Gut microbes are believed to play a critical role in most animal life, yet fitness effects and cost benefit-tradeoffs incurred by the host are poorly understood. Unlike most hosts studied to date, butterflies largely acquire their nutrients from larval feeding, leaving relatively little opportunity for nutritive contributions by the adult’s microbiota. This provides an opportunity to measure whether hosting gut microbiota comes at a net nutritional price. Since host and bacteria may compete for sugars, we hypothesized that gut flora would be nutritionally neutral to adult butterflies with plentiful food, but detrimental to semi-starved hosts, especially when at high density. We held field-caught adult Speyeria mormonia under abundant or restricted food conditions. Since antibiotic treatments did not generate consistent variation in their gut microbiota, we leveraged inter-individual variability in bacterial loads and OTU abundances to examine correlations between host fitness and the abdominal microbiota present upon natural death. We detected strikingly few relationships between microbial flora and host fitness. Neither total bacterial load nor the abundances of dominant bacterial taxa were related to butterflies’ fecundity, egg mass, or egg chemical content. Increased abundance of a Commensalibacter species did correlate with longer host lifespan, while increased abundance of a Rhodococcus species correlated with shorter lifespan. Contrary to our expectations, these relationships were unchanged by food availability to the host and were unrelated to reproductive output. Our results suggest the butterfly microbiota comprise parasitic, commensal, and beneficial taxa that together do not impose a net reproductive cost, even under caloric stress.

Ravenscraft2019_DataqPCR data (bacterial 16S and butterfly ef1a), microbiome data (16S OTU tables; bacterial taxonomy, phylogenetic tree, and representative fasta sequences of the OTUs), cumulative butterfly metadata (over the lifespan) and daily butterfly metadata in .csv and .txt formats. See README file for detailed descriptions of individual files.