Abstract

The timing of exposure to testosterone produces activational and organizational effects in vertebrates. Evidence in female birds and mammals suggests that high serum testosterone levels, and the exposure to high testosterone levels during early development, are negatively associated with female fertility and sexual attractiveness to males. We tested these hypotheses by examining associations between serum testosterone levels (an indirect index of activational effects), anogenital distance (AGD, a direct index of organizational effects), the number of male mating partners attained by females, and the number of offspring weaned. We examined 160 wild degu (Octodon degus) females during the mating season, and 308 females during the nursing season, across an 11-year study. During the mating season, we found no association between female serum testosterone levels or AGD on the number of male mating partners, and on the number of offspring weaned. However, we found that the number of male mating partners was positively associated with the number of offspring weaned, suggesting that females attain direct fitness benefits from polyandrous mating. During the nursing season, we recorded a positive association between female AGD and the number of offspring weaned. This indicates that testosterone organizational effects positively impact female fertility. This finding is unique and adds to the theoretical framework of female phenotypical masculinization via intrauterine position phenomenon. © The Author(s) 2025.