Energy Expenditures by Caribou while Cratering in Snow

The rate of energy expenditure by caribou (Rangifer tarandus granti) digging in snow for lichens was determined by heart rate telemetry and an analysis of cratering mechanics. Based on a significant linear relationship between energy expenditure and heart rate, the mean cost per digging stroke in light, uncrusted snow was 118 J, whereas in denser (0.36 g/cm3) snow with a thin, hard crust the mean cost was 219 J/stroke. The cost of cratering through snow compacted by a snowmobile was 481 J/stroke. A comparison of metabolic and mechanical energy required for cratering suggested that caribou have evolved an energetically-efficient mechanism for obtaining food from beneath the snow layer. J. WILDL. MANAGE. 49(4):987-993 The ability of caribou to obtain lichens and other forages from beneath the snow by digging is well known. The amount of time caribou and reindeer (R. t. tarandus) reportedly spend digging for food in winter varies from only a few minutes per day in areas of shallow or patchy snow (Boertje 1981, Duquette 1984) to approximately 2 hours/day where snow is deep and continuous (Segal 1962, Thing 1977). During the winter months, caribou often obtain energy by catabolizing body fat and protein (McEwan and Whitehead 1970, Dauphine 1976), and thus it is important to the body condition and survival of the animal that the energy cost of obtaining food be low or be compensated for by the energy derived from the forage. Previous estimates of the energy expenditure associated with digging in snow by caribou and reindeer (Makarova and Segal 1958, Thing 1977) did not include the energy costs of digging in different snow types and at different rates. The most frequently used field method for measuring energy expenditure, using a gastight muzzle mask to measure oxygen consumption (White and Yousef 1978), cannot easily be used to measure digging costs because the mask restricts the animal's ability to smell and/or eat the forage. In addition, the valves in the mask may freeze or become clogged with snow, thereby affecting energy cost determinations. Heart rate telemetry has been used successfully with several domestic and wild ungulate species to predict energy expenditure (e.g., Webster 1967, Yamamoto et al. 1979, Renecker and Hudson 1983, Nilssen et al. 1984, Richards and Lawrence 1984) and appeared to be suitable for measuring cratering costs under various conditions. This study was supported by State of Alaska funds for organized research through the Inst. of Arct. Biol. and by a Graduate Fellowship in Resource Problems from the Univ. of Alaska to the senior author. The heart rate transmitters were implanted by R. A. Dieterich. We thank B. C. Anderson, S. L. Zeylmaker, N. J. Ferron, and D. R. Ashby for assisting with data collection and training and handling of captive caribou. R. D. Cameron and D. R. Klein provided helpful comments on the manuscript. We also thank E. A. Sturm and D. A. Borchert for pre-