Rapid GIT transit time in volant vertebrates, with implications for convergence in microbiome composition

Flying birds and bats have simplified gastrointestinal tracts (GITs) and low intestinal mass to support flight. While previous work showed reduced GIT passage times in birds relative to other vertebrates, GIT passage times have never been collectively quantified for bats. We conducted a meta-analysis of published digesta passage times across vertebrates, comparing volant and non-volant vertebrates, while considering the effects of body mass and diet. We hypothesized that, like flying birds, bats have significantly faster digesta passage times relative to nonvolant vertebrates, likely due to their adaptations for flight. Our study supports this, revealing significant differences in passage times among flying and non-flying groups, with bats exhibiting faster transit times compared to non-volant vertebrates. Using a phylogenetic comparative analysis, we show that flight and diet have a strong effect on GIT transit times across diverse taxa, while body mass plays a more limited role. Accelerated transit times in bats likely promote the rapid turnover of gut contents, which may contribute to their distinct GIT microbe compositions. Unique among mammals, bat GIT microbiomes are dominated by Pseudomonadota bacteria, a pattern also observed in flying birds. We hypothesize this convergence may result from rapid GIT transit times quantified here for volant taxa.