Seasonal dynamics in the mammalian microbiome between disparate environments.

Host-associated bacterial microbiomes can facilitate host acclimation to seasonal environmental change and are hypothesized to help hosts cope with recent anthropogenic environmental perturbations (e.g., landscape modification). However, it is unclear how recurrent and recent forms of environmental change interact to shape variation in the microbiome. The majority of wildlife microbiome research occurs within a single seasonal context. Meanwhile, the few studies of seasonal variation in the microbiome often restrict focus to a single environmental context. By sampling urban and exurban eastern grey squirrel populations in the spring, summer, autumn, and winter, we explored whether seasonal rhythms in the grey squirrel gut microbiome differed across environments using a 16S amplicon sequencing approach. Differences in the microbiome between urban and exurban squirrels persisted across most of the year, which we hypothesize is linked to anthropogenic food consumption, but we also observed similarities in the urban and exurban grey squirrel microbiome during the autumn, which we attribute to engrained seed caching instincts in preparation for the winter. Host behaviour and diet selection may therefore be capable of maintaining similarities in microbiome structure between disparate environments. However, the depletion of an obligate host mucin glycan specialist ( Akkermansia ) during the winter in both urban and exurban squirrels was among the strongest differential abundance patterns we observed. In summary, urban grey squirrels showed different seasonal patterns in their microbiome than squirrels from exurban forests; however, in some instances, host behaviour and physiological responses might be capable of maintaining similar microbiome responses across seasons.