High Rates of Energy Expenditure and Water Flux in Free-Ranging Point Reyes Mountain Beavers Aplodontia rufa phaea

Published by and copyright of the University of Chicago Press. The definitive version of this article is available at: http://www.jstor.org/stable/10.1086/521204We measured water flux and energy expenditure in free-ranging\ud Point Reyes mountain beavers Aplodontia rufa phaea by using\ud the doubly labeled water method. Previous laboratory investigations\ud have suggested weak urinary concentrating ability,\ud high rates of water flux, and low basal metabolic rates in this\ud species. However, free-ranging measurements from hygric\ud mammals are rare, and it is not known how these features\ud interact in the environment. Rates of water flux (210+/-32 mL\ud d-1) and field metabolic rates (1,488+/-486 kJ d-1) were 159%\ud and 265%, respectively, of values predicted by allometric equations\ud for similar-sized herbivores. Mountain beavers can likely\ud meet their water needs through metabolic water production\ud and preformed water in food and thus remain in water balance\ud without access to free water. Arginine-vasopressin levels were\ud strongly correlated with rates of water flux and plasma\ud urea : creatinine ratios, suggesting an important role for this\ud hormone in regulating urinary water loss in mountain beavers.\ud High field metabolic rates may result from cool burrow temperatures\ud that are well below lower critical temperatures measured\ud in previous laboratory studies and suggest that thermoregulation\ud costs may strongly influence field energetics and\ud water flux in semifossorial mammals.This research was supported by National Science Foundation grant 0213095