The Norway Rat: The Norway rat is the largest of the commensal rodents. It carries many transmittable diseases. They are primarily ground nesters and are most active at night. Pregnancy lasts 23 days and produces 7 to 8 young. They reach sexual maturity in 2-5 months and live up to 1 year. Have poor vision but excellent hearing, smell, touch and taste. Will travel up to 150 feet for food and water. An opening of ½ inch will allow entry into a structure.