Female-biased sex ratios are associated with higher maternal testosterone levels in nutria (Myocastor coypus)

Under various ecological conditions, producing a biased sex ratio may be adaptive. However, the factors that translate specific ecological conditions into internal processes remain an enigma. A potential mediator is maternal testosterone, which may reflect physical, reproductive, and social conditions. The nutria (Myocastor coypus) is a polygynous rodent, invasive in many parts of the world, which shows fluctuating sex ratios. Using necropsies of 82 pregnant culled nutrias, we found that in early pregnancy, offspring sex ratios are more male-biased than in later pregnancy. Since sex ratios decrease with pregnancy age, male fetuses in our study population may be terminated. In 68% of the litters, the heaviest fetus was a male, suggesting that males are the "expensive" sex. We also found that while maternal weight was not associated with testosterone, heavier females and those with lower testosterone had male-biased sex ratios. Litters of high testosterone females had female-biased sex ratios. To the best of our knowledge, this study is the first to show a negative association between maternal testosterone and male-biased sex ratios. Testosterone, through its role in reproduction, might be mediating maternal internal and external conditions by adjusting intra-uterine sex ratio.