Figure 3) The involvement of food intake and gut
microbial oscillations in mediating both metabolism and innate immunity
raises several questions regarding their function across a range of
ecological contexts. The figure visualizes the expected rhythms of
mucosal commensals like under six ecological contexts.
Yet, microbiota-independent mechanisms may be expected in species where
feeding, metabolic, and immune requirements are uncoupled. For example,
ectotherms exhibit circadian rhythms in body temperature and activity62–64, and have some level of circadian cycles in
metabolism 65 and immunity 66,67,
but feeding patterns are often not circadian (e.g. for large reptiles
such as snakes and crocodiles). In these cases, does the gut microbiota
undergo diurnal oscillations, and is the entrainment of innate immunity
completely independent of the gut microbiota? Given findings from
laboratory mice, one might expect that diurnal rhythms of the gut
microbiota to be strongest after feeding (Fig. 3). In social or
gregarious animals, microbiota are often shared and pathogen exposure
increases 68–70, providing another example where
pathogen exposure may not be closely correlated with food intake.
Therefore, peaks in pathogen exposure or activation of immunity may not
be limited to mealtimes. This raises the question as to whether social
animals have altered circadian rhythms in immune function compared to
solitary species, and whether such adaptations are mediated by the gut
microbiota.
Considering microbial rhythms in the context of metabolic and immune
requirements throughout the day may provide a useful framework to
predict the strength and the functional role of gut microbial
oscillations that goes beyond light and temperature cycles.
Nevertheless, investigating microbial oscillations across latitudes and
in environments with extreme light or temperature conditions (e.g. cave,
arctic, or desert animals) will aid our understanding of the
circumstances under which microbial rhythms occur. For example, gut
microbiome rhythms in meerkats may be particular strong due to the arid
environment they inhabit 26, which is characterised by
steep temperature differentials between day and night. This extreme
fluctuation induces nightly torpor in small desert mammals71, and whilst it is unclear whether meerkats undergo
a similar process, it might be expected that extreme temperatures exert
metabolic constraints that both influence and are influenced by the gut
microbiota.