Researchers know that many mammals can adjust the ratio of male and female young depending on the surrounding conditions at the time of conception but how precisely this is accomplished remains a matter for debate. A recent study carried out with equine embryos by Christine Aurich, DVM, PhD, and colleagues at the University of Veterinary Medicine, Vienna, Austria, has provided important information on how the survival of female embryos could be enhanced under conditions that would otherwise tend to favor the birth of males.

According to the team, because of the process involved in sperm cell formation, there should be an equal chance that a mammalian egg will be fertilized by "male" sperm, carrying a Y chromosome, as by a "female" sperm, carrying an X chromosome. The symmetry of the system ensures that roughly the same number of males and females are born, which is helpful for the species’ long-term survival. Surprisingly, though, many mammals do not produce equal numbers of male and female offspring.

The discrepancy could theoretically be explained by 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. It might seem as though male embryos are better able to survive under conditions of high energy intake, but how does this work?

Jana Beckelmann, a PhD candidate in Aurich’s laboratory, recently published evidence that a particular protein, insulin-like growth factor-1 (IGF1) might somehow be involved. From an examination of about 30 equine embryos,