Maternal Testosterone and Offspring Sex-Ratio in Birds and Mammals: A Meta-Analysis

Sex allocation theory predicts that parents should bias offspring sex to maximize their fitness in a given context. Quantifying the fitness benefits of offspring sex-ratio biases would be facilitated by a better knowledge of their underlying mechanism(s) and associated costs. The hypothesis that steroid hormones are involved in sex determination has gained in popularity recently. Being influenced by external stimuli and involved in a range of physiological processes, they could be a ubiquitous mediator of environmental conditions influencing sex-ratio with low fitness costs. Previous studies indicated that higher maternal testosterone levels led to the overproduction of sons around conception in both birds and mammals. We conducted a systematic review (including meta-analysis) of these studies and, as predicted, we found a weak positive and significant overall effect of maternal testosterone on the proportion of sons. Neither taxa, nor the type of study (experimental/observational), or the timing of timing testosterone manipulation/measure were significant predictors of offspring sex-ratio, which may be explained by low statistical power in addition to low variability between effect sizes. Our meta-analysis provides evidence for a general positive influence of maternal testosterone around conception on the proportion of sons across birds and mammals, although less confidently so for the latter. It begs for more large-scale experimental studies, especially on mammals, and ideally in the wild. It may also have some important consequences for the poultry industry.