No evidence that gut microbiota impose a net cost on their butterfly host

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 ab...