send
Send Letter to Editor

Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Small mammals like the masked shrew are both prey and predator in the food chain. © Loren Ayers
Small mammals like the masked shrew are both prey and predator in the food chain.

© Loren Ayers

April 2006

Small for one, and one for small

Researchers champion the cause of the little mammals and strive to bring population studies into the 21st century.

Kathryn A. Kahler

The big book of little creatures
Trapping to track small mammals
Neat little packages
A hefty price tag for small mammal research
The short list of small mammals

Their names conjure up diminutive pictures in the mind's eye – least shrew, pygmy shrew, least chipmunk, pocket gopher. But these tiny creatures play an important role in the bigger scheme of things. They are part of the biodiversity we strive to protect and the web of life in which we are entwined.

Small mammals serve as a forage base for larger carnivores like coyotes, foxes or birds of prey. Insectivores like shrews and moles help control insects. Small mammals disperse seeds stuck to their fur, carried in their droppings or cached in winter stores. The tiny tunnels created by burrowing mammals help aerate soil and allow water to percolate through, making a more suitable place for seeds to take root. Small mammal waste deposited in burrows fertilizes plant roots. Burrows also provide underground nesting sites for important pollinating insects such as bumblebees.

Ecologists monitor small mammal populations as environmental indicators of changing landscapes. Tracking fluctuations in the number of animals and the distribution of species helps assess the effects of agriculture, development, forest management, and other human activities on natural habitats. For instance, the red-backed vole prefers cool, moist, closed-canopy forests with plenty of large woody debris. By studying how these small mammals respond to forest harvesting and succession, foresters can determine whether red-backed voles can be used as indicators of forest sustainability.

Despite their importance, small mammals receive scant attention from the Department of Natural Resources and other research agencies. Mostly it's an issue of numbers: Following a single wolf pack of six, eight or even 15 members is easier than keeping tabs on a dispersed population of pocket gophers. But soon, thanks to the efforts of two Wisconsin ecologists, small mammals may receive more priority in research activities.

The big book of little creatures

Loren Ayers, an ecologist with DNR's Bureau of Endangered Resources, believes much can be learned from small mammal surveys and inventories. When it comes to funding projects, though, these mammals aren't just small – they're invisible.

"How does DNR use small mammal information in land management? We don't," says Ayers. "There are exceptions. We are working to improve the situation, and right now, we're focusing on infrastructure."

The "infrastructure" Ayers refers to will provide the means and information to update Mammals of Wisconsin by Hartley H. T. Jackson. Based on field work done from the early 1900s to 1952, and published in 1961, the book is still considered the definitive source of mammal information in Wisconsin. Therein lies the problem: Our data on the range, population and locations of small mammals is outdated.

"The primary purpose of most of our small mammal work right now is to update Jackson's Mammals of Wisconsin," says Ayers. "We're gathering information, trying to learn and get up to date about what's out there."

Determining what's out there can be done in several ways. Some of the specimens Jackson studied in his original research came from examining small mammal skulls found in owl pellets, the indigestible portion of an owl's diet that it regurgitates in a compact mass. Most often, however, researchers survey small mammal populations by setting up traplines.

"One or two people can't come close to inventorying all the small mammals in Wisconsin," Ayers observes. "So we train wildlife biologists and others who have an interest in small mammals, provide them with equipment and reimburse their travel expenses for doing inventories."

Ayers and Richard Bautz, a small mammal expert under contract with the Department of Natural Resources, have trained DNR wildlife biologists, student interns, U.S. Forest Service personnel, members of the Great Lakes Fish and Wildlife Commission, researchers from the Marshfield Clinic investigating infectious diseases, and private individuals. In exchange for training and support, the agency receives valuable information about small mammals and their distribution, habitat associations, population status and relative abundance.

Trapping to track small mammals

Both live traps and lethal traps are used to capture small mammals: live traps when researchers want to mark or tag an animal for recapture, or to survey the types of animals living in an area; lethal traps when collecting specimens to study anatomy, or to dissect to study food habits, disease or other biological factors.

A popular live trap for catching mouse-sized mammals is the Sherman box trap, made of aluminum or galvanized steel and measuring 3 by 3 ½ by 9 inches. Bait is placed in the back of the trap and the door is folded in, compressing a spring secured by a small latch. Lured into the trap, the animal steps on a treadle that releases the door, which snaps shut. Firm fruits or vegetables like apples or potatoes, or peanut butter mixed with oatmeal are commonly used baits.

Catching animals like rabbits, weasels and ground squirrels requires larger Sherman traps or Tomahawk traps. Tomahawks are made of wire mesh with a similar spring mechanism to trigger the door latch.

Another popular live trap is the pitfall trap. Typically made from coffee cans or plastic containers, a pitfall trap is buried so the top of the container is flush with the ground; the animal walks in and falls to the bottom. Pitfalls are most effective if placed next to fallen logs animals follow as natural travel paths. If fallen logs are not available, a drift fence can be constructed of metal flashing or rigid plastic, and pitfall traps placed at either end. Animals walking through the woods bump into the drift fence and follow it into one of the traps, much in the same way fyke nets lead fish to collection nets for lake surveys.

When surveying an area for small mammals, an array of live traps works best placed near or under brush piles, along fallen logs, near burrow entrances, along streams, at bases of trees, or in hollow stumps.

Traps are shaded with plastic or sheet metal canopies to protect the captured animals from sun and precipitation. In winter, a trap must contain enough food to keep the animal alive until the trap can be checked. Shrews, for example, can eat their body weight in food every day, so they need a trap well stocked with provisions to survive. Flying squirrels are nocturnal and will need to eat if caught overnight. Nesting materials placed inside the trap, like wads of cotton, wool or leaves, help small mammals maintain body heat until they are checked and released.

Traplines are marked overhead with brightly colored plastic tape or strips of cloth so they can be located with ease. Coordinates of trap locations are mapped on a diagram or recorded using GPS equipment. Traps are checked frequently to prevent small mammals from becoming bait themselves if discovered by larger predators like raccoons, weasels or bears.

Other methods of documenting species involve setting up tracking stations and hair traps. Tracking stations can take a variety of forms. In some, an area is covered with carbon paper, graphite spray, talcum powder or another medium the animal walks through to reach the bait. White paper or sticky contact paper is laid between the graphite and the bait, and as the animal makes its way across, a pattern of tracks appears. A piece of PVC pipe placed along a known travel path or runway is another common tracking device; graphite spray at one end, sticky paper at the other, the bait in the middle.

Hair traps or tubes collect samples for lab identification. Hairs cling to double-sided tape or another sticky medium as the animal rubs against the trap trying to get the bait.

Many museums and colleges maintain collections of study skins and skeletons, usually taken by lethal trapping as research and educational aids. Snap traps like traditional mouse traps are available in different sizes. The Museum Special Trap, sized between a mouse trap and a rat trap, does not crush the animal's skull, which is often the most valuable part of the specimen for identification and subsequent museum work.

Neat little packages

Small mammals make excellent teaching specimens because they are easily caught and very convenient to work with. "What makes this group of mammals so well-suited for study," Bautz recently told a group of undergraduates in the UW-Madison Wildlife Ecology program, "is that they come in neat little packages."

Neat they are, but a little nasty, too. Trapped animals need to be handled cautiously to protect both the animal and the handler. Students were advised to wear surgical masks and gloves as a precaution against airborne viruses like hantavirus, which causes Hantavirus Pulmonary Syndrome (HPS), a respiratory disease that is fortunately rare in Wisconsin. The deer mouse (Peromyscus maniculatus) is the primary carrier of hantavirus and can transmit it without showing any signs of being sick. Deer mice can also carry deer ticks, the source that carries and transmits Lyme disease. Thorough hand-washing is absolutely essential after handling any wild animal, especially before eating or drinking; Bautz reminded the students to carry waterless hand cleaner for use in the field.

Richard Bautz shows students how to identify, handle, mark and release small mammals for study. © Robert Queen
Richard Bautz shows students how to identify, handle, mark and release small mammals for study.

© Robert Queen

A nip from the tiny teeth of a mouse or shrew won't inflict much damage, but a bite from a larger, more aggressive species like the red squirrel can be serious. Wearing leather gloves provides good protection against biters. Bautz also has designed several simple devices to prevent larger animals like gray squirrels from chomping handlers. One is a "squeeze box" or modified Tomahawk trap: Bautz inserted a piece of plywood cut to the same dimensions as the trap floor. When a squirrel is inside the trap, the plywood piece is gently pushed up to restrain the animal between the wood and the top of the trap. An ear tag can be applied without touching the animal. It's less stressful for the squirrel and handlers alike.

Ensuring the animals' safety is a primary concern. Throughout the demonstration Bautz worked quickly and avoided excessive handling that might have stressed the animals. When removing an animal from a live trap, Bautz placed a plastic bag around the opening and held it tightly because these tiny creatures are adept at finding the smallest of escape routes. He then carefully released the trap door and allowed the animal to work its way into the plastic bag, grabbing the bag firmly just above the animal.

To mark an animal for recapture studies, Bautz first snipped one corner out of the plastic bag before transferring the animal from the trap. He gently worked the animal into that corner, and maneuvered its head out of the hole so it could breathe, but also be restrained safely. He then took body measurements, and put a dot of yellow paint on one ear or applied a numbered ear tag before releasing the animal.

When transferring an animal from the bag to a plastic jar or cage, Bautz gently worked it to the top of the bag, grabbed it by the scruff of the neck, then dropped it into an economy-sized peanut butter jar and quickly screwed on the lid, in which a square had been cut out and covered with wire mesh to provide air. Once in the jar, each animal was identified, measured, weighed and its sex determined. Any captured animal exhibiting signs of stress (such as panting or fatigue) or injury should immediately be released, Bautz said; red squirrels in particular are prone to stress because they are such a high-energy, aggressive species.

A hefty price tag for small mammal research

Ayers says expense is one reason small mammals are not studied as often as research biologists would like. One Sherman trap costs about $15 and one person can set a trapline of 100 traps per day. In 2005 museum snap traps cost $5.50, but now have gone up to $7.90. The price increase means researchers more often have to resort to using inexpensive, over-the-counter standard mouse traps, which are less effective.

"Other associated equipment can cost hundreds of dollars more," says Ayers. "A GPS unit, for example, is $300. It's an equipment and labor-intensive process."

To address the lack of funding, the small mammal research received help from the federally-funded State Wildlife Grants (SWG) program. One such grant funded a compilation of historic mammal distribution data to supplement Jackson's records with more modern data. Ayers and assistants scoured museum collections and teaching collections from colleges and universities in Wisconsin and throughout the country, reports from research projects, and other agencies such as the U.S. Fish and Wildlife Service, the USDA Forest Service and the National Park Service.

The project converted Jackson's records to electronic format and combined them with the new data to produce a mammal distribution model and predict ranges for 44 small to medium-sized mammals statewide based on vegetative cover and suitable habitat. The maps are being used to plan additional field inventories and may be useful for making land management decisions. In the future they may help wildlife teams and agency partners form State Wildlife Action Plans. These plans aim to conserve wildlife species before they become rare, while it is still economical to protect them and the habitat they need to thrive.

Ayers' program also partners actively with Wisconsin NatureMapping, a citizen monitoring project. At Wisconsin Nature Mapping citizens can report wildlife observations of common, readily identifiable species, such as 13-lined ground squirrels; gray, fox and red squirrels; snowshoe hares and badgers. Nature mappers trained at the Beaver Creek Reserve in Fall Creek may also use the reports to complement their projects.

Ayers hopes a newly-funded State Wildlife Grant project will move small mammal surveying forward in a hurry. Genetics-based wildlife inventories have proven to be viable and effective alternatives to live trapping and lethal trapping, which are labor-intensive and expensive. The goal of the new $88,000 state wildlife grant is to make DNA tissue sampling and species identification a routine option for mammal inventory, monitoring and research projects. Partners include researchers from the Marshfield Clinic Research Foundation, the U.S. Forest Service North Central Research Station, and Northland College. The project will use genetic tissue sampling when conducting field inventories of six mammals listed as "Species of Greatest Conservation Need" and 16 mammals listed as "Species with Information Needs" to see if the technique can be applied to a broader range of species. A genetic databank of 2,500 specimens will be accessible to research organizations and agencies by September 2007.

"We're hopeful this project, in conjunction with the work we're doing with our other partners, will move us down the road to a revision of Mammals of Wisconsin," says Ayers. "These little animals are just too important to continue to rely on 45-year-old information."

Kathryn A. Kahler is production and circulation manager for Wisconsin Natural Resources magazine with offices in Madison.

The short list of small mammals
The following are species most commonly considered small mammals in Wisconsin:

arctic shrew, Sorex arcticus
masked shrew, S. cinereus
pygmy shrew, S. hoyi
water shrew,S. palustris

least shrew, Cryptotis parva
northern short-tailed shrew, Blarina brevicauda
eastern mole, Scalopus aquaticus
star-nosed mole, Condylura cristata

least chipmunk, Tamias minimus
eastern chipmunk, T. striatus

Franklin's ground squirrel, Spermophilus franklinii
thirteen-lined ground squirrel, S. tridecemlineatus

northern flying squirrel, Glaucomys sabrinus
southern flying squirrel, G. volans

pocket gopher, Geomys bursarius
western harvest mouse, Reithrodontomys megalotis

white-footed mouse, Peromyscus leucopus
deer mouse, P. maniculatus

southern red-backed vole, Clethrionomys gapperi

prairie vole, Microtus ochrogaster
meadow vole, M. pennsylvanicus

southern bog lemming, Synaptomys cooperi
northern (or woodland) jumping mouse, Napaeozapus insignis
meadow jumping mouse, Zapus hudsonius

ermine (or short-tailed weasel), Mustela erminea
long-tailed weasel, M. frenata
least weasel, M. nivalis