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 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 the gut flora, we leveraged inter-individual variability in bacterial loads and OTU abundances to examine correlations between and 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; this relationship was unchanged by food availability. Contrary to our expectations, we detected no negative impact of gut microbiota upon semi-starved hosts, even at high bacterial densities. Less...