Energetic cost of heating ingested food in mammalian herbivores

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 t his process. Because the specific heat capacity of cellulose is only one-th ird that of Liquid water, the quantity of energy that is lost depends prima rily on the temperature and amount of water (free or contained in plants) t hat is ingested. Using the doubly labeled water method with meadow voles (M icrotus pennsylvanicus) studied under field conditions, I show that the cos t 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 fract ion of individual energy budgets diverted to heating ingested food should b e similar for small and large herbivores, and I explore some consequences o f this result for our understanding of the winter ecology of mammalian herb ivores.