The process involved in the formation of sperm cells involves symmetry, the equal chance that a mammalian egg will be fertilized by "male" sperm, carrying a Y chromosome or a "female" sperm, carrying an X chromosome, and that symmetry means that roughly the same number of males and females are born, which is necessary for the long-term survival of a species.

Yet that does not happen. Many mammals do not produce equal numbers of male and female offspring and the explanations are numerous -  such as differential fertilization efficiencies of male and female sperm (Y chromosomes are smaller than X chromosomes so perhaps male sperm can swim faster?) or by different rates of survival of male and female fetuses in the uterus. Male embryos seem better able to survive under conditions of high energy intake, but how does that work?

A researcher is claiming that a particular protein, insulin-like growth factor-1 (IGF1), might be involved. Examining dozens of embryos, Jana Beckelmann at the University of Veterinary Medicine in Vienna noticed that during early pregnancy (between eight and twelve days after fertilization) the level of messenger RNA-encoding IGF1 was approximately twice as high in female embryos as in male embryos.  

IGF1 is known to have important functions in growth and to inhibit apoptosis, or programmed cell death. As IGF1 treatment of cattle embryos produced in vivo improves their survival, it is likely that the factor has positive effects on the development of the early embryo in the horse. So why should female embryos contain more of the factor than males?

 Beckelmann
says the difference could relate to the fact that female embryos have two X chromosomes, which might produce more of a factor required for the expression of the IGF1 gene (which is not encoded on the X chromosome) than the single X chromosome in males is able to generate. Beckelmann was also able to confirm that the IGF1 protein was present in the embryos, confirming that the messenger RNA is actually translated to protein. 

Losses in early pregnancy are unusually high in the horse and it is believed that female embryos are especially prone to spontaneous abortion. Male embryos are known to be better able to survive under high glucose concentrations, so well-nourished mares preferentially give birth to male foals.

Beckelmann says, "We think the higher IGF1 concentrations in female embryos might represent a mechanism to ensure the survival of the embryos under conditions that would otherwise strongly favour males." If this is so, the ratio of the sexes in horses is the result of a subtle interplay between environmental and internal factors, including insulin-like growth factor-1.