Breeding

The brown rat can breed throughout the year if conditions are suitable, a female producing up to five litters a year. The gestation period is only 1-2 months and litters can number up to fourteen, although seven is common. The maximum life span is up to three years, although most barely manage one. A yearly mortality rate of 95% is estimated, with predators and interspecific conflict as major causes. Brown rats live in large hierarchical groups, either in burrows or subsurface places such as sewers and cellars. When food is in short supply, the rats lower in social order are the first to die. If a large fraction of a rat population is exterminated, the remaining rats will increase their reproductive rate, and quickly restore the old population level.