Metabolic rate in common shrews is unaffected by temperature, leading to lower energetic costs through seasonal size reduction

Small endothermic mammals have high metabolisms, particularly at cold
temperatures. In light of this, some species have evolved a seemingly
illogical strategy: they reduce the size of the brain and several organs
to become even smaller in winter. To test how this morphological strategy
affects energy consumption across seasonally shifting ambient
temperatures, we measured oxygen consumption and behaviour in the three
seasonal phenotypes of the common shrew (Sorex araneus), which differ in
size by about 20%. Body mass was the main driver of oxygen consumption,
not the reduction of metabolically expensive brain mass. Against our
expectations, we found no change in relative oxygen consumption with low
ambient temperature. Thus, smaller body size in winter resulted in
significant absolute energy savings. This could only partly be explained
by a change in activity budgets. Our findings highlight that these shrews
manage to avoid one of the most fundamental and intuitive rules of ecology
allowing them to subsist with lower resource availability and successfully
survive the harsh conditions of winter.