The mammalian microbiome is a key determiner of host fitness due to its essential role in many physiological processes. However, the extent to which natural variation in the microbiome impacts fitness in wild populations is unknown. My current research explores how within and between individual variation in the gut microbiome affects host phenotypes. To address these questions, I am combing field studies in wild house mice with laboratory and gnotobiotic techniques. My previous research focussed on competitive and reproductive signalling in bank (Myodes glareolus) and field voles (Microtus agrestis).