Manipulating sex ratios for conservation: short-term risks and long-term benefits

Manipulating family sex ratio is often possible, either through non-invasive methods like changing sex-determining ecological or social factors, or through more invasive methods such as hormone treatment of embryos or sperm sexing prior to using assisted reproductive technologies. If the number of available eggs limits population growth, the production of relatively more daughters than sons may eventually lead to increased population growth in terms of absolute numbers. However, any deviation of the effective sex ratio from equality increases the rate of inbreeding and the loss of genetic variance in the next generation. I show here that there is a range of female biased sex ratios where increased population growth outweighs the effect of an enhanced inbreeding rate during the first generation or the first few generations after the start of a sex ratio manipulation programme. This is especially so in small and declining populations, where some sex ratio manipulations not only increase the effective population number N-e but also shift the population quickly into population numbers that are safe against the Allee effect. Consequently, an optimal sex ratio manipulation with respect to the genetic quality of a population means sending an endangered population first through a genetic bottleneck to achieve increased N-e and hence decreased rates of inbreeding, in the long run.