Question

My sister and I have been having an argument about the reproductive rate of mice. Somewhere she read the statistic that a single pregnant mouse can result in 10,000 mice in a year, if the mice are left unchecked. I say that’s way too many mice. Can you provide the scientific answer to how fast mice can reproduce?

Answer

I can try, but don’t ask me to do the math. I’ve seen this riddle worked out a couple of times. The answer is always different, but it always boggles the mind! Keep in mind that the answer assumes ideal conditions of food, temperature, competition, and fertility, and year-round breeding. If food is in short supply, reproduction drops off. At cooler temperatures, litter size is smaller. Mice living outside have a shorter breeding season. The answer also assumes, I believe, that all of the offspring are going to survive long enough to mate and reproduce—extremely unlikely in the real world.

• A female house mouse will give birth to an average of 6 young (range is 2-13) approximately 19 days after mating.
• That female can mate again just two days after giving birth.
• Her young become sexually mature and can mate at 2 months of age. From each litter that means 3 more female mice producing 6 young each.
• That first female can produce 6-10 litters of young in a year, resulting in 42-60 offspring. Each of her 21-30 female offspring can do the same, and each of their female pups will do likewise.

Drumroll Please!

And the answer is…that one female house mouse can theoretically give rise to 5,082 mice in one year. However, a mathematician came up with an answer of 5 million mice in one year—a significant difference! This is reproduction data for mice in a perfect mouse world where no one dies and food is plentiful. Remarkably, all of this first female’s children, grandchildren, great-grandchildren, and great-great-grandchildren can have young in the same year! In a perfect world without predation or competition, a mouse can live for two years, but most mice live for only several months.

While all of these numbers are theoretical, one real world experiment did begin with 24 mice, supplied with abundant food and water, and housed in outdoor pens. These 24 mice multiplied to 2,000 mice after only 8 months.

Here’s a quiz for you. Say you brought one baby mouse home from a pet store and took good care of it. How many mice would you have at the end of 6 months? (Answer: Just one; it takes two to tango!)