jumping mouse (Zapus hudsonius), a species of the mountain west, and the western jumping mouse (Z. princeps), widely distributed across northern North America. They are hard to tell apart in the field, and both of them have been recorded in Southeast, mostly on the mainland.
Information on their ecology and behavior comes from studies that have been done elsewhere in their range, so it is possible that things may be a little different here in Southeast.
Very different from deermice (but just as cute!), jumping mice have huge hind feet, small front legs, and a wonderfully long tail (for balancing when they jump). Reports of their jumping ability indicate distances of 3 to 6 feet, when the animals are eluding a disturber, and more leisurely jumps of several inches when unstressed. They can also walk on all fours, dig, climb shrubs, swim and dive.
Both species favor habitats that have good cover, including grassy or sometimes shrubby meadows from sea level to the alpine, especially close to water or where the humidity is high. They are nocturnal, so we seldom see them. They are omnivorous, eating seeds, subterranean fungi and terrestrial invertebrates.
Their weight varies a lot (in the neighborhood of an ounce) because they put on fat in the fall, so much that their weight can almost double in preparation for winter hibernation. They hibernate from fall to spring, lowering their metabolism to conserve energy, and allowing their body temperature to drop close to freezing. Hibernation nests are dug in shallow soil or under a log, down to a depth of about 20 or 30 inches or so. After plugging the entrance of the nest burrow with dirt, the mice curl up in a nest of dry plant material and go to sleep. Winter mortality can be high: as much as 25 percent of adults may die, and up to 75 percent of juveniles (especially those born late in the summer), largely due to insufficient fat storage and starvation. Although potentially they can live for several years, most live only about two years.
When soil temperatures warm up in spring, jumping mice emerge and are soon ready to mate. The nest is a ball of dry grass or other vegetation, under a log or a clump of herbage, or in a burrow. There may be one or two litters per year, depending on the season, with an average of four or five young per litter. Females can often produce a litter when they are one year old, but some delay this until age two, especially if food is in short supply; in a really good season, females may reproduce in the same summer they are born. The young are born blind, deaf, furless, without claws or teeth, and weigh less than one gram (there are 28 grams to an ounce). But after a month or so, they become self-sufficient.
• Mary F. Willson is a retired professor of ecology.
