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.