LOCOMOTORY ENERGETICS AND METABOLIC FUEL RESERVES OF THE VIRGINIA OPOSSUM

Marsupials have lower resting metabolic rates than placental mammals, but it is not clear whether particular species can extend this energet ic advantage to locomotion. Some active marsupials have a low cost of locomotion, but other more sedentary species, such as the Virginia opo ssum, appear to run very inefficiently. Steady-state rates of O-2 cons umption (VO2) and CO2 production (VCO2) were measured at rest and duri ng horizontal treadmill exercise in wild-caught, trained opossums. Ave rage daily VO2 in undisturbed animals was 7.73+/-0.40 ml O-2 kg(-1) mi n(-1) (5.67+/-0.20 ml O(2)kg(-1) min(-1)during light and 9.84+/-0.81 m l O-2 kg(-1) min(-1) during dark hours, means +/- S.E.M., N=6). Net co st of locomotion ranged between 6.16 and 8.99 Jkg(-1) s(-1) as speed i ncreased and was always higher than for an average mammal of equivalen t mass. Net cost of transport decreased as speed increased and was 15- 80% higher than for an average mammal. During aerobic locomotion, most of the energy was provided by carbohydrate oxidation, which accounted for 60-95% of VO2 as speed increased. Glycogen and triglyceride reser ves were quantified in the major storage depots to estimate potential survival time and travelling distance. Enough metabolic fuel was store d to survive for at least 1 week without eating, and 95% of this energ y was in adipose tissue triglycerides. However, maximal travelling dis tance was less than 2km because opossum locomotion is mainly supported by carbohydrate reserves, which represented only 4% of the available energy. We conclude that aerobic, ground locomotion of Virginia opossu ms is associated with two major energetic handicaps because their part icularly high cost of transport and the nature of the main oxidative f uel they consume are both incompatible with prolonged locomotion.