Changes in gut capacity with lactation and cold exposure in a species with low rates of energy use, the pine vole (Microtus pinetorum).

Interspecific differences in the adaptive capacity of the gut may contribute to interspecific differences in rate of energy use and life-history traits. We tested the hypothesis that changes in gut capacity when energy demands are elevated are similar in species with low average rates of energy use compared with a species with higher average rates of energy use. We measured changes in gut capacity in Microtus pinetorum, a species with low average rates of energy use, and compared these with published data of changes in gut capacity of other rodent species with higher average rates of energy use. We quantified food ingestion, daily metabolic rate, resting metabolic rate, and gut organ masses, lengths, and functional volumes in nonreproducing, lactating, and cold-exposed females. Cold-exposed females had significantly higher, and lactating females moderately higher, mass-independent daily metabolic rates than control females. No significant changes in the mass or length of the intestinal organs occurred with cold exposure. Length, but not mass, of the gut was significantly greater in lactating females and functional volume was greater in cold-exposed females, compared with control females, independent of body mass. These changes in gut capacity were much less extensive than those reported for other rodent species. Interspecific differences in gut capacity were not attributable to differences in rates of ingestion and energy need among species. A large adaptive capacity of the gut or maintenance of a large reserve capacity may be a requirement for high rates of energy use and may contribute to the positive interspecific correlations that exist between rates of growth and reproduction and energy use for maintenance metabolism.