Low fertility in mice selected for small body size (L) at 6 weeks of age was due to fewer matings, more ovulation failures in females mating, a lower ovulation rate and a lower fertilization rate than in unselected mice (C2) maintained as a control. The L males apparently had less libido than C2 males. This may have contributed to fewer matings in the L line. Failure to mate or failure to ovulate, if mating, could not be explained by differences in age or in body weight. It is suggested that a thyroid deficiency was responsible.
Females of the line selected for large body size (H) at 6 weeks of age ovulated more ova than C2 females. Essentially no foetal death occurred, after implantation, except in the H line. A lower percentage of the foetuses was living at 18 days of gestation in females of this line than in the females of the C2 line.