5) The effect of urbanisation on gut microbial rhythms
Urbanization is rapidly altering wildlife environments and activity patterns. Medium to large mammals are becoming more nocturnal to escape human disturbance 99 whilst small mammals that are normally nocturnal are active around the clock in urban areas100. Artificial light is causing birds and bats to extend and reduce their activity periods, respectively101,102, and is also associated with altered physiology and immune responses 103–105. Urban habitats also offer different diets, with many urban animals becoming scavengers or being provisioned by humans 106, and are associated with pollution 107 and higher pathogen diversity 108 than natural habitats. How these shifts in behaviour and exposure to pathogens and pollution are affecting health for both humans and wildlife via circadian mismatching is an outstanding question of urgent need of attention109–111, given ongoing and rapid human encroachment into natural habitats.
How might urbanization affect the gut microbiota, and what are the consequences for wildlife health? Accumulating evidence from across phylogenetically-diverse species suggests that urbanization generates a more ‘humanized’ gut microbiota, with a higher proportion of opportunistic pathogens 112–117. Yet, whether urbanisation is altering microbial rhythms is still unclear. In humans, urbanisation is associated with a loss of seasonal rhythms in the gut microbiota 78,118, indicating that biological rhythms might be disrupted by urban lifestyles. Wildlife health may be negatively affected by urbanisation and artificial light if changes to activity patters (e.g., timing of feeding) or altered diet disrupts gut microbial oscillations (Fig. 3). Constant light or dark leads to a loss of microbial rhythms in both chickens 32 and mice35, and this alteration is at least in part due to sensory signalling from the brain rather than changes to feeding times119. Diets high in fat also dampen microbial rhythms and thereby lead to dysbiosis – an imbalance in the microbiome that has negative health outcomes 39,120,121. Together, these indicate that urbanisation may alter microbial rhythms via multiple mechanisms.