Vertebrate Pests: The Mild Winter's Effect

Are mild winters playing a role in causing ‘year-round’ rodent infestations in urban areas?" This question is frequently asked of mammalogists around the world. But to answer this question, we must first examine the effects of severe winters on urban rodent populations that initially exist outside buildings, but eventually disperse inside.

THE COLD THREAT. In the wild, winter (in the temperate regions of the world) is a life-threatening period for feral populations of small mammals that do not hibernate. In fact, natural mortality is highest for mouse populations during the winter seasons. This is because the smaller the mammal, the greater the surface area relative to volume. Consequently, small mammals are subject to lethal hypothermia (low body heat) if they cannot easily and quickly find shelter from the cold, or they cannot constantly feed to generate body heat.

Nevertheless, rodents employ several strategies for adapting and/or coping with cold — even freezing — temperatures. First, like most all other animals, rodents seek shelter from the cold. In areas away from buildings, rodents use ground burrows and insulated nests to stay warm. When they must venture out to find food, they may select travel pathways that shield them from the dangerous winds by hunkering along rocks, darting in and out of tree hollows, scurrying below dead vegetation snow cover and so forth.

Heavy snow cover, in fact, is a benefit to many small mammals because the snow itself offers a protective cave from the extreme cold, as well as provides some conservation of heat in the immediate zone around a rodent’s body (consider Eskimos and igloos). An animal burrowed deeply into or below the snow faces an ambient temperature of about 32°F whereas the ambient temperature above the snow is many degrees lower.

In towns and cities, wild rodents foraging about nearby buildings will respond quickly to any opportunity to use man-made structures, especially heated ones. The garage door of a suburban home, if not properly closed, will leak heat. Cold-threatened rats and mice find heat currents escaping from any door gaps areas highly attractive, and predictably will follow the current.

Rodents also possess several behavioral, morphological and physiological mechanisms for adapting to cold stress. One example of this is that a rodent can curl up in a ball or hunch its body up so that the rodent’s body shape becomes spherical instead of linear. This position brings the ratio of the outer surface to the volume to a minimum, and is thus more advantageous for the rodent to conserve body heat. Additionally, in a hunched position, those parts of the body that are not heavily fur covered (e.g., feet, ears, etc.) are better protected than when the rodent is stretched out in longitudinal position.

Morphologically, mice exposed to cold on a daily basis can adapt over a period of just several weeks. Research has shown that cold-adapted mice can develop a 20 to 40 percent improvement in their fur insulation. This has been verified in the field for many years via reports by pest professionals noting that mice infesting meat freezers at temperatures of -10°C contain "long coats" of fur that drag along the floor. But meat freezer mice cannot survive simply as a result of growing long fur coats. These rodents build nests within the insulated walls of the freezers and are thus insulated from the dangerous cold, except for the times they are foraging about for food. Also, in ongoing freezer infestations, it is a certainty that the mice have daily access to food.

COLD AND REPRODUCTION. Even though winter culls out many rodents, some rodents (more so house mice than either of the rats) can cope and adapt to cold environments, provided they have access to shelters when needed, good nest locations, and most importantly, are able to find and gather food regularly throughout the cold spell. Obviously, eating results in metabolism for energy and for maintaining body heat. Still, the cold imparts a high cost.

When food (protein) is scarce, and energy must be channeled as much as possible into simply staying alive, less energy is available for breeding.

Consequently, rats and mice existing outside of buildings coincide their breeding cycles with the warmer seasons. In general, the breeding peaks of rats and mice occur during the late spring and early fall. Approaching early winter, breeding drops off, and depending on the severity of the winter, may cease all together. (Indoors, both rats and mice are capable of breeding year-round.)

Thus, it is not only that severe winters help keep populations in check via "winter kill" — cold winters also prevent reproduction in those rodents that were able to cope and adapt and survive a harsh winter. An exception to this is when mice in cold environments have unlimited access to food, such as mice infesting meat freezers. These mice can breed successfully despite low temperatures.

With this background, we can surmise the effect of recurring mild winters on rodent populations in a particular region. Rats and mice during mild winters might breed further into the fall/winter season, or begin breeding earlier during the spring, resulting in less stunting of the total reproductive potential. Additionally, insects, which serve rodents as an important source of protein, may be more abundant and more easily attainable during warmer periods. Protein is essential for breeding.

CONCLUSION. Let’s return to the original question. Do mild winters contribute towards the severity of rodent populations around the globe? Most probably. Can we accurately measure the effect? Not easily. Moreover, several other environmental and sociological factors have been contributing to the “success” of the commensal rodents over the past decade of mild winters.
These include urban sprawl, increased numbers and distribution of food-serving establishments, continuing deterioration of city infrastructures and buildings, and, most importantly, a general lack of awareness by the public for the need to pay for services that truly sets the goal of striving towards elimination of local rodent populations (See Vertebrate Pests: The Rodent Explosion, part I, Nov. 2000 and Vertebrate Pests: The Rodent Explosion, part II, Jan. 2001).

To this point it is a sad fact that most of the buying public wants pest control done cheaply and without their help in reducing filth, clutter or implementing rodent-proofing, while at the same time expecting magical results. The result: many urban rodent colonies are harvested down, but never eliminated. Our urban rodent populations are kept on a “low boil” year after year.

During the years of severe winter, the potential explosive growth of these low boil rodent populations (made up of both residual indoor populations and dispersing immigrants coming from outside areas) is suppressed. But when only harvesting of these low boil populations occur repeatedly concurrently with mild winters — watch out.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com.