In general terms, an animal’s home range is that area in which it obtains its food, has its nest and rears its young. An animal usually becomes familiar with its home range and spends most of its time in restricted parts of it.
In textbooks, we see the home ranges of commensal rodents listed as 50 to 100 feet for the Norway rat, 100 to 300 feet for the roof rat and 10 to 30 feet for the house mouse. But it is important to keep in mind that these distances are provided as mere guidelines.
Within the many different local environmental and structural conditions in the field, home ranges may shift up and down in distance dramatically. In other words, home ranges tend to be site specific. As professionals, we need to be able to "read" our client’s buildings. By doing so, we can determine when a home range is likely to be longer or shorter than the general guidelines. This small consideration during an inspection can make the difference between success and failure in baiting and trapping programs.
HOME RANGE NECESSITIES. Let’s examine rodent home ranges a bit closer. Home range sizes are affected by the abundance, location and availability of resources in a rodent’s area. In many cases, the presence and location of food is a primary factor responsible for rodents being regularly active in a particular area. In general, home ranges often become shorter when food is abundant, readily available and can be gathered in concealment. Population densities can also affect the size of the home range.
Another important component is the spatial dimension of the home range. In and around buildings, home ranges may be horizontal, vertical or may include both dimensions. A mouse may travel 24 feet from nest to food along the perimeter floors of a kitchen every day. In another building, the mice may travel daily 24 feet vertically by climbing from one floor to another. Thus it is important during rodent inspections to always consider the various vertical utility lines and pipe chases within buildings.
An essential element of the home range issue is to realize that home ranges are commonly shorter than stated "averages." A mouse in a home, for instance, might travel each evening 15 to 20 feet around the kitchen, out to the garage and back to its nest. Trap or bait placements following label directions of 8 to 12 foot spacings will likely result in the mouse encountering devices placed at these distances.
AN EXAMPLE. A severe infestation of mice in a large office modular complex I visited recently illustrates this point. Out of about 120 offices cubicles, 80 percent of the mice were originating from only about 15 percent of the office areas. What areas comprised the 15 percent? Those areas nearby the coffee stations and break rooms (food and warmth) and those office cubicles in which clutter was abundant. In the cubicle areas, the mice were simply living within cluttered, seldom-used desk drawers. In some of the other cubicles, the mice were nesting within old filled boxes that had been stored (and forgotten) beneath the modular desks. The home range of these mice included only the desk area and the immediate floor areas around the cubicle floor. The mice were foraging for leftover snacks and then taking them back to their nest in the back of the drawer. The other mice were infesting ceiling areas and venturing larger distances down to desks, counter tops and floor areas to collect food and then climbing back to the ceiling.
In addition to asking employees where they see mice, it is useful to check with the cleaning staff as to which office cubicles are the most cluttered and untidy and focusing your control efforts there. This way you account for those animals with short home ranges. Other examples of short home ranges include infestations within supermarkets beneath shelving; within pallets inside seed warehouses; within poultry egg-laying barns; and swine-rearing barns. In many areas within these accounts, the home ranges are often less than 10 feet.
Of course, rats may also use short home ranges. The rats in a city alley way I inspected last year were traveling only a distance of 21 feet back and forth from their burrows to a nearby dumpster. Home ranges may also be larger than the stated averages. This occurs especially in accounts where food and shelter are abundant, but the rodent infestation is brand new and there are only one or a few rodents using the area.
For example, it is common for rats to forage up to 400 feet or more daily from their nest. Such rats may only be seen or heard only sporadically throughout the course of weeks, frustrating both the client and the hired pest professional.
UNUSUAL TRAVELS. Sometimes, dispersal movements are confused with home ranges. When rodents are disturbed from their established home ranges, they may travel unusually long distances to seek better a habitat (and then establish new home ranges). The Norway rat has been recorded moving as far as two miles in one night and mice up to 1¼ miles in one night.
Many of us have witnessed new rodent infestations occurring in buildings as a result of building demolitions and construction work several blocks away from the newly infested site. But such dispersal events are incidental occurrences and not typical of the daily travels of city rats and mice.
In summary, use the listed average home range distances as mere guidelines while on any rodent job. Be prepared to read the situation and tailor the control program accordingly. When food and harborage are abundant and in close proximity, rodents are likely to travel significantly less than their average range. If a home range is vertically oriented, from the ceiling to a counter top or some other elevated surface (e.g., food warehouse racks), baiting or trapping solely on the floor of the building may not affect "off-floor" rodents. Strive to ensure your baiting and trapping points reflect the site specific home ranges. The extra effort will help reduce callbacks and rodent control failures.
The author is president of RMC Pest Management Consulting and can be reached at 765/939-2829 or rcorrigan@pctonline.com.
