Construction of low-variability litters of preweaning mice

Mouse litter variation at weaning stems from a combination of genetic makeup, intrauterine experience, and postnatal maternal care. The 1st 2 factors may be matched at birth by making up separate litters out of littermate pairs whose birth weights are equal (+/-.02 g). Maternal competence may be matched by exchanging the mothers between the 2 cages each day. These tactics yield litters at weaning whose variability has been very much reduced. The range of litter-average body weights has been reduced from about 40 percent to about 5 percent and the range of litter-average brain weights has been reduced from about 15 percent to about 1-2 percent.