Body temperature variation in free-living and food-deprived yellow-necked mice sustains an adaptive framework for endothermic thermoregulation

Abstract

In many mammalian species, variation in body temperature (Tb) exceeds the values suitable for defining homeothermy, making it justifiable and even necessary to resort to the term “heterothermic”. However, Tb data are only available for ca. 1% of extant mammalian species. We investigated variations in Tb in wild free-living and experimentally food-deprived yellow-necked mice Apodemus flavicollis, during the temperate-zone autumn-winter period. In line with the adaptive framework for endothermic thermoregulation, we hypothesised that Tb in the mice should be adjustable with the energetic cost-benefit trade-off associated with maintaining homeothermy. In laboratory conditions, mice clearly entered a state of daily torpor when food-deprived. Our study thus makes it clear that A. flavicollis is a heterothermic species in which food deprivation results in switching between endothermic and poikilothermic thermoregulation. We also assumed that, in free-living mice, heterothermy increases with elevated environmental challenges, e.g. when the ambient temperature (Ta) decreases. Consistent with this was the inverse correlation noted between variation in Tb in free-living mice and Ta, with most individuals clearly becoming torpid when Ta decreases below 0 °C. It is the increased cost of food hoarding under cold conditions that most likely triggers a state of torpor as a last result. Overall, our study indicates that yellow-necked mice can provide a further example of species sustaining an adaptive framework for endothermic thermoregulation.