Rats do not live long -- 2 to 3 years, tops -- but during their short lives they can produce many family members. Once they're sexually mature, at the age of 3 or 4 months, a male-and-female brown rat pair can have as many as 2,000 babies in the course of a year.
Rat Reproduction Cycle
A female rat is receptive to impregnation about 15 times a year, for a six-hour period each time. In that short, six-hour time span, a female can mate with many males. A mother rat has many litters during the year, with a dozen or more babies in each one.
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