Bioenergetics of North Plains Rodents

Abstract

Maintenance metabolic rates, based on food consumption experiments of captive rodents of 10 species, ranged from 1.6 to 4.0 times the estimated basal rate. The use of exercise wheels increased the energy expended for maintenance. Production, as evidenced by weight gain, comprised only a small part of energy flow. Assimilation efficiencies of rodents living in the wild on natural diets were determined by the ash tracer method. Omnivorous and granivorous species demonstrated better assimilation than foliage herbivores. Gravid and lactating females of most species exhibited increased assimilation. Food consumption in the wild, as measured by the mean weight of the stomach contents, varied inversely with the mean minimum temperature at the time of trapping. Gravid and lactating females were hyperphagic. Caloric values of the stomach contents of these species ranged from 4.6 to 5.7 kcal/g. The nitrogen indices indirect estimates of protein content of the food from the stomachs of omnivorous species were consistently larger than those of foliage herbivores. Only crude estimates of ingestion rates and energy expenditures for maintenance and productivity can be made at this time because we lack information regarding activity cycles and thermal environments encountered by these rodents. INTRODUCTION In order to describe the energy dynamics of rodent populations one must have some measure of: 1) the amount of energy expended for maintenance and production, 2) the assimilation efficiency, and 3) the caloric value of the food. Estimates of maintenance energy expenditure by small mammals living in the wild have been made from respirometry and food consumption experiments (Odum, Connell, and Davenport, 1962; McNab, 1963b). Estimates of production in wild populations have been made from growth and reproductive rates (Golley, 1960; Odum et al., 1962). Assimilation efficiencies of captive rodents have been measured by subtracting the feces production from the food intake (Golley, 1960; Smith, 1962; Jameson, 1965). The energy value of a variety of ecological materials including rodent foods has been determined by calorimetry (Golley, 1960; Odum et al., 1962; Pearson, 1964). We have calculated the maintenance energy expenditure, assimilation efficiencies, and the caloric value of the food utilized by rodents at several sites in North Dakota, Minnesota, and Wyoming. The species studied included: deer mice (Peromyscus maniculatus), meadow voles (Microtus pennsylvanicus), red-backed mice (Clethrionomys gapperi), meadow jumping mice (Zapus hudsonius), grasshopper mice (Onychomys leucogaster), feral house mice (Mus musculus), Ord kangaroo 1 Present address: Department of Biological Sciences, University of Idaho, Moscow 83843. 2 National Science Foundation Undergraduate Research Participant.