Body-Composition Dynamics in Meadow Voles (Microtus Pennsylvanicus) of Southeastern Colorado
Unangst, Edward T., Jr., Wunder, Bruce A., The American Midland Naturalist
ABSTRACT.-Body-composition dynamics in meadow voles (Microtus pennsylvanicus) were examined by noninvasive EM-SCAN(R) methods for 12 mo. At the population level, body mass and fat-free mass varied seasonally with highs in spring and summer and lows in autumn and winter. In autumn and winter, losses in body mass and fat-free mass occurred in individual adults (40+ g summer body mass), whereas voles maintained 2-3 g of lipid mass year-round, equating to 4% body fat in summer and 7% body fat in winter. Thus, changes in body composition were primarily due to changes in fat-free mass and not changes in lipid mass. Voles may rely more on energy sparing (reduction in fat-free mass and body mass) than on energy storing as a primary means of over-,vinter survival. Compared with results from more northern populations of arvicoline rodents, the lesser change in total body mass observed in this study suggests a latitudinal influence on seasonal change, possibly due to reduced environmental circumstances.
Seasonal changes in total body mass are highly variable in many nonhibernating small mammals. In particular, over-wintering strategies that such mammals employ include energy storing (increased body fat, food caching) or energy sparing (nesting, social aggregations, decreased body size). When small mammals reduce body size in winter (Dehnel effect), associated changes in body composition (fat-free and lipid mass) are adaptive to reduce energy requirements at a time when food energy is limited (Dehnel, 1949; Fuller et al, 1969). At the population level, changes in body mass and composition have been attributed to: (1) selective death or emigration of heavier individuals; (2) mass loss in adult individuals; (3) recruitment of light individuals; or (4) slower growth rates in subadults (Iverson and Turner, 1974). In addition, small mammals at high latitudes lose weight in the winter suggesting: (1) a nonadaptive stress response; (2) a need to physically fit into the subnivean space; (3) a reduction in physiological wear for endurance until breeding; and (4) a reduction in energy needs at low resource availability (Hansson, 1990). In subarctic arvicoline rodents, body mass reduction in winter often can range from 30-50%, with negligible change in absolute lipid mass (Sealander, 1966; Iverson and Turner, 1974; Anderson and Rauch, 1984; Derting and Noakes, 1995; Zuercher et al, 1999). Whether the extent of mass loss during overwintering is as significant in arvicoline rodents in more southern locations remains undocumented.
We undertook this study to investigate body mass and body-composition dynamics in meadow voles (Microtus pennsylvanicus) near the southern limit (southeastern Colorado) of their North American distribution (Hoffmann and Koeppl, 1985) with noninvasive bodycomposition estimation techniques (EM-SCAN Model SA-2). Additionally, we investigated if changes in body composition (fat-free and lipid mass) occur seasonally using both population-based and individual (recapture) data to address meadow vole overwintering strategy. Our second objective was to investigate the possible effects of latitude on the degree of change in body mass and body composition of arvicolines. We hypothesize that more southerly arvicoline populations will exhibit smaller changes in body mass and body composition compared with more northerly populations because of less environmental stress.
MATERIALS AND METHODS
Acknowledgments.-This work was completed as partial fulfillment of requirements for the Ph.D. of E. T. Unangst, Jr. and was supported by a grant to ETU from the Directorate of Faculty Research, United States Air Force Academy. …