Island Science

Island’s Little Critters

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

Baby Meadow Vole

My friend has had a vole problem this year in her garden. For some reason, every time she tells me the stories of voles enjoying her organic creations I envision a Looney Tunes cartoon of carrots and cabbages disappearing one by one down holes. Nantucket doesn’t have a very diverse population of small mammals, but the ones who remain are wily and have adapted to our sandier soil.

A trip to the Field Station reveals a honeycomb of mole tunnels crisscrossing the roads and paths. Nantucket has very few mammals, but we do have several species that spend a great deal of their time living a fossorial or underground life, such as moles, voles, and shrews. These animals have evolved underground to find protection, warmth, and enhance their capability to reproduce. Their existence help support many of our raptor populations.

Voles are members of the order Muridae (rodents) which includes mice, rats, voles, and lemmings. The most common specimen found on Nantucket is the meadow vole (Microtus pennsylvanicus). Voles can live either above or below ground and they dig snow tunnels in the winter, which is called subnivean behavior. Voles are small mammals that tend to have large heads, compact bodies, and short noses. Microtus means “small ear” and the voles of this genus have short rounded ears that barely protrude from the fur surrounding them. The meadow vole is a typical representative of this group and has dull fur that is grizzled chestnut to yellowish brown on the upper parts of the body, silvery gray on the lower parts. The meadow vole has a relatively “chunky” build and a short tail which is about one-third of the total body length, sparsely haired. and scaly. The front paws have four toes and their hind paws have five, in case you’re counting.

Meadow voles make nests in clumps of grass, using materials such as dry grass, sedges, and weeds. From their nests, they build “runways,” like tunnels beneath the grass and plants. If you look closely you will see many of these in mowed fields. Meadow voles are most active at night during the summer and during the day in the winter. They are less active when there’s a full moon. They also dig underground burrows where they store food for the winter and females give birth to their young. Although these animals tend to live close together, they are aggressive towards one other. This is particularly evident in males during the breeding season and the females year-round who can be very territorial.  Supposedly, if a vole feels threatened, it will stamp its hind feet, much like a rabbit.

The meadow vole is also called a field mouse or a meadow mouse and very often voles are mistaken for mice. The vole has a very short lifespan in the wild, sometimes less than a year although they can live up to a year-and-a-half. Its main predators on Nantucket are raptors, snakes, and feral cats. Many voles are snatched up by hawks and owls, particularly barn owls. In fact, the welfare of barn owls, short-eared owls, and northern harriers is literally tied to the presence or absence, of this species. Meadow voles are considered one of the most prolific species in the world (take that rabbits!) having up to 12 litters a year with anywhere from 1-12 youngsters per litter. Voles eat various plants such as grasses and sedges, and they will quickly recolonize a newly opened field. Meadow voles do not hibernate and they eat constantly, often up to their own weight in food a day (easier to do with summer gardens). Voles prefer green vegetation in the summer and switch to mostly grains and seeds in fall and winter.

I am sure you have heard of the famous Muskeget vole. The vole found only on Muskeget Island is considered by some scientists to be a subspecies of Microtus pennsylvanicus, although other researchers list it as a separate species, Microtus breweri, which is also known as the Muskeget vole or beach vole. Subspecies and species offshoots occur when there is genetic isolation which is a natural result of mammals living on islands separated by water.  The beach vole is larger, more grizzled, and pale brown when compared to its darker, slightly smaller cousin. Perhaps due to fewer predators, the Muskeget beach vole is a better parent and less aggressive. Female beach voles produce a slightly smaller litter than meadow voles (3.5 offspring versus 4.0 on average). According to Don E. Wilson’s 2003 book “The Smithsonian Book of North American Mammals,” the beach vole is a legitimately separate species as established by a statistical analysis of skull and tooth characteristics and that is good enough for me for now, although I wouldn’t be surprised if I change my mind in a year. The typical diet of a Muskeget beach vole is simply beach grass—they don’t have a lot of choice in the matter.

Shrews are members of the family Soricidae in the Order Insectivora (insect eaters). Insectivores are the descendants of the most primitive placental mammals and are the predecessors of all other placental mammals.These insect-eating mammals have long-pointed, flexible snouts with a finely developed sense of smell and touch. Their sight and hearing are poorly developed, and they all have musk glands, like the weasel family.  They have five clawed toes on all four feet, in contrast to rodents, like mice, that have only four toes on their forefeet (aren’t you glad you were paying attention to the vole description above?).

These very small, non-hibernating mammals have high metabolic rates and may consume up to twice their body weight per day (see: voles are the biggest pigs out there). Most insectivores are ferocious predators with an insatiable appetite and a constant need to forage. Shrews have a very short life span and high reproductive rate. Their small size enables them to access food sources unavailable to most mammals or birds.

Shrews are fairly vocal, with many sounds made that are above the hearing range of humans.  Like whales and bats, water shrews, wandering shrews and masked shrews utilize high frequency ultrasonic sounds for hunting, orientation, protection, and communication. Shrews are very aggressive and usually solitary, only meeting to mate. They are very nervous; known to die from fright from loud noises, even from thunder.  Normally, our local shrews will feed on prey found in its burrows.  They have a very diverse diet, preferring earthworms, but also consuming insects, centipedes, millipedes, spiders, slugs, snails, mice, frogs, salamanders, minnow, crayfish, roots, berries and nuts, and even other shrews (seems somewhat greedy).  Shrews are equal opportunity carnivores, sometimes half of the shrew’s diet may consist of meadow voles. In fact, research indicates that meadow vole and northern short-tailed shrew populations may fluctuate together in cycles.

The species of shrew found on Nantucket, Muskeget and Tuckernuck is the Northern short-tailed shrew, Sorex brevicauda. Some records indicate we also have the Cinereus shrew (Sorex cinereus) on island and across Massachusetts another four species of shrews can be found.

While moles are disdained by some land owners, their benefits are numerous. They consume larvae and adults of many pest insects, such as Japanese beetles. In addition, their tunneling activity loosens the soil, aerating it and mixing deeper soils with surface material, all of which improve soil quality.  Moles are also members of the mammalian order Insectivora. Predators of moles include snakes, skunks, foxes, weasels, coyotes, hawks and owls. On Nantucket, moles have only snake and avian predators to worry about, along with dogs and feral cats.

Moles are larger than shrews, usually with proportionally shorter tails.  They have enlarged forefeet, reduced hind feet and pelvis, and short thick fur that can be rubbed backward or forward, which helps it wriggle through tunnels. The eyes are poorly developed, to the point that they can only detect light from dark. Hearing is excellent, and the sense of smell is well-developed.  Moles are active both day and night and don’t hibernate or aestivate, but will spend up to ten hours a day sleeping. Like shrews, they have a voracious appetite, feeding primarily on earthworms and other invertebrates.  The extremely sensitive snout of the mole is unmatched in the animal kingdom.  It is covered with a dense array of nervous receptors connected to nerve cells supported by a rich supply of blood vessels.  In addition, the snout, paws, tail and back of the head have sensitive bristles, like cat whiskers, that aid the mole in detecting objects. Moles have twice as much blood and hemoglobin as other mammals of similar size. This allows moles to breathe more easily in underground environments with low oxygen.

Moles are “fossorial” mammals; they spend practically all their lives underground. They make two types of tunnels. The pushed-up type we see are temporary feeding tubes, normally used for only a few weeks. Most food is obtained by prey falling into the feeding tunnels.  Tunnels will follow the grubs and worms (shallow in summer, deeper in winter). The second, deeper ones (down to three feet) are more permanent. These are used for sleeping, escaping from most predators and avoiding the cold of winter.  Nesting and resting chambers may be attached to these deep tunnels.  Where food supplies are favorable, generations of moles may inhabit the same tunnels. Moles will occasionally make vertical shafts to the surface to disperse excavated soil, especially in heavier, clay soil, often spreading out the soil to conceal the tunnel. In ideal soils, moles can tunnel at a rate of over 1 and ½ feet per minute. Moles often consume an amount of food equal to 60 to 100% of their body weight daily. To satisfy this demand, a mole will dig up to 150 feet of new tunnels a day.

Moles differ from shrews in several characteristics.  Moles have white teeth; not brown-tipped.  Moles have enormously enlarged, long-clawed forefeet.  The smelling ability of moles is more limited than shrews. Star-nosed moles can be gregarious, sharing tunnels and paths with other moles and even with shrews like the least and northern short tailed shrew (sounds like a TV show).  Because of the protection afforded by the fossorial habit, only one annual litter of two to six offspring are produced by moles.  On Nantucket, the moles living underground (also in the Insectivora order) are the Eastern mole Scalopus aquaticus.

Our large populations of moles on Nantucket are mainly due to the abundance of grub species and our relatively soft soil and warm winters. These creatures factor into our food chain, controlling insects species, overturning our soil, sometimes in a food turning way, providing fodder for raptors and snakes.  And, except when they are cleaning out our gardens, they are often pretty darn cute.

Articles by Date from 2012