The brown rat has only been recorded in Britain since the early 18th Century. It is thought to have been introduced in shipping from Russia, and did not originate in Norway
It is now by far the more abundant of the two rats species and is widely distributed in both urban and rural areas. It occurs both indoors and outdoors away from human habitation, and is the species often associated with sewer systems.
Reproduction and Life Cycle
They are capable of reproducing at the age of about 3 months. Pair bonds are not formed as mating is carried out on an opportunistic and promiscuous basis. When a female becomes sexually receptive, her sent attracts all local males. Gestation period is only 3 weeks and due to postpartum oestrus this can occur every 3 weeks. Unlike most mammals that have to wait until the original litter is weaned rats can mate again after giving birth but this only occurs when conditions are favourable for the young to survive. So therefore rats could give birth every 24 to 28 days, and this can give rise to very rapidly increasing populations.
About 50 -70 % of all rats in the UK are carriers of this disease. They excrete the bacterium (Lepotospirosis) through their urine into the environment, where it can survive for up to 45 days in fresh water or damp conditions. The bacteria can enter the human body via damaged skin (such as cuts and abrasions) or the mouth and nose.