Associations between energetics and over-winter survival in the short-tailed field vole Microtus agrestis

1. Over-winter mortality in small mammals is influenced strongly by low amb ient temperatures. Individuals with greater thermogenic capacities might th en be expected to survive better than those with lower thermogenic capaciti es. 2. To test this hypothesis, short-tailed field voles Microtus agrestis (Lin naeus) were captured during the winters of 1995/96 and 1996/97 at two field sites near Aberdeen (57 degreesN). The captured animals were tagged and ta ken back to the laboratory, where their resting metabolism (RMR), thermogen ic capacity (NA induced metabolism excluding RMR) and body mass were measur ed. 3. Body mass, RMR and thermogenic capacity did not differ significantly bet ween the start and end of winter in voles that were captured at both times. 4. Body mass varied significantly over the winter months, being lowest in J anuary and highest in March. Thermogenic capacity also varied over the wint er and the variation was Linked significantly with changes in ambient tempe rature, suggesting that either voles with greater thermogenic capacity were more likely to be active on cold nights, or voles were flexible in their t hermogenic capacities RMR did not vary significantly over the winter. 5. Animals that survived the winter had a significantly higher residual RMR than those that died (or permanently emigrated) but the survivors did not have significantly greater body masses, RMRs, thermogenic capacities or res idual thermogenic capacities.