Male social environment and female masculinization level interact to explain female testosterone levels in a communally rearing rodent

<p><span>Testosterone is known to modulate several morphological and behavioral phenotypic traits in males. However, females also synthetize testosterone and other androgens which influence the development and function of female tissues. In vertebrates, mating and social interactions can in turn modulate individual female testosterone levels, generally being higher in females that compete for males and/or that monopolize reproduction. Female testosterone levels may also be higher in females with masculinized phenotypes or when females are exposed to the energetic demands of reproduction. We used data from a long-term study (2009–2019) in a natural population of the communally rearing rodent <em>Octodon degus</em> to examine the potential associations between female serum testosterone levels and season, female masculinization morphotype, and the individual composition within social groups. We quantified the size and composition of social groups, including the masculinization morphotype of adult male and female group members. We found that female testosterone levels were higher in masculinized vs. feminized females and were also higher during the offspring-rearing season than the mating season. Female testosterone levels also varied with the number and morphotype of male group members, but not with female or offspring social environments. Together, our results suggest that female testosterone levels are more sensitive to intersexual than to intrasexual interactions. Our results also reveal that female and male testosterone levels do not significantly differ between the sexes, a notable finding previously reported only in rock hyraxes. We discuss how the complex social system of degus could be driving this physiological similarity between the sexes.</span></p>