Selection among males for the genetic improvement of female fertility.

The quantitative physiological study of genetic variation in reproductive performance has shown that differences in the litter size and incidence of lambing in sheep are associated with variation in the release of LH. This variation is detectable in young animals of both sexes, and the injection of LHRH may aid its recognition. In the male it is associated with variation in mating behavior and testis growth. The ovarian activity of both sheep and mice has been shown to change following selection for testis growth. The use of measures of testis growth as criteria for selection to improve the reproductive performance of females is discussed.