Social environment and masculinization level interact to explain male testosterone levels in a social rodent: Part 1: The male side

In vertebrates, male testosterone levels vary across the year being generally higher during the mating relative to the offspring rearing season. However, male testosterone levels may also be influenced by male anogenital distance (AGD) length (a proxy of prenatal androgen exposition), and by the social group environment. In social species, it has been proposed that high levels of testosterone could be incompatible with the development of an amicable social environment. Thus, in these species, it is predicted that males have relatively low levels of testosterone. Our goal was to examine the potential association between male serum testosterone levels, male AGD length, and the social environment in the rodent Octodon degus under natural conditions. We quantified male serum testosterone levels during the mating and offspring-rearing seasons, and we determined the number of females and males in each social group, as well as the composition of groups, in terms of the AGD length of the female and male group mates, from 2009 to 2019. Our results revealed that male testosterone levels covary with the season, being highest during offspring rearing season. Additionally, male testosterone levels vary with male AGD length and female and male social group environments. More importantly, male degus exhibit low levels of testosterone that are indistinguishable from female levels during offspring-rearing season. Similar to other highly social mammals, where males and females live together all year, male amicable behavior could be the best male mating strategy, having as a proximal mechanism, a reduction in circulating testosterone levels.