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Norway Rat, Brown, House, 'Wharf & Sewer Rat'
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COMMON NAME: Norway / brown / house / wharf / sewer rat
SCIENTIFIC NAME: Rattus norvegicus (Berkenhout)
CLASSlORDERIFAMILY: Mammalia / Rodentia / Muridae
INTRODUCTION: The Norway rat is the largest of the commensal rodents and is the most common rat in the temperate regions of the world. The Norway rat not only damages / destroys almost all materials it comes into contact with by gnawing. It will eat and contaminate stored food, but it is also of immense human health importance as a vector or carrier of many diseases. It is believed to be of central Asian origin, but is now distributed throughout the world and found throughout the United States.
RECOGNITION: The adult (with combined head and body length) is from 7-9.5” in length with the tail length of from 6-8” being added to the previous dimension. It usually weighs up to about 20 oz. Its fur is coarse and shaggy with color ranging from brown to brownish-gray with scattered black hairs mixed throughout. The underside is gray to yellowish white. It has a blunt muzzle with small eyes and ears. It is covered with dense short hairs I an even coat. The body is stocky with a scaly tail. The tail is shorter than the head and body combined. Adult fecal droppings are up to 3/4 " in length, capsule-shaped, and has blunt ends.
SIGNS OF INFESTATION:
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Gnaw marks. One can tell the age of gnawings by judging that new gnawings or holes tend to be rough whereas, old gnawings are smooth from wear. Old openings are typically greasy looking due to the oils and dirt from the coats of rodents entering and exiting the opening.
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Droppings. Fresh droppings are soft and moist whereas, old droppings are dried, hard and crumble easily; adult Norway's droppings are about ¾ with blunt ends vs. adult roof' rat’s being about 1/2 “ with pointed ends. Droppings are easy to identify and it is important to identify the type of rodent we are dealing with.
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Tracks/footprints. Front foot is 4-toed and the print is in front of a usually longer hind footprint with 5 toes. Recent, fresh tracks are clear and sharp whereas, old tracks are at least partially obscured by dust and surrounding debris.
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Rub marks or dark, greasy markings on vertical surfaces are common, especially around the entrance where several rats entering and leaving over a period of time. Fresh marks are soft, greasy, and easily smeared whereas, old marks are dry with grease and flaky.
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Burrows. Are found in earthen banks, under concrete slabs, under railroad ties, or landscaping timbers and under walls. If the tunnel is active, it is generally clear and free of dust and cobwebs. The main opening usually has a hard packed soil for the bottom with rub marks generally visible.
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Runways. Norway rats consistently follow the same paths, usually along walls, stacked merchandise, etc. They have poor vision. Active runways have a greasy appearance, are free of dust and cobwebs, and have fresh tracks and/or droppings present.
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Damaged goods. Norway rats prefer meat, fish, and cereal (dry dog food is a favorite). On the other hand, roof rats prefer fruits, vegetables, and cereals such as corn meal and livestock feed.
BIOLOGY: Norway rats reach their sexual maturity in about 5 months. Their pregnancy lasts an average of just over 20 days. The young are born live, blind and naked at birth. A thin coat of hair appears in about 7 days with the eyes opening in about 14 days. They are weaned at approximately 4 weeks. The average number of litters is between 3-6 per year, each litter containing an average of about 7 young, but they average about 20 weaned offspring / female / year. Adults live an average of only 12months, but much longer in captivity.
They have rather poor vision and are color blind, but their senses of hearing, smell, touch, and taste is well developed. Their touch is via their vibrissae or long whiskers and is so keen it is almost indescribable. Norway rats are excellent runners, climbers, jumpers, and swimmers.
A Norway rat requires about an ounce of food and from 1/2-1 oz of water every day, with the water coming from a nonfood source. This volume of consumption averages an output of droppings from about 50-160 droppings and 1/2 oz of urine produced each day. They urinate or defecate with almost every step.
Historically, the disease most commonly thought of involving rats (roof rats primarily) is plague which is transmitted via fleas leaving an infected rat and attacking man. Plague has not been found in rats in the United States for many years. Other transmittable diseases include murine typhus via fleas (also from dust caused through droppings and urine), infectious jaundice/ leptospirosis/Weil's Disease via urine in water or food, rat-bite fever via bites, trichinosis via undercooked pork, and food poisoning or Salmonellosis via droppings. Another problem is through tropical rat mite dermatitis, which is caused by mites when they feed on humans coming directly from rats.
HABITS: Rats are primarily nocturnal in habit and they are very cautious. Although they constantly explore their surroundings, they shy away from new objects and changes. Outdoors, Norway rats prefer to nest in burrows in the soil along railroad embankments, stream/river banks, piles of rubbish, under concrete slabs, etc. The burrow will have at least 1 entrance hole and at least 1 emergency exit, which is often hidden under grass, debris, etc. These are social animals and often many burrows will be located within a given area. An opening of greater than 1/2" is required for entry into buildings. Indoors, Norway rats usually nest in basements and the lower portions of buildings in piles of debris or merchandise as long as it is not disturbed. Although Norway rats prefer the ground or lower levels of buildings and sewers, on occasion they may be found in attics, on roofs, and in other high places.
Although they will eat practically anything, Norway rats prefer meat, fish, and cereal. If the food material eaten proves to be disagreeable, they are quick to develop food/bait shyness. Once they find an acceptable/preferred food, rats tend to eat their fill at one sitting/place and will return time after time. They almost always require a nonfood or separate source of water. Norway rats will travel up to 150 ft from their harborage for food and/or water. They will gnaw through almost anything to obtain food and/or water, even plastic or lead pipes.
Once established, Norway rats tend to follow the same route or pathway between their harborage and food and/or water sources. As often as possible, they follow vertical surfaces, which they navigate via their vibrissae or long whiskers contacting runways. Most runways along vertical surfaces will usually include dark rub marks on the vertical surfaces where their oily fur makes contact. Their runway will be free of debris, and outdoors, the grass will be worn away to the bare soil.
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