White-footed Mice

White-footed mouse from Squirrels and Other Fur-Bearers

Illustration from Squirrels and Other Fur-Bearers by John Burroughs (1909)

“I think mice are rather nice.” So began the children’s poem by Rose Amy Fyleman that I read to my three sons when they were young.

Fyleman was an English writer who lived in earlier times (1877-1957) and her mice were not the primary hosts for the larvae and nymphs of black-legged (Lyme disease) ticks or the possible carriers of hantavirus. Unfortunately, our winsome, large-eyed, big-eared white-footed mice are. They can be covered with the larvae and nymphs of black-legged ticks and build up huge concentrations of the Lyme disease bacteria Borrelia burgdorferi, which doesn’t sicken them, and thus remain the principal reservoir for the disease.

They also enjoy life in our old farmhouse, and we are compelled to keep traps laced with peanut butter especially in early autumn when they are seeking warm winter homes. Live-trapping and moving them are not an option even if we wanted to. The late, great mammalogist William J. Hamilton, Jr. once live-trapped and marked white-footed mice and released them a mile away from their home territory. A few days later they were back, exhibiting an amazing homing ability.

In addition, they know their home ranges of approximately one-fifth of an acre so well that they can quickly find hiding places whenever they are needed.

White-footed mice in an old bluebird box by Mark Lethaby

White-footed mice in an old bluebird box by Mark Lethaby (license)

But as Hamilton once wrote about the white-footed mouse, “It often takes up residence in houses; the first evidence of its presence may be a boot half filled with cherry pits or hickory nuts.” In our house, I find sunflower seed stashes not only in old boots but stuffed under the cushions of our sofas and chairs. And occasionally I’ve uncovered shredded sweaters or shirts balled up into nests on some forgotten closet shelf or in an old bureau drawer.

However, most white-footed mice find homes outdoors in abandoned squirrel, woodchuck, or bird nests, bird nest boxes, tree cavities, half-rotten stumps, rock piles or even in a ball of leaves underground. Recently, researchers have noted that they especially like the humid conditions under the rapidly expanding invasive shrub Japanese barberry, which is also favored by black-legged ticks, still another connection with Lyme disease and white-footed mice.

White-footed mice use their long tails as props and balancing organs, to climb trees, although they are not as adept as their longer tailed congener’s deer mice. White-footed mice (Peromyscus leucopus) and woodland deer mice (P. maniculatus) look alike, and often live in the same habitat in our Appalachian forests. The length of their tails can sometimes be their distinguishing characteristic because the tails of white-footed mice are slightly less than half the total length of their bodies while the eastern woodland deer mice have tails more than half their total length. But the best way to tell them apart is by comparing their skulls. Since I’m not inclined to measure their tails or study their skulls, perhaps some of the mice in our home are deer mice because they too can create havoc inside homes and camps and are carriers of hantavirus, although so far they’ve not been indicted as primary hosts for black-legged ticks.

Both species of mice are abundant over a wide range of North America, but white-footed mice have drastically increased their distribution over the last few decades and now are found in the eastern two-thirds of the United States except for Florida, as far west as portions of Wyoming, Colorado, and eastern Arizona, south through eastern Central America and north in Ontario, Quebec, southern Nova Scotia, and even Labrador.

White-footed mouse by Patrick Coin

White-footed mouse by Patrick Coin (license)

Back in the 1980s, J.O. Wolff did extensive studies of both white-footed mice and deer mice in the Appalachians of southern Virginia and found that female white-footed mice lived in separate home ranges while males had ranges overlapping one or more females with which they mated. It seems as if white-footed mice are usually polygamous, but can also be monogamous, and even in at least one known case polyandrous. These choices may protect their mothers and their young because males can recognize their mates and children and will not kill them. They even often help raise their offspring, although females do most of the work.

Mating begins in early March and continues through late October in Pennsylvania. Their gestation period is usually 23 days unless a female is still nursing a previous litter. Then gestation can be anywhere from five to 14 more days. Most litters contain four or five young but can be from two to eight.

Born tiny and naked, they develop rapidly, and at three weeks of age they are able to leave their nest, although if they are threatened they can leave at 16 days. Previously, if their mother is disturbed while nursing, she runs from the nest with her young clinging to her teats and sometimes falling off as she heads for the nearest hiding place.

These young can breed as early as 46 days of age, but most are two months or 60 days old before they do. Still, it is easy to see how many would survive if they did not have many predators such as skunks, mink, weasels, raccoons, foxes, coyotes, bobcats, black bears, and black rat snakes.

These “most abundant and ubiquitous rodents in Pennsylvania,” according to mammalogist Joseph F. Merritt, also have catholic food tastes, eating whatever fruits, seeds, and small creatures are available. A short list includes various grass seeds, raspberry seeds, shadberries, the fruits of viburnum species, hickory nuts, basswood seeds, and conifer seeds, and in the summer they add meat to their diets in the form of caterpillars, ground beetles, snails, centipedes, occasional small birds, and even other small mammals including young white-footed mice. But their favorite foods appear to be pitted wild black cherry seeds, acorns, and the seeds of jewelweed. The latter taste like walnuts and have turquoise-blue endosperms that turn their stomach contents turquoise-colored in late summer.

northern red oak (Quercus rubra) acorns by free photos & art

northern red oak (Quercus rubra) acorns by free photos & art (license)

On a study of the impact of small mammals on northern red oak regeneration by Colleen A. DeLong and Richard H.Yahner from October 1989 until December 1990 in Huntingdon County, Pennsylvania, they attributed most red oak acorn loss to white-footed mice from 67% to 88% in autumn and 94% to 100% in spring in a section of mature forest where they had planted 400 red oak acorns. Certainly, we’ve had fewer mice in our traps lately, and it may be because we haven’t had a good acorn crop in three years. In fact, many wild fruit and nut crops were sparse or nonexistent in 2013, including wild black cherries.

But by mid-autumn white-footed mice have collected caches of food to get them through the winter. Hamilton discovered that they are also fond of storing clover seed and beech seeds, and once found almost a peck of beechnuts they had stashed in a beech tree cavity in New York State.

Here in Pennsylvania, some white-footed mice have periods of torpor from late December until early February, but most remain active. They stay warm by nesting with other white-footed mice and/or deer mice huddled in tree cavities or by nesting underground.

During all seasons of the year, though, in our home, “they run about the house at night; they nibble things they shouldn’t touch, and no one seems to like them much,” as the poet Fyleman wrote. She may have thought mice were nice, but despite their beguiling appearance, I think they are an attractive nuisance.

Little Loggers

Last winter I spent more time watching meadow voles beneath our feeders than I did birds. The heavy snowfall in early December provided perfect cover for them and when most of it melted later in the month, the voles’ runways were easy to see. Several voles had nests near our feeders and often their dark gray heads poked out of them to grab a seed or two.

One Saturday afternoon I sat at our bow window watching birds while I listened to the Metropolitan Opera radio broadcast, but I ended up being more interested in meadow vole behavior. On that day, they ventured farther from their nests along their open runways to eat birdseed, and I often mistook them for dark-eyed juncos until they moved. By finding and then focusing my binoculars on a nest entrance, I was able to get excellent views of the plump, beady-eyed creatures sitting there, running along their runways, or feeding with gray squirrels, juncos, and mourning doves. The squirrels chased the birds and each other but didn’t seem to see the voles. Maybe that’s because these nervous little engines of energy move incredibly fast. Once I saw two, one right after the other, dive into a nest entrance.

Well into January, I continued my vole watching. Probably there were more than two but only a couple were out at the same time. They would pick up a seed with their front paws and, sitting on their back haunches, eat it much as a squirrel might (they are, after all, both members of the Order Rodentia). The voles used our discarded Christmas tree, along with the tree sparrows, song sparrows, and juncos, as cover when they ventured out to nibble dried and still-green grasses on the periphery of the feeder area. Juncos startled them whenever they flew in or foraged near their nest entrances, and the voles always darted back into their nests. But they also paid attention to the birds’ frequent alarms, and when the birds flew up in a panic, the voles dashed for cover.

After a month of vole-watching, snow and ice once again sealed them off from the outside world, an ideal situation as far as the voles were concerned because they were safe from many of their enemies, especially avian predators such as hawks, owls, blue jays, and crows. Even many of their larger enemies–foxes, opossums, skunks, and feral house cats–would have found it difficult to break through the thick ice layer that covered the foot of snow on our mountain during much of February.

In the meantime, the voles lived in their surface runways beneath the snow, where their other major predator–weasels–could have chased them down, or in their five-to eight-inch-in- diameter, globular-shaped nests of grasses where they huddled together to conserve energy during the coldest days of winter. Most often, such groups consist of juveniles staying with their mothers although occasionally one or two adult males may join them. They also ate the roots, tubers, leaves, seeds, fruits and grasses they had previously cached above and below ground in preparation for winter.

In late February two fifty-degree days quickly melted the icy snow cover, and once again the meadow voles were visible below the feeders as they ran along their open runways. But even more amazing were the immense number of vole runways that meandered through the dried grasses of First Field like the mazes in children’s magazines and activity books. These patches of torn-up, matted grasses that scrolled themselves across the landscape had been painstakingly constructed by the voles’ sharp teeth as they snipped off any green sprout that surfaced. Slightly wider than a garden hose, their previous under-the-snow passageways were now exposed to the sunlight and the eyes of predators. Their many domed, grassy nests were also open to the outside world.Vole runways did not cover all of First Field. Voles particularly like the thick cover of bluegrass and First Field still harbors pockets of it that were planted decades ago so that was where many of the nests and runways were concentrated. They also like moist areas of dense vegetation, made up primarily of grasses and sedges. Both the lower portion of our once-lawn, a former wetland, and a three-acre wetland at the bottom of First Field above the stream, were crisscrossed by vole runways. Along the runways, occasional piles of little, brownish-green pellets marked the voles’ communal toilets.

By late March, the meadow voles had begun breeding as the promiscuous males competed for the attention of promiscuous females. After a gestation period of 21 days, a female has her first of eight or nine litters in a season. Those litters range in size from one to 11, with an average, in Pennsylvania, of five to seven. She is bred almost immediately after bearing a litter and has a mere three weeks to tend her young, which are born blind, pink, hairless, and helpless, before she has another litter.

At one week, the young are already covered with fur and their eyes are open. At two weeks, they are weaned, and the following week they are on their own. The females of a litter can breed at four weeks of age and the males at five. All this breeding makes the meadow vole the most prolific mammal in Pennsylvania. Without a wide variety of predators, they would quickly overrun their habitat, especially every third or fourth year when their numbers are high. Back in 1924, one captive female, observed by Vernon Bailey, a mammalogist for the United States Biological Survey, produced 17 litters in one year and a daughter from her first litter had 13 litters that same year.

The meadow vole, whose scientific name in 1815 was Mus pennsylvanica (Pennsylvania mouse) for its type locality in meadows below Philadelphia, is now Microtus pennsylvanicus or Pennsylvania small ear, referring to the vole’s tiny ears. Also popularly known as the field or meadow mouse, it is no friend of the white-footed mouse of field and forest. When vole numbers are high, mouse numbers are low which may explain why we had no mice in our old farmhouse last year. Researchers aren’t sure how the voles keep mice out, but they suspect that the much larger and more pugnacious voles may attack and chase any mice they find. Certainly, I frequently observed the voles chasing each other from the birdseed.Both mice and voles are a necessary part of the food chain, supplying endless meals for larger creatures. But do they serve other purposes in the natural world?

Ecologist Richard S. Ostfeld and his associates at the Institute of Ecosystem Studies in Millbrook, New York, have been studying the effect mice and voles have on tree regeneration in old fields. He built nine, one-third of an acre enclosures in old fields and filled them with high (400), medium (175), or low (80) densities of voles per two and a half acres. In each enclosure, he planted tree seedlings of species that colonize old fields in the eastern United States and discovered that the high-density voles killed 95 percent of the seedlings, the medium-density 80 percent, and the low-density 65 percent. They showed a definite preference for red maple, white ash, and the invasive tree-of-heaven and disliked white pine and red oak. Even those seedlings that they didn’t eat, they clipped off near ground level, leaving distinctive, diagonally cut stumps. For some reason, which the scientists haven’t figured out, voles like to keep their homeland free of tree seedlings.

A separate study of white-footed mice found that they only ate tree seeds. Between the mice and voles, establishing a forest in an old field seemed almost impossible.

The next enclosures Ostfeld built were at the boundary between forest and field, since trees usually invade old fields at the edge of the forest. He left the forest end of the enclosures open, figuring that the voles would stay in the field and that mice would move between field and forest. Again he established the same densities of voles as the previous set of enclosures and again they ate the same kind and number of tree seedlings. The mice turned up their noses at those species and instead ate the seeds of red oaks and white pines.

Over the years, Ostfeld found that few tree seedlings of any species survived if vole numbers were high and mice numbers low, but many tree seedlings thrived if mice numbers were high and vole numbers low. As an ecologist, Ostfeld was fascinated by the influence of the “little loggers,” as he calls voles, on the natural world.

“These rodents…play a strong role in preserving attractive vistas and maintaining the open habitats favored by such other wildlife as deer, turkeys, woodcocks, and bluebirds,” he wrote in Natural History magazine. “And meadow voles, by excluding white-footed mice from some habitats, may reduce the risk of Lyme disease, which is carried by ticks that feed off (and are infected by) these mice.”

Could that be why we have not, so far, seen a tick on our mountain? Or why the wetter portions of our field have not been invaded by any tree seedlings in the 32 years we have lived here?

Everything is indeed connected to everything else as more than one ecologist has observed. And unraveling those connections remains a daunting task even for scientists. Our fields, after all, are not the fields that Ostfeld studied and our voles and mice may prefer and dislike different tree species.

The complexities of the natural world continue to fascinate me and I have Never Enough of Nature, as the late, great scientist Lawrence Kilham entitled one of his books. Who would have suspected that meadow voles, in addition to providing food for many predatory birds and mammals, could not only control mice numbers but the regeneration of forests?