Body Composition and Reserves of Energy of Microtus pinetorum from Southwest Virginia

Abstract

Body composition of Microtus pinetorum was explored in relation to month of capture, sex, age and reproductive status. There were significant bimonthly differences in concentrations of body water, ash and caloric value of live weight biomass (LWB) and dry weight biomass (DWB). There was no difference between males and females for any body component. Juveniles had higher mean annual concentrations of fat and water, and higher mean annual caloric values of DWB and LWB than adults. Pregnant and lactating females had lower mean annual concentrations of fat and caloric values, of LWB and DWB than nonbred adult females. Reproduction and growth during periods of nutritional stress resulted in significant decreases in concentrations of body fat and reserves of energy. INTRODUCTION Determinations of seasonal changes in body composition and reserves of energy are fundamental to understanding the transfer of energy through a small mammal population. Previous studies have shown body composition of rodents differs among species, seasons, habitats, sexes, ages, reproductive conditions, geographic distributions and environmental conditions (Gorecki, 1965; Hayward, 1965; Sawicka-Kapusta, 1968; Evans, 1973; Fleharty et al., 1973; Schreiber and Johnson, 1975; Bergeron, 1976; Kolodziej-Banach, 1976; Jagosz et al., 1979; Millar, 1981). Only Cengel et al. (1978) examined body composition (reserves of fat) in relation to season, habitat and sex in Microtus pinetorum. Body fat has been suggested as a physiological indicator of changes in food quantity and quality of small rodents (Judd et al., 1978; Merson, 1979; Merson and Kirkpatrick, 1981). Cengel et al. (1978) found a significant relationship between the nutritional quality in the habitat and the amount of pine vole body fat. Our study quantified further the body composition and reserves of energy of wild pine voles. The objective was to determine the sensitivity of body composition and reserves of energy to changes in season, reproductive status, age and sex, and to assess the relationship between body composition and the level of nutritional demand for energy. METHODS Adult and juvenile Microtus pinetorum were snap-trapped with apples as bait from May 1978 to March 1979 in two rural apple orchards located in Botetourt Co., in SW Virginia. Voles > 20 g in body mass were considered adults, those < 20 g, juveniles (Paul, 1970). Trapped voles were weighed, split midventrally, and the alimentary tract and the liver removed. Mesentery fat adhering to the alimentary tract was trimmed away and replaced in the body cavity. The weighed carcasses were frozen and then lyophilized for 48 hr. The difference between mass at the time of trapping and mass after lyophilization was concentration of water (expressed as percent live body mass). Each dry carcass was ground to a fine mix. Approximately three-fourths of the homogenate was analyzed for fat (expressed as percent dry body mass) by 16 hr of ether 1 Present address: Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station 77843. 2 Present address: U.S. Forest Service, 3825 E. Mulberry, Room 259, Fort Collins, Colo. 80524.