Non-model species deliver a non-model result: Nutria female fetuses neighboring males in utero have lower testosterone

Ruth Fishman, Yoni Vortman, Uri Shanas, Lee Koren

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


Neighboring fetuses may impact their siblings in various respects, depending on their in utero location and sex. The effects of the intrauterine position (IUP) are widely studied in model organisms, especially laboratory bred murine strains that are characterized by short gestations and altricial offspring. In some species, the proximity to a male fetus and its higher circulating testosterone masculinizes neighboring female fetuses. In utero testosterone exposure might be manifested as higher testosterone concentrations, which contribute to a variation in morphology, reproductive potential and behavior. In this study, we examined the influence of neighboring an opposite sex fetus on testosterone levels in a feral animal model characterized by a long gestation and precocious offspring. Using necropsies of culled nutria (Myocastor coypus), we accurately determined the IUP and quantified testosterone immunoreactivity in fetal hair. We found that as expected, both male and female fetuses neighboring a male in utero had longer anogenital distance. However, females adjacent to males in utero showed lower testosterone levels than male fetuses, while testosterone levels of females without a male neighbor did not differ from those of males. This surprising result suggests an alternative mode by which local exogenous steroids may modify the local fetal environment. Our study emphasizes the importance of examining known phenomena in species with different life histories, other than the traditional murine models, to enhance our understanding of the evolutionary mechanisms that are driving sexual differentiation.

Original languageEnglish
Pages (from-to)105-109
Number of pages5
JournalHormones and Behavior
StatePublished - May 2019

Bibliographical note

Publisher Copyright:
© 2019 Elsevier Inc.


  • Androgens
  • Anogenital distance
  • Fetal hair-testing
  • Intrauterine position
  • Myocastor coypus
  • Sex differences


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