The Life of the White Mouse. 



197 



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. 



Females suckling young frequently fail to at once again become 

 pregnant, due to some influence from the mammary glands which 

 inhibits ovulation, and in all cases where more than two young are 

 being suckled and the female becomes pregnant, the implantation 

 of the embryos is retarded for about nine days, until the young 

 cease to suckle, and a corresponding prolongation of the gestation 

 period occurs, as compared with that of non-suckling females. 

 Both of these phenomena appear to be protective for the parent 

 organism, since females whose young are removed at birth usually 

 bear litters in such rapid succession that they die of exhaustion 

 before the termination of the reproductive period. 



Males and females which were never allowed to pair have 

 lived the same length of time as other animals, and have shown 

 no peculiarities of behavior or appearance. 



