Many mammalian herbivores remain active throughout winter. To satisfy daily energy needs, they ingest large quantities of cold food that subsequently must be warmed to body temperature. Some energy is inevitably lost during this process. Because the specific heat capacity of cellulose is only one-third that of liquid water, the quantity of energy that is lost depends primarily on the temperature and amount of water (free or contained in plants) that is ingested. Using the doubly labeled water method with meadow voles (Microtus pennsylvanicus) studied under field conditions, I show that the cost of heating ingested water represents 4.7–12.9% of the daily energy budget of nonreproductive individuals. Whether ingested water is liquid or frozen is critical because of the high cost of melting ice. I show that the fraction of individual energy budgets diverted to heating ingested food should be similar for small and large herbivores, and I explore some consequences of this result for our understanding of the winter ecology of mammalian herbivores.