Field metabolic rate, movement distance, and grazing pressures by western grey kangaroos (Macropus fuliginosus melanops) and Merino sheep (Ovis aries) in semi-arid Australia
Details of the energy (food) requirements of wild herbivores are essential for understanding their role in ecosystems generally, and for evaluating their potential impact on co-occurring domestic herbivores, as in the case for Australia's largest grazing herbivores, the marsupial kangaroos, which co-exist with domestic livestock, particularly the wool breed Merino sheep. Using the doubly labelled water method we compared the field metabolic rate (FMR) of the western grey kangaroo (Macropus fuliginosus melanops) with that of the merino sheep, grazed sympatrically in a large naturally-vegetated enclosure. We combined our FMR studies with fine-scale GPS-monitoring to reveal important and significant impacts of daily movement patterns on the energetics of western grey kangaroos, but not the sheep. The daily FMR was substantially higher for kangaroos with longer daily movements. These individuals elevated their FMRs relative to predicted basal or minimal metabolic rates (i.e. FMR:BMR) by 4-5 times that expected for mature, non-reproductive mammals. Notably, those kangaroos that exhibited FMR:BMRs typical of mammals generally (around 2-3 times) had daily energy requirements around 6245 kJ. This level of energy expenditure was less than one third of that of the domestic sheep (22,799 kJ d−1). Even when compared on a common body mass of 35 kg, the western grey kangaroo energy expenditure was only 0.46 that of the merino sheep. Moreover, for typically-sized mature females kangaroos (25 kg), which make the bulk of kangaroo populations, the comparative energy requirement was just 0.31 that of a mature, non-breeding merino ewe.