The life of the white mouse

Abstract

White mice may give birth to young every month in the year, but most of the litters come during the warmer months. Males and females are both sexually mature when about six weeks old, and in females of that age, ovulation occurs regardless of whether or not there has been a previous pairing. The gestation period of non-suckling mice is from 18 to 21 days, so that the first litter may be born when the parents are about two months old, and in all cases of normal, paired animals a litter has appeared before they were three months of age. This first litter may be large (12) or small (2). If the first litter includes more than five young (the average size of litters among white mice) the succeeding litters from that female are usually above the average number of young, but the reverse of this proposition is sometimes true and at other times not. Sixteen litters seems to be the limit for one female to bear, and some stop before reaching that number. The total number of young produced may reach 80, and appears to bear no relation to the size or the total number of litters. Females which are allowed to suckle their young, cease to bear at 18 to 22 months of age, after producing 12 to 16 litters, while those females whose young are removed as soon as found usually die before they cease to bear, but in the rare instances where such females survive, their litters come nearer together, but reproduction stops at an earlier age, so that the total number of litters produced is within the limits stated above. Females appear to be somewhat shorter lived than males, but animals of both sexes if healthy at birth, given reasonably good care, and protected from contagious diseases, have an expectation of life of about two years.