Hypotheses on how selection for some traits in rodents led to correlated responses in offspring sex ratios.

Selective breeding of some species of rodent has sometimes occasioned unexpected and unexplained correlated changes in sex ratio (proportion male) at birth. Experiments in selective breeding for blood pH, sexual drive, body weight, sexual dimorphism in adult body weight, and for resistance to a dietary toxin have all been reportedly accompanied by this strange phenomenon. Here attempts are made to explain this. The overall form of explanation is that selection for any one of these characters is incidentally accompanied by selection for hormone concentrations which in turn are responsible for the variation in sex ratio. For example one may suppose that selection for sexual drive ipso facto selects for offspring sex ratios because, (1) Androgen levels are partially responsible for sexual drive and (2) Androgen levels are also (weakly) genetically determined and (3) Lastly, ex hypothesi, androgen concentrations are partially responsible for offspring sex ratios. If this line of reasoning were correct, the selection for sex drive would be expected to be accompanied by correlated changes in offspring sex ratio.