Age, senescence and the gut microbiome in a wild animal population
Background: In humans, gut microbiome (GM) differences are often correlated with, and sometimes causally implicated in, ageing. However, it is unclear how these findings translate in wild animal populations. Furthermore, studies that investigate how GM dynamics change within individuals (as opposed to among individuals), and with declines in physiological condition, are needed to fully understand links between chronological age, senescence, and the GM, but have rarely been done. Here, we use longitudinal data collected from a closed population of Seychelles warblers (Acrocephalus sechellensis) to investigate how bacterial GM alpha diversity, composition, and stability are associated with chronological age and the year leading up to death. We hypothesise that GM diversity and composition will differ, and variability will increase, in older adults, particularly in the terminal year prior to death, as it becomes dysregulated due to host senescence.  Results: GM alpha diversity and composition remained largely invariable with respect to adult age and did not differ in an individual's terminal year. Furthermore, there was no evidence that the GM became more heterogenous in senescent age groups (individuals older than 6 years), or in the terminal year. Instead, environmental variables such as season, territory quality, and time of day, were the strongest predictors of GM variation in adult Seychelles warblers. Conclusion: We found no evidence to suggest that host senescence is associated with GM restructuring within a natural population. This contrasts with studies on humans, captive animal populations, and some (but not all) studies on non-human primates, suggesting that GM deterioration may not be a universal hallmark of senescence in wild animal species. Further work is needed to disentangle the factors driving variation in GM-senescence relationships across different host taxa.