John Watson, age 11, of Visalia, California, for his question:

How many babies does a mouse have per year?

In one year, a pair of little house mice could produce thousands and thousands of offspring. This could happen if every baby survived and carried on the family system of multiplication. Mice do not hibernate and females deliver a litter every two months or so. There are one to 12 babies per litter, though the average is six or seven. These figures indicate that mice can create a population explosion in a big way. As a rule, other factors reduce their numbers. But last year a plague of surplus mouse populations swarmed over parts of Australia. ~x

Mice, of course, are small members of the rodent clan. They include the ordinary house mouse, the white mouse used in labs, pet mice in assorted colors plus about 130 other species of the wilds. All of them are fantastic breeders, though the wild brethren usually have less chance to survive and multiply beyond bounds. The offspring of captive lab mice and pampered pet mice have a better chance of survival. House mice also can produce population explosions    that is if they share the homes of folk who don't give a hoot about decent sanitation.

Chances are, today's question arose from the keeping of pet mice. In this case, it is important to know what to expect if the parents are allowed to breed as often as they choose. A young female is ready to mate at the age of two months and bears her first litter of half a dozen babies about two weeks later. After two months she bears her second litter. Barring accidents and restrictions, she gives birth to six or seven litters a year. This can be expected through her normal life span of about three years.

This results in about 120 babies a year, or 360 during her lifetime. However, meantime her daughters, granddaughters and so on are bearing litters every two months. At the age of four months she expects litters of grandchildren and two months later she expects great grandchildren. This is multiplication on a very grand scale. If all her descendants survive, there could be thousands and thousands of them around to participate in the celebration of her first birthday.

A few of these baby pet mice can be donated to friends    but not this many, Obviously, the owner must take steps to restrict his mouse population, though this is not easy. Males and females can be kept in separate cages, but at the age of two months it may be hard to tell which is which. What's more, little mice have clever ways to get from cage to cage. The owner of a pair of pretty pet mice must be prepared to face these problems    and several others besides.

Pet mice are gussied up cousins of the drab little house mouse. And the house mouse is a pest. He ruins our stores of grains by devouring some and befouling much more with his wastes. He also spreads polio and other diseases. And this little pest is enchanted by your pet mice. When possible, both the males and females sneak into your cages to mate with their glamorous cousins. So please remember, all plain and fancy mice are rated as pests and an owner has a responsibility to keep them in check.