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 deg rees C. Their maximum cold-induced rate of energy assimilation is abou t 90 kJ/d, which is twice the energy expenditure at 23 degrees C and f ive times their basal metabolic rate. Cold-acclimated individuals have an enlarged assimilation of 113 kJ/d. The pattern of energy budget li mitations depends on sex: males adopt a more frugal strategy of energy use. In females acclimated to a fiber-diluted diet, the size of the a limentary tract and the maximum cold-induced rate of energy assimilati on are increased. In males the effect is opposite. There is a strong c orrelation 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. The se results are consistent with the hypothesis that energy budgets are limited by the process of food digestion and/or absorption in the smal l intestine. The intraspecific correlation between basal metabolic rat e and maximum rate of energy assimilation and between basal metabolic rate and the size of the alimentary tract was not high.