Links between gut microbiome and senescence in birds: patterns, mechanisms and future directions

Senescence is a widespread process, yet its onset and rate vary markedly across species, populations, individuals, and traits. To date there is still limited information on the mechanisms underlying avian senescence trajectories and importantly, we lack mechanistic data on non-model and wild species. In this mini-review article, we explore a novel mechanism, the gut microbiome, as a putative contributor to avian senescence. Current evidence from non-avian systems suggests a bi-directional relationship between cellular senescence and the gut microbiome, whereby age-related changes in e.g. in the immune system signalling and gut barrier function alters gut microbiota, which in turn promotes systemic ageing. Studies on captive birds also support this, with poultry models suggesting an association between the gut microbiome and age/senescence. In wild birds, evidence remains scarce; gut microbiome differences between developing young and adult individuals have been observed, however the first cross-sectional and longitudinal studies show inconsistent results. Here, we offer here a novel mechanistic perspective that places host-microbiome interactions as a putative mechanism influencing senescent trajectories of animals, providing a roadmap for future studies in eco-evolutionary avian ageing research. We argue for 1) more longitudinal studies, 2) using a combination of functional microbiota analyses and experimental (e.g. transplant) manipulations to reveal causal relationships between microbiota and senescence, and 3) expanding beyond gut bacteria, to include other host sites (e.g., reproductive microbiome), and other microbial taxa (e.g., fungi and archaea). We hope this framework will advance our understanding of senescence in wild bird populations.