Body Composition, Energy Content, and Lipid Cycles of Four Species of Rodents

Gross body composition, energy content, and lipid cycles of Peromyscus maniculatus , Reithrodontomys megalotis, Sigmodon hispidus , and Microtus ochrogaster were studied. Ash values for adults ranged from 14.2 to 16.1 per cent of dry body weight, and water content varied from 61.5 to 74.2 per cent. Lipid content ranged from a low of 14.6 per cent dry body weight in M. ochrogaster to a high of 25.6 per cent in S. hispidus . Annually, kilocalories per gram live weight averaged 1.61 in P. maniculatus , 1.65 in R. megalotis , 1.74 in S. hispidus , and 1.42 in M. ochrogaster . Energy content of subadults and adults of S. hispidus differed significantly during summer, autumn, and winter; no such differences were noted for age categories of the other species. The four species exhibited three different types of lipid cycles: in both P. maniculatus and R. megalotis , lipid content was lowest in summer and highest in winter; in S. hispidus , the lowest readings were in spring and the highest in winter; in M. ochrogaster , lipid content fluctuated on a monthly basis but exhibited no annual cycle. We postulate that annual lipid cycles reflect the Zoogeographic history of the species, and might thus serve as a measure of physiological adaptation to climatic conditions unlike those that existed at the respective times and places of evolution of the four species.