Limits to Sustained Metabolic Rate: The Link between Food Intake, Basal Metabolic Rate, and Morphology in Reproducing Mice, Mus musculus

Abstract

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 proceed to peak lactation. Nine unmated animals and eight of each of the two mated groups had their basal metabolism (oxygen consumption at 28° C) measured and were then killed by means of a CO₂ overdose, and the wet masses of 14 different body components measured. Body mass of the control animals averaged 26.5 g, not significantly different from animals in the other two groups immediately postmating. At peak gestation 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 (BMR) of the control animals (0.313 W) was significantly lower than the BMR of both pregnant (0.78 W) and lactating (0.95 W) animals. Food intake of controls was 97.9 kJ d⁻¹ (3.6 X BMR). At peak gestation food intake 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 reproduction, the peak food intake would have represented 12.2 times BMR. Morphological changes associated with the reproductive cycle mainly included increases in the masses of the alimentary tract, liver, kidneys, heart, and lungs. A principal components analysis revealed four major dominant axes of variability in the morphology. The first principal component was dominated by the combined alimentary effects and explained 71.8% of the variation in BMR. These data are consistent with the hypotheses that maximal sustainable metabolic rate reflects a link between structure of the alimentary tract, maximal daily food assimilation, and BMR. That is, as mice progressed through reproduction, their increasing 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.