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General Information about Rats & Mice


In general, Norway rats and black rats have litter sizes of six to twelve young, but up to 22 pups have been recorded. The gestation period ranges from 21 to 25 days. Assuming she lives for a year, the adult female typically produces three to six litters, although under exceptionally good conditions, as many as 12 litters are possible. The number of litters per year, and the number of pups per litter, are dependant on the food supply, harborage, age and condition of the female, competition, temperature, climate, and other factors. In warmer temperature climates, for example, the roof rat is more fecund; whereas the Norway rat is more fertile in colder temperature regions. In general rats are capable of breeding every month of the year.

The pink young are helpless, blind, and naked at birth with their external ears sealed down. Fine hair appears on the body in about a week. The eyes and ears open in 12 to 14 days. Young rats become sexually mature at about three months of age at which time they are mostly independent of their mother. The sexually active males are receptive to mating at any time, but the females are receptive only during part of a four to six day regular estrous cycle. The female may be impregnated a few hours after birth of a litter. Under ideal conditions, it is possible for the females to give birth every 24 to 28 days, but this is rare.


Because rats can adapt themselves to a great many environments, their behavior can vary considerably. For this reason It is important to be cautious when attributing certain set habits to one species or the other under different conditions. The black rat for example, is a better climber than the Norway rat, but the latter may at times utilize attics and other aerial areas while roof rats inhabit sewers and/or construct and use ground burrows.


Generally rats have two peak activity periods; one within the hour following sunset, and again just before dawn. If rats were not mostly nocturnal animals, we would soon be apprised of their tremendous numbers, and this is true as people who work “night shifts” in our cities can certainly attest. On the other hand rats occasionally alter or even reverse their activity periods from nighttime to daytime depending on human activity, competition and the availability of resources. Rats seen during the day do not always indicate a severe infestation.


For rats, shelter is either found within ground burrows, or within natural (trees) or man-made (building voids, sewer lines, etc.). Because the Norway rat originated in the treeless, grassy steppes of Central Asia, it is primarily adapted to digging ground burrows for it’s shelter. Norway rats will nest in underground burrows even when nesting places are available within adjacent buildings. Roof rats nest in trees, dense hedges, and vines on fences. They also target attics,Wall voids, and soffits of buildings. They are not opposed to burrows, however they only burrow in the absence of the Norway rats do not actually live in sewers, rather they find a break in the sewer wall and excavate a nest cavity in the dry side braches or in nearby surrounding soil. The sewer itself serves as a highway as well as a source of food.

Side and aerial view of Norway rat burrow in poultry pen. As rat colonies become extended with many interconnecting channels and entrance holes.

These burrows, if left untreated by a Pest Control Specialist, could cause damage not only outdoors but indoors also. There is also always the threat of disease and or developing aggression.

In severe infestations, it is common for large areas of ground to be totally undermined with burrows. Experienced professionals often relay stories of ground caving upon walking over large rat colonies during inspections. Around farms, extensive burrowing has caused collapse of grain bins and various other types of foundations. In the more residential areas, severe infestation can potentially cause HVAC damage as well as electrical, plumbing, and foundation being compromised, etc.


The Norway Rat is the most important urban pest for most parts of the world. The Norway Rat did not originate in Norway. Presumably, it’s name is based on where this rat was classified, with the prepared study specimens later being sent from Norway. It is called the brown rat, sewer rat, barn rat, wharf rat, water rat, and grey rat. The Norway Rat originated in Central Asia in the area north of the Caspian Sea in the former USSR. It is more highly developed species than the roof rat and is adapted to the dry, treeless, grassy plains of it’s native area. As more aggressive Norway Rat spread outward from Asia, the more primitive black rat disappeared over much of it’s original range. The Norway Rat first appeared in Europe about 1730. Soon after, it was carried by trading ships to America around 1740. In America it spread just as quickly and now can be found throughout the US. The Norway Rat is a relatively large rat although many exaggerations are made by the public and press about it’s size. An average adult measures about 16 inches in length from it’s nose to the end of it’s tail, and weighs about 12 ounces. The length of the tail is shorter than the body. The typical pelage color is grayish brown, but it may vary from a pure grey to a blackish or reddish brown. Because of the variations in color, the rats often cannot be separated by color alone.

Above: This is a picture is of animal feed set out for squirrels, and as a consequence it attracted rats into the backyard of a home. Never spread seed or grain onto the top of your soil, and be mindful of grain and feed falling from bird feeders.


Much is said about rats having “favorite foods” in urban environment. For the most part, the common rodent is an opportunistic omnivore. This simply means they tend to consume many different types of foods as they encounter them. In urban areas, rats will accept locally available food that provides their necessary nutrition, and many variations occur in the actual foods consumed. In city sewer systems, for example, rats can be fed well as a result of the food disposed of via the many and ever increasing sink garbage disposal units. In the wild, rats consume various types of grains, nuts, vegetation, and seeds. Insects, slugs, snails, and other invertebrates are also commonly taken, as are birds. Fish and other small mammals as opportunities arise. Rats will also sift through feces of other, such as dogs, horses, and livestock, and pick out undigested food particles. Rats commonly feed at dusk and just prior to dawn. Rats consume about 10% of their body weight daily. It is important to note that if a serious rat infestation is present in an area, it is usually indicative that the rats have ample food easily available to them (e.g., a possible sanitation problem).


A house mouse foraging for food.
(Photo: Syngenta Professional Products)

The house mouse is the number one rodent pest in most parts of the world. The renowned house mouse expert RJ Berry wrote “The house mouse is a weed; quick to exploit opportunity and able to withstand local adversity and extinction without harm to the species. This means it has to be able to breed rapidly, tolerate a wide range of conditions, and adjust quickly to changes in its environment. These traits are responsible for the success of the species in so many parts of the world. House mice are extremely common within cities and towns, but they also live as field rodents away from any buildings. But they should not be confused with voles (e.g., meadow mice), white footed mice, deer mice, or shrews, all of which are entirely different animals.z


The roof rat, ship rat, black rat, and/or house rat is a arboreal native of the forests of Southeast Asia. The rat followed the caravan routes across India into the eastern Mediterranean region and entered Europe about the time of the Crusades during the outbreaks of the plague known as the “Black Death”. The black rat arrived in the Americas about the same time as the early explorers in the later part of the 15th century.

The roof rat is not always black, but can be brown-backed with the belly varying from gray to cream-colored or white. The roof rat is the most numerous and significant rat pest species. This rat live among vegetation in exterior areas, as well as within buildings. The roof rat and the Norway rat coexist peacefully due to the fact that the Norway rat, although more aggressive, and stronger, is mostly a ground dwelling species.

Dense growth of trees, shrubs, and vines, woodpiles, sheds, and accumulated yard rubbish all contribute to infestation by this rat. Note however, the roof rat may occur in habitats more commonly associated with the Norway rat, such as sanitary sewers in California and Arizona. The roof rat seems to be thriving and spreading inland away from coastal zones due to the affinity by urbanites for “green” and wildlife friendly yards that incorporate lush landscaping, fruit and nut trees and bird feeders.