Living at the physiological limits: field and maximum metabolic rates of the common shrew (Sorex araneus).

Shrews (genus Sorex, small insectivorous mammals) are well known for their extremely high basal metabolic rates (BMRs) even when corrected for their small body size. We measured energy expenditure of the common shrew (Sorex araneus) under natural conditions (field metabolic rate [FMR]) by doubly labeled water method to test whether FMR is proportional to high BMR in this species. The study was performed in summer in northeastern Poland. In addition to the FMR, we also measured maximum metabolic rates induced by cold exposure and by intense activity (MMRCOLD and MMRRUN, respectively) to evaluate the aerobic reserve (MMR-FMR) in S. araneus. This aerobic reserve was used as an indicator of the potential for metabolic constraints. The FMR averaged 2.31+/-0.32 L CO2 d(-1) (+/-SD) or 58.1+/-8.0 kJ d(-1) in 8.2-g animals. This figure constituted 216%-258% of a value predicted for a "standard" mammal of the same body mass and was the highest mass-specific field metabolic rate in mammals. Because of the high BMR level in S. araneus, the FMR to BMR ratio (2.4) was not far off mammalian standards (median value of 3.1). The rate of water efflux determined in S. araneus (20.2 mL H2O d(-1) or 2.46 mL H2O g(-1) d(-1)) exceeded all figures reported to date in other mammals and was apparently linked to the high FMR level and relatively high water content of shrews' food. Maximal metabolic rates (MMRRUN of 18.1+/-1.6 mL O2 g(-1) h(-1) and MMRCOLD of 23.5+/-1.9 mL O2 g(-1) h(-1)) were not high in proportion to BMR or FMR that resulted in relatively narrow aerobic reserve in S. araneus: 20% when calculated against the MMRRUN and 39% when compared with the MMRCOLD. Our study reveals that S. araneus has high energy costs of living and operates close to its physiological limits.