Activational and organizational effects of testosterone on the number of mating partners and reproductive success in males of a social rodent

The timing of exposure to the steroid hormone, testosterone, produces
activational and organizational effects in vertebrates. These activational
and organizational effects are hypothesized to relate with the number of
female mating partners and reproductive success in males. We tested this
hypothesis by examining 151 wild degu (Octodon degus) males across a
10-year study. We quantified the association between adult serum
testosterone levels (i.e., an indirect index of adult activational
effects) and anogenital distance (AGD) length (i.e., a direct index of
fetal organizational effects), and their interaction on the number of
female mating partners and reproductive success. We found no evidence of
an association between adult male serum testosterone levels and the number
of female mating partners, or between adult male serum testosterone levels
and reproductive success. However, male AGD was positively associated with
reproductive success, but not so with the number of female mating
partners. Additionally, the positive association between male AGD and male
reproductive success was mediated by the number of mates. Our findings do
not support major roles of activational or organizational effects of
testosterone on the number of female mating partners and its consequences
on male reproductive success. Instead, our results suggest that compared
with individual male attributes, the female social environment plays a
more important role in driving male reproductive success.