Fig. 1

Schematic overview of direct and indirect pathways via which season may impact the gut bacterial community. Yellow boxes indicate the three important aspects of seasonal environmental variation (diet, temperature, day length) that may impact the gut bacterial community. Green boxes indicate the host components of the indirect pathways. E.g., seasonal day length variation impacts the circadian system of the host, which will impact the gut bacterial community composition and function via seasonal variation in feeding patterns and circadian rhythms in hormone expression. On its turn, the gut bacterial community (blue box) impacts the host circadian system short chain fatty acids and bile acids production. Black arrows indicate the route of seasonal pathways modulating the gut bacterial community, grey arrows indicate modulation of the host by the gut bacterial community