Limits to the energy budget in a rodent, Peromyscus maniculatus: Does gut capacity set the limit?

Nonreproducing Peromyscus maniculatus acclimated to 23 degrees C and a standard mouse food can maintain a positive energy balance at -10 degrees C. Their maximum cold-induced rate of energy assimilation is about 90 kJ/d, which is twice the energy expenditure at 23 degrees C and five times their basal metabolic rate. Cold-acclimated individuals have an enlarged assimilation of 113 kJ/d. The pattern of energy budget limitations depends on sex: males adopt a more frugal strategy of energy use. In females acclimated to a fiber-diluted diet, the size of the alimentary tract and the maximum cold-induced rate of energy assimilation are increased. In males the effect is opposite. There is a strong correlation between an individual's maximum cold-induced rate of energy assimilation and the size of the alimentary tract, liver, and kidney; the correlation is strongest for the mass of the small intestine. These results are consistent with the hypothesis that energy budgets are limited by the process of food digestion and/or absorption in the small intestine. The intraspecific correlation between basal metabolic rate and maximum rate of energy assimilation and between basal metabolic rate and the size of the alimentary tract was not high.