LIFE PROCESSES IN CAPTIVE GRAY RATS 37 



below the mean in litters cast when females were very young, 

 and significantly lower when females were at the height of 

 reproductive activity at 5 to 9 months of age, deviations being 

 more than three times their probable errors. When fertility 

 began to decline, variation in litter size increased gradually, 

 reaching its highest point in litters produced near the end of 

 reproductive life. These changes in the variation of litter 

 size as the age of the mothers advanced are very similar to 

 those reported by Little ('33) in a strain of inbred mice. 



The age of the female, seemingly, is a factor that profoundly 

 influences her entire reproductive life. It determines to a 

 great extent not only the beginning and the end of her sexual 

 activity, but also produces cyclic changes in litter production, 

 in litter size, and in the variation of litter size. It may prove, 

 also, to have an effect on the functioning of the germ cells, 

 as Little ('33) has suggested is probable. 



THE SEX RATIO 



Little is known regarding the sex ratio in gray rats living 

 in their natural habitat. The proportion of the sexes found 

 in large numbers of trapped adults gives no information of 

 value on the sex ratio in the newborn, since mortality during 

 early postnatal life may have taken a heavier toll of one sex 

 than of the other. 



Two small series of data have been recorded that give the 

 sex proportions in the young of wild females brought into 

 captivity: Miller ('11) found a sex ratio of 82.1 males to 100 

 females in fifty-one offspring of a caged wild female ; the sex 

 ratio in 139 young obtained at the beginning of this investiga- 

 tion from matings of wild rats was 98.6 males to 100 females. 

 This evidence, meager though it is, indicates that a sex ratio 

 below equality may be normal for wild rats. Since these 

 animals are polygamous, a preponderance of females among 

 them would not be detrimental to the survival of the race. 



Sex ratios for individuals in the first ten generations of 

 captive grays were given in the former report on this strain, 

 and discussed with reference to various factors that might 



