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The development of small temperature-sensitive radiotransmitters has allowed researchers to measure skin temperature (Tsk) or body temperature (Tb) and quantify temporal patterns of torpor use by free-ranging animals (eg, number of bouts, length of bouts). However, simply addressing temporal patterns of heterothermy limits the scope of potential research questions because temporal patterns may not correlate with energy savings. Our objective was to devise a predictive model for thermoregulatory energy expenditure, using Tsk and ambient temperature (Ta) as independent variables. We used open-flow respirometry and temperature telemetry to quantify the metabolic rate (MR) and Tsk of big brown bats (Eptesicus fuscus) over a range of Ta from 0 C to 40 C. We calculated regression equations relating Ta, Tsk, and MR for each of four different thermoregulatory states: steady-state normothermia, cooling, steady-state torpor, and warming. Our approach may prove useful for quantifying thermal energetics in other free-ranging heterothermic endotherms.