Social environment and anogenital distance length phenotype interact to explain testosterone levels in a communally rearing rodent: Part 1: The male side

In vertebrates, male testosterone levels vary across the year being generally higher during the mating season relative to the offspring rearing season. However, male testosterone levels may also be associated with male anogenital distance (AGD) length (a proxy of prenatal androgen exposition), and influenced 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, season, 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 season, being highest during the 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 year-round, male amicable behavior could be the best male mating strategy, thus leading to a reduction in circulating testosterone levels.

Social environment and anogenital distance length phenotype interact to explain testosterone levels in a communally rearing rodent: Part 2: The female side

Testosterone is known as a "male" hormone; however, females also synthetize testosterone, which influences female sexual and aggressive behavior. In female vertebrates, as in males, testosterone levels can vary seasonally. However, female testosterone levels may also be related with female anogenital distance (AGD) length phenotype (a proxy of prenatal androgen exposure), and the social group environment. We used data from a long-term rodent study (2009-2019) in a natural population of degus (Octodon degus) to examine the potential associations between female serum testosterone levels, season, female AGD phenotype, and social group composition. We quantified female serum testosterone levels during the mating and offspring rearing seasons, and we determined the number of females and males in social groups, as well the composition of groups, in terms of the AGD of the female and male group mates. Our results indicate that female testosterone levels vary with season, being highest during the offspring rearing season. Additionally, female testosterone levels were associated with the number of male group-members and the AGD of male group-members but were not associated with female social environment and focal female AGD phenotype. Together, our results suggest that female testosterone levels are sensitive to intersexual interactions. Our results also reveal that female and male testosterone levels do not differ between the sexes, a 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.

One for all and all for one: phenotype assortment and reproductive success in masculinized females

Homophily by morphological and behavioral traits has been described in several species of vertebrates, but its functional consequences remain poorly studied. Homophily by plurally breeding females may improve direct fitness by enhancing reproductive success. Female mammals may exhibit phenotypical masculinization due to exposure to androgens during early development, a condition that is associated with maternal performance during subsequent breeding. Our goal was to assess whether female composition (in terms of masculinization) of plurally breeding groups influences female fitness in a natural population of degus (Octodon degus). We assessed if plurally breeding female degus assort themselves by anogenital distance (AGD), an accurate measure of masculinization level. We also quantified if homophily by AGD phenotype affects female reproductive success and the reproductive output of the group. Plurally breeding groups typically included similarly masculinized (i.e., long AGD) females or similarly feminized (short AGD) females, indicating a strong degree of homophily. Females weaned more offspring in plurally breeding groups with more masculinized females. Additionally, standardized variance in the number of offspring weaned decreased in plurally breeding groups with mostly masculinized females, indicating greater reproductive equality in these groups. We conclude that female degus organize into homophilic social groups of similar AGD, and that social groups of masculinized females exhibit a higher reproductive success.

Parental care in male degus (Octodon degus) is flexible and contingent upon female care

Parents in many animal species provide care to their offspring as a mechanism to enhance their own fitness. In mammals, this behavior is expressed mostly by the females, but also by males of some species. Proximally, rates of paternal offspring care have been linked to organizational and activational effects of testosterone. Specifically, intrauterine position of male fetuses is associated with differential exposure to testosterone, leading to development of males with different levels of masculinization (assessed through differences in the length of the anogenital distance (AGD). The relative roles played by organizational and activational effects of testosterone on male parental care remain unclear. In this study, we aimed to determine if male sex-biased uterine environment and testosterone levels across the breeding period explain variation in paternal care in the social rodent, Octodon degus. Neither quantity (time with the offspring) nor quality (frequency of grooming and retrieving) of paternal care was affected by male sex-biased uterine environment, nor did paternal care significantly differ across the different stages of male reproduction. In contrast, paternal care was associated with maternal care. Quantity of male care decreased with increasing quantity of maternal care, and quality of male care increased with increasing quality of maternal care. While serum testosterone did not differ between males with different sex-biased uterine environment, male testosterone tended to increase during mating and decrease when pregnant females or offspring were present.

The number of species of degus (genus Octodon) is currently underestimated: An appraisal of species limits and their phylogenetic relationships (Rodentia: Hystricomorpha: Octodontidae)

As currently understood, the genus Octodon contains five species degus, lunatus, bridgesii, pacificus, and ricardojeda. Previous phylogenetic studies suggest that genus specific diversity is underestimated. In order to evaluate the taxonomic diversity of Octodon, we implemented unilocus (cytochrome-b) and multilocus (cytochrome-b + 4 nuclear genes) species delimitation methods. Octodon degus was recovered as a sister of the other species of the genus. The unilocus bGMYC and mPTP methods, based on cytochrome-b sequences, delimits 11 and 7 candidate species respectively, and both methods fail to recognize O. pacificus from O. ricardojeda. Results of the multilocus analysis (BPP) vary as a function of the dataset used. When the five genes are used 11 species are delimited, while eight species are delimited when only the nuclear genes are used. Octodon bridgesii is shown as comprising at least two species (one on the Pacific coast and the typical form found on the Andean slopes), while O. ricardojeda may comprise two species (one on the Chilean side of the Andes and the other in Argentina). Likewise, both multilocus matrices recover O. pacificus as a distinct species. This shows that species diversity of Octodon is underestimated. Remarkably, many of the delimited species based on genetic data are morphologically differentiated in cranio-dental characteristics. However, a pair of species has not achieved morphological differentiation, being cryptic species. Finally, the incongruence between mitochondrial and nuclear phylogenies suggests that processes such as incomplete lineage sorting and/or introgression have been present during the radiation of the genus.