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 tactices yield litters at weaning whose variability has been very much reduced. The range of litter‐average body weights has been reduced from about 40% to about 5% and the range of litter‐average brain weights has been reduced from about 15% to about 1‐2%.