Rodents breed more rapidly than you want to think. Rats reach sexual maturity at just 3 or 4 months, meaning that a population can swell from two rats to around 1,250 in one year, with the potential to grow exponentially. Mice, on the other hand, can have up to 10 litters containing up to 14 pups in each. Needless to say, they breed at a shockingly rapid rate, which is why an infestation can be dangerous.