![]() ![]() The average female rat has 4 to 6 litters per year and may successfully wean 20 or more offspring annually. These seasonal trends are most pronounced in more severe climates. Breeding often peaks in spring and fall, with reproductive activity declining during the heat of summer and often stopping completely in winter, depending on habitat. ![]() They become completely independent at about 3 to 4 weeks and reach reproductive maturity at 3 months of age.įemales may come into heat every 4 or 5 days, and they may mate within a day or two after a litter is born. They can eat solid food at 2 1/2 to 3 weeks. Newborn rats are hairless and their eyes are closed, but they grow rapidly. Litters of 6 to 12 young are born 21 to 23 days after conception. Nests may be lined with shredded paper, cloth, or other fibrous material. Norway rats usually construct nests in below-ground burrows or at ground level. Their sense of taste is excellent, and they can detect some contaminants in their food at levels as low as 0.5 parts per million. Rats use their keen sense of smell to locate food items and to recognize other rats. Therefore, for safety reasons, baits can be dyed distinctive colors without causing avoidance by rats, as long as the dye does not have an objectionable taste or odor. Rats have poor eyesight, relying more on their hearing and their excellent senses of smell, taste, and touch. Some individuals may be active during daylight hours when rat populations are high. They usually become active about dusk, when they begin to seek food and water. In the United Kingdom, brown rats are an important reservoir for Coxiella burnetii, the bacterium that causes Q fever, with seroprevalence for the bacteria found to be as high as 53% in some wild populations. Similar to other rodents, brown rats may carry a number of pathogens, which can result in disease, including Weil’s disease, rat bite fever, cryptosporidiosis, viral hemorrhagic fever, Q fever and hantavirus pulmonary syndrome. In fact it is common for breeding wild brown rats to weigh (sometimes considerably) less than 300 g (11 oz) Stories of rats attaining sizes as big as cats are exaggerations, or misidentifications of other rodents, such as the coypu and muskrat. Exceptionally large individuals can reportedly reach 900 to 1,000 g (32 to 35 oz) but are not expected outside of domestic specimens. Adult body weight averages 350 g (12 oz) in males and about 250 g (9 oz) in females. The length is commonly in the range of 20 to 25 cm (8 to 10 in), with the tail a further 18 to 25 cm (7 to 10 in), thus being roughly the same length as the body. The brown rat is a rather large true murid and can weigh twice as much as a black rat and many times more than a house mouse. The fur is coarse and usually brown or dark grey, while the underparts are lighter grey or brown. ![]()
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