Rats and mice have extremely high reproductive potential.  Factors affecting rat populations are age at puberty of 6 to 11 weeks, a year-round breeding season and an estrous cycle of 4 to 6 days.

Other reproductive factors include a gestation period of approximately 21 days, an average litter size of eight (four to twenty) and two to four litters per year.

All these factors indicate why a rodent population increases so rapidly under favorable conditions.

It has been demonstrated repeatedly that when nothing is done to eliminate the food source or available shelter after rats or mice have been controlled in a given area, populations soon recover because of increased birth and rearing success.  Conversely, rodents living in an environment of limited food and shelter reproduce at a slower rate.

Three population forces determine the size of a rodent population at any given time.  These are reproduction, mortality and movements.

Reproduction increases the population; mortality makes it smaller and movement work in either direction.  The relative balance of these population forces determines whether a population is growing, shrinking or staying the same.