Short answers, pulled from the story.
The English naturalist John Berkenhout published a book titled Outlines of the Natural History of Great Britain in 1769 where he assigned the scientific name Mus norvegicus to the brown rat. This name suggested the animal originated from Norway although historical records show no evidence that the species came from that northern European country.
A typical adult brown rat weighs between 300 and 600 grams with a body length reaching up to 25 centimeters excluding the tail. The largest individuals can reach weights of one kilogram but these are rare outside domestic settings while the heaviest live specimen on record weighed over 1.4 kilograms.
Modern science now places the origin of Rattus norvegicus in central Asia and China rather than the Norwegian origin theory proposed by John Berkenhout in 1769. Earlier observers had called the creature the Hanover rat linking pest problems of 18th-century England with the House of Hanover before American scholar Alfred Henry Miles published a text in 1895 suggesting the species traveled from Persia to England less than two hundred years prior.
Reliable reports document presence in Ireland by 1722 and England by 1730 after the species spread globally from northern China and Mongolia during the Middle Ages. North America received the first brown rats between 1750 and 1755 while France saw records in 1735 followed by Germany in 1750 and Spain in 1800.
Under ideal circumstances populations could triple every eight weeks allowing numbers to jump from two to fifteen thousand within one year. Gestation lasts only 21 days with litters containing up to 14 pups though seven is common and females can become pregnant immediately after giving birth while nursing another litter.