At Home: A Short History of Private Life

by Bill Bryson

When they are not eating, rats are likely to be having sex. Rats have a lot of sex—up to twenty times a day. If a male rat can’t find a female, he will happily—or at least willingly—find relief in a male. Female rats are robustly fecund. The average adult female Norway rat produces 35.7 offspring a year, in litters of 6 to 9 at a time. In the right conditions, however, a female rat can produce a new litter of up to 20 babies every three weeks. Theoretically, a pair of breeding rats could start a dynasty of 15,000 new rats in a year. That doesn’t happen in practice, because rats die a lot. Like a lot of other animals, they are more or less programmed by evolution to expire fairly easily. The annual mortality rate is 95 percent. A determined extermination campaign will normally reduce rat populations by 75 percent or so, but once the campaign stops the rat population will recover in six months or less. In short, an individual rat hasn’t got great prospects in life, but his family is effectively ineradicable.