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 ambient temperatures. Individuals with greater thermogenic capacities might then be expected to survive better than those with lower thermogenic capacities. 2. To test this hypothesis, short-tailed field voles Microtus agrestis (Linnaeus) 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 taken back to the laboratory, where their resting metabolism (RMR), thermogenic capacity (NA induced metabolism excluding RMR) and body mass were measured. 3. Body mass, RMR and thermogenic capacity did not differ significantly between 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 January and highest in March. Thermogenic capacity also varied over the winter and the variation was Linked significantly with changes in ambient temperature, suggesting that either voles with greater thermogenic capacity were more likely to be active on cold nights, or voles were flexible in their thermogenic 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 residual thermogenic capacities.