Rats breed so quickly that in just 18 months, 2 rats have created over 1 million relatives.
Before their offspring are born, rats build nests from any material that can be foraged from the area, including branches, grass, trash, and paper. These nests are usually built in crevices, in rotting trees or in buildings. Rats, generally, are baby-making machines. Female rats can mate around 500 times in a six-hour period and brown rats can produce up to 2,000 offspring in a year, according to Discover Magazine. Brown rats can have up to 22 young at once, though eight or nine is more the average. Tropical rats tend to only have one to six babies at once.