City comfort: weaker metabolic response to changes in ambient temperature in urban red squirrels

Abstract

The ecophysiological responses of species to urbanisation reveal important information regarding the processes of successful urban colonization and biodiversity patterns in urban landscapes. Investigating these responses will also help uncover whether synurban species are indeed urban ‘winners’. Yet we still lack basic knowledge about the physiological costs and overall energy budgets of most species living in urban habitats, especially for mammals. Within this context, we compared the energetic demands of Eurasian red squirrels (Sciurusvulgaris) from the core of an urban environment with those from a nearby forest. We measured oxygen consumption as a proxy for resting metabolic rate (RMR) of 20 wild individuals (13 urban, 7 forest), at naturally varying ambient temperature (Ta) in an outdoor-enclosure experiment. We found that the variation in RMR was best explained by the interaction between Ta and habitat, with a significant difference between populations. Urban squirrels showed a shallower response of metabolic rate to decreasing Ta than woodland squirrels. We suggest that this is likely a consequence of urban heat island effects, as well as widespread supplemental food abundance. Our results indicate energy savings for urban squirrels at cooler temperatures, yet with possible increased costs at higher temperatures compared to their woodland conspecifics. Thus, the changed patterns of metabolic regulation in urban individuals might not necessarily represent an overall advantage for urban squirrels, especially in view of increasing temperatures globally.

What's going to be on the menu with global environmental changes?

“…For example, stomach content analysis revealed populations of wild boar ( Sus scrofa ) living in urban areas of Spain had higher consumption of anthropogenic food sources and larger body mass compared with non‐urban wild boar (Castillo‐Contreras et al, 2021). Responses to urbanization may also be maladaptive, such as Eurasian red squirrels ( Sciurus vulgaris ) showing weaker metabolic responses to changes in ambient temperature and a decreasing reliance on hibernation, potentially as a consequence of urban heat island effects and year‐round food abundance (Wist et al, 2023).…”