A Gut Reaction to City Life: Stress-Microbiome Interactions in Urban Squirrels

Wildlife living in urban environments must overcome abiotic and biotic challenges unlike anything experienced in their evolutionary pasts. It is hypothesized that amongst vertebrates, the hypothalamic-pituitary-adrenal (HPA) axis, the hormone signalling pathway responsible for regulating an organism's stress response, is likely integral for coping with the array of novel stressors found in urban environments. More recently, it has been suggested that an organism's bacterial microbiome may facilitate rapid ecological adaptation to novel conditions, since, the metagenome has the capacity to respond much quicker to environmental change than the host genome. Academic evidence provides a foundation for the existence of an intimate bi-directional relationship between the HPA axis and the bacterial microbiome; wherein the microbiome can influence programming of the HPA axis early in life, but HPA activity, through stress induced immunomodulation, has the capacity to elicit changes in bacterial community structure. To date, little work has gone into characterizing this relationship in free-ranging wildlife. Urban environments may be used as a form of globally mass-replicated experimental system with which to investigate HPA axis-microbiome interactions within an eco-evolutionary context. In 2016, the CitiSci(urid) Project performed preliminary characterization of patterns in the covariation between multiple measures of stress physiology, and microbiome community structure in urban and natural populations of eastern grey squirrels. Oral and fecal bacterial communities differed between urban and natural populations, and in both cases the most prevalent bacterial groups were correlated with measures of stress physiology.