Chuen Zhang Lee , Sarah F Worsley, Charli S. Davies, Ece Silan, Terry Burke, Jan Komdeur, Falk Hildebrand, Hannah L Dugdale , David S Richardson

Studies on wild animals, mostly undertaken using 16S metabarcoding, have yielded ambigous evidence regarding changes in the gut microbiome (GM) with age and senescence. Furthermore, variation in GM function has rarely been studied in such wild populations, despite GM metabolic characteristics potentially being associated with host senescent declines. Here, we used seven years of longitudinal sampling and shotgun metagenomic sequencing to investigate taxonomic and functional changes in the GM of Seychelles warblers (Acrocephalus sechellensis) with age and senescence. Our results suggest that taxonomic GM species richness declines with age and in the terminal year, with this terminal decline occurring consistently across all ages. Taxonomic and functional GM composition also shifted with host age. However, all the changes we identified occurred linearly with adult age, with little evidence of accelerated change in late life or during their terminal year. Therefore, the results suggest changes that changes in the GM with age are not linked to senescence. Interestingly, we found an increase in the abundance of a group of transposase genes with age, which may accumulate passively or due to increased transposition induced as a result of stressors that arise with age. These findings reveal taxonomic and functional GM changes with age in a wild vertebrate and provide a blueprint for future wild functional GM studies linked to age and senescence.