The fertility of mice selected for large or small body size

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Abstract
The fertility of two unrelated strains of mice (strainsNandC) which had both been selected for large and small body size has been studied.The fertility of pairs of mice in the large or small lines of strainCwas unimpaired by selection. In strainN, some of the pairs in the large and small lines, but not in the control line, were sterile. Sterility in the large line was due to the low libido of the males, and not to female infertility. Sterility in the small line was probably due to hypo-functioning of the anterior pituitary of some females: the oestrous cycle was delayed or absent, some mice failed to ovulate after mating, and a high proportion of those mating had no implanted embryos at 12 days' gestation. Oestrus and ovulation could be induced in mice of the small line of strainNby exogenous gonadotrophins, and the proportion of mice with implanted embryos was considerably increased by progesterone supplements.The number of eggs found after natural mating was considerably higher in large mice than in small mice, and was significantly correlated with body weight in the small line of strainConly. The egg-number/body-weight ratio was higher in the lines of strainCthan in those of strainN, though the ratios were similar when carcass fat was subtracted from total body weight.The amount of endogenous follicle-stimulating hormone secreted by the mice of the five lines was estimated by inducing ovulation with various amounts of exogenous gonadotrophins, and comparing the number of eggs found after each dose with the mean number ovulated after natural mating. Estimates of the amount of follicle-stimulating hormone secreted by mice of strainCwere higher than those for mice of strainN.Differences in the rates of growth and in the numbers of eggs ovulated after natural mating indicate a higher level of pituitary activity in strainCthan in strainN.