Reproductive microbial diversity is associated with competitive phenotypes in socially polyandrous jacanas

The composition of host-associated microbial communities may reflect the overall status of the host, including physiology and reproductive success. New hypotheses suggest that reproductive microbiomes can mediate sexually selected phenotypes, which may be particularly important for species with mating systems that feature strong sexual selection. These dynamics have been particularly understudied in female animals. Using 16S rRNA sequencing, we compared the cloacal microbiome of females and males from two socially polyandrous bird species that vary in the strength of sexual selection, Jacana spinosa (Northern Jacana) and J. jacana (Wattled Jacana). We hypothesized that the strength of sexual selection would shape cloacal microbial diversity, such that the more polyandrous J. spinosa would have a more diverse microbiome, and that microbiomes would be more diverse in females than in males. If the reproductive microbiome is indicative of competitive status, we also hypothesized that cloacal microbial diversity would be associated with competitive traits, including plasma testosterone levels, body mass, or weaponry. We found a positive relationship between microbial alpha diversity and testosterone and weaponry in female J. spinosa. We found no differences in microbial alpha diversity between species or sexes, but we did find that microbial beta diversity significantly differed between species. Our results suggest that the cloacal microbiome may be a key component of the competitive phenotype in socially polyandrous jacanas, supporting the hypothesis that the interplay between sexual selection and microbial communities can shape the physiology and fitness of wild avian hosts.