Annual changes in blood parameters, organ weights, and total body lipid inPeromyscus maniculatusandMicrotus ochrogasterfrom a prairie environment

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Abstract
Annual cycles in hematological parameters, organ weights, and total body fat were measured in two species of mice from a prairie environment. Hematocrit was higher in winter in both Microtus ochrogaster and Peromyscus maniculatus. Plasma protein concentration decreased in winter in both species. Liver, kidneys, and adrenal glands were larger in female P. maniculatus than in males. Greatest liver and kidney size occurred during lactation. Total body lipid was about 5 to 7% of live body weight throughout the year, providing enough energy to survive only about one day of fasting. Adaptive shifts in bodily functions can be expected to coincide with periods of unusual energy expenditure such as lactation and winter. In these mice conserving energy seems to be a more important survival strategy than storing it.