Multiple hypotheses seek to explain the adaptive value of a biased sex ratio. A possible mediator for such a process is maternal testosterone, which may be involved in female reproduction, by stimulating follicular growth and development, and may also reflect body, reproductive, and social conditions. Several studies in our lab test the association between maternal condition, steroids, and fitness. In rats, we recently found that maternal hair testosterone, but not corticosterone, predicted litter sex ratios. Younger mothers with large litters and older mothers with small litters tended to have male-biased litters. The possible trade-off between litter size and sex ratio may be mediated by testosterone and influenced by body fat and composition, which influence the individual’s well-being. In a separate study, using necropsies of 82 pregnant culled nutrias we found that as pregnancy progresses, litters became more male-biased. We also found that bigger females and those that had lower testosterone had male-biased litters. Litters of high testosterone females had female-biased litters, hinting at a possible fitness cost associated with female testosterone. Other studies in the lab examined the association between testosterone and maternal behavior in rock hyrax, and testosterone and sex ratios in humans.