Rats reach sexual maturity within 3-4 months whereas mice reach sexual maturity in about 8-10 weeks;

Mice can produce about 6 - 10 litters per year with 5 - 12 young per litter;

Rats can produce about 4 - 6 litters per year with 6 - 10 young per litter;

With both rats and mice - the problems can escalate quickly.