- Norway Rat -


Norway Rat

Rattus norvegicus


Tell me a little bit about rats

Also known as the house rat, brown rat, wharf rat, gray rat, sewer rat, and water rat, the Norway rat was first introduced to the U.S. by European settlers and trading ships around the year 1775. Since then, the Norway rat has become the most prevalent rat in the United States. This rat is better adapted than other species to survive and produce young in cold climates.


How can I identify a rat?

Norway rats are large and robust, weighing between 12 and 16 ounces. Although reports of larger rats have been documented, these instances are rare. Length of the head and body combined average 7 to 9.5 inches and the tail measures 6 to 8 inches. Their fur is coarse and ranges from reddish to grayish brown; many variations and color combinations are possible. The tails of Norway rats are mostly devoid of fur and scaly. Their eyes are small, they have blunt noses, and their ears are small and covered with short hairs. To identify a Norway rat by its droppings, look for capsule-shaped excrement measuring ¾” to 1” in length.
Some interesting characteristics of Norway rats include amazing physical abilities. They are excellent swimmers and can stay submerged in water for up to three minutes. They can swim a ½ mile in open water, tread water for three days, and travel through sewer lines against substantial currents. Also, they can jump from a standing position 3 feet vertically and can drop down 50 feet without any threat of injury.


What is the biology of rats?

The Norway rat is generally a ground dwelling animal. It frequently nests outdoors in underground burrows. On farms, they may live in barns, livestock buildings, granaries, and silos. In cities, rats may spend their entire lives in buildings or outside in the ground where space is available. They inhabit all types of building structures including warehouses, hotels, food facilities, residences, and stores. They are also commonly found in dumps and sewers and can be found living near ponds, lakes, or in parks.
Rat nests found in buildings are typically on lower floors. However, if an infestation is large, they may move up to attic areas, upper floors, or into suspended ceilings. They can also be found in wall voids, underneath floors, and in crawl spaces. In some city dwellings, rats have been bold enough to live in furniture in occupied homes. When building their nests, almost any soft material is sufficient for their bedding.
When rats burrow outside, they usually nest near the side of foundation walls. The underground burrows start out measuring 12 to 20 inches in length, but grow as the number of rats populating the burrow grows. After time, underground tunnels may connect several burrows. When this happens, there is always one main entrance and two additional openings called bolt holes, which are used for sudden escapes.
Rats require 1 ounce of food daily, which is 10 times the amount needed by mice. They eat all kinds of food, although they generally prefer nutrition with high amounts of carbohydrates and protein. Household trash is an excellent source for achieving a balanced diet. Cereal grains, meats, fish, livestock feed, and fresh fruits and vegetables are ideal nutritional sources.
If rats are living outside, they will search for food outside. However, if there is no food available outside, they will enter indoor locations at night and then reenter their outdoor burrows once their food search is complete. If a rat has taken up residence in a field or wooded area, he or she may kill and eat small animals, birds, and insects. In sewers, rats are fond of American cockroaches.
Rats need ½ to 1 ounce of water daily if feeding on dry food and less if feeding on moisture-rich food. Unlike mice, rats cannot survive very long without water. To ensure the necessary amount of hydration, rats will obtain water from sinks and toilets, puddles, condensation on pipes, and early morning dew. Rats will resort to extreme lengths to obtain their food and water. They have excellent climbing abilities and will climb up stairs, pipes, wires, and walls to get into buildings.
Rats are nocturnal, and generally feed at dusk and just before dawn. If you see rats during the day, this is a sign of a heavy infestation. It can also mean that the rats are really, really hungry or that they were disturbed.
Like mice, rats have territories. The radii of these territories usually range from 50 to 150 feet from the nests. When there is abundant food available and the rat population is high, territories will be lower in area. If the opposite is true, rats can travel up to 300 feet in search of food and water. Rats in urban areas tend stay in the yards in which they find food and water.
Although rat families may share food and water sources, this atmosphere of sharing will disintegrate once the population rises. Competition and fighting increase along with the population, and this results in a population divided into “social orders.” These orders are comprised of dominant rats and weaker rats. Dominant rats get first selection of lodging and food. The weaker rats are forced to live the farthest away from food sources. As a result, the weaker rats have to feed when the dominant rats are inactive, during the day. For this reason, daytime rat spottings are a sign of heavy infestation.
Rats often have a condition called “neophobia,” which means “fear of the new.” After exploring and becoming acquainted with their surroundings, rats are acutely aware if anything is out of place or newly introduced, such as traps and baits. Neophobia can last anywhere from several hours to several days– this is why traps and baits may take several days to become effective.
Breeding peaks in the spring and fall of every year and tapers off in the summer and winter.
After mating and a gestation period of about 22 days, the mother rat gives birth to a litter of 8-12 pups. The young are naked and blind at birth. Their eyes open in about 9-14 days, and they are weaned 10 to 15 days later. Rats reach sexual maturity at an average age of 3 months. Under ideal conditions, maturation can happen at 8 weeks. Females can go into heat every 4 or 5 days, and have the ability to mate within a day or two after a litter is born. The average female rat has 4 to 7 litters per year, and may successfully wean 20 or more pups annually.
Captive rats may live up to 3 years. In the wild however, they usually live only 5-12 months. Rats face destruction by predators, other rats, people, disease, and stress.