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 sexratiosaremoremale-biasedthaninlaterpregnancy.Sincesex 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 Bexpensive” 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. Significance statement For decades, the mechanisms behind offspring sex ratios have been of interest across disciplines. Maternal testosterone has been implicated in mediating maternal condition, thus influencing secondary sex ratios. Here, we investigated the reproductive parameters of a culled nutria and integrated it with maternal hair testosterone levels to test the association between long-term testosterone and sex ratios. Our most surprising result was that high maternal testosterone levels were related with female-biased sex ratios. This is contrary to previous studies in other species and counter-intuitive. Heavier females tended to have male-biased litters. We also found that the proportionate representation of males within litters declined over the course of pregnancy. Male fetuses were usually the heaviest fetus, suggesting that they are the more Bexpensive” sex. We believe that our study provides new insights in this long-debated issue and will contribute to understanding the reproductive costs involved with maternal testosterone across animal models.