LIMITS TO SUSTAINED METABOLIC-RATE - THE LINK BETWEEN FOOD-INTAKE, BASAL METABOLIC-RATE, AND MORPHOLOGY IN REPRODUCING MICE, MUS MUSCULUS

Thirty virgin female white (MF1) mice (Mus musculus) were divided into three groups: an unmated control group; a second mated group, allowed to proceed to peak gestation; and a third mated group, allowed to pro ceed to peak lactation. Nine unmated animals and eight of each of the two mated groups had their basal metabolism (oxygen consumption at 28 degrees C) measured and were then killed by means of a CO2 overdose, a nd the wet masses of 14 different body components measured. Body mass of the control animals averaged 26.5 g, not significantly, different f rom animals in the other two groups immediately postmating. At peak ge station body mass averaged 50.5 g, immediate postpartum mass was 35.5 g, and by the end of lactation it was 38.4 g. Basal metabolic rate (BM R) of the control animals (0.313 W) was significantly lower than the B MR of both pregnant (0.78 W) and lactating (0.35 W) animals. Food inta ke of controls was 97.9 kJ d(-1) (3.6 x BMR). At peak gestation food i ntake was 119.3 kJ d(-1) (2.4 x BMR), and at peak lactation it was 405 .5 kJ d(-1) (5.96 x BMR). If BMR had not increased throughout reproduc tion, the peak food intake would have represented 12.2 times BMR. Morp hological changes associated with the reproductive cycle mainly includ ed increases in the masses of the alimentary, tract, liver kidneys hea rt, and lungs. A principal components analysis revealed four major dom inant axes of variability in the morphology. The first principal compo nent was dominated by the combined alimentary, effects and explained 7 1.8% of the variation in BMR. These data are consistent with the hypot heses that maximal sustainable metabolic rare reflects a link between structure of the alimentary tract, maximal daily food assimilation, an d BMR. That is, as mice progressed through reproduction, their increas ing food requirements were beyond the capacity of their guts to supply them. This led to hypertrophy of the alimentary tract and associated organs, with a consequent increase in BMR.