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We outline a computer model of heat and mass transfer through flesh, fat and porous fur for endotherms of any dimensions. We then validate it with a series of laboratory studies. Finally, we explore applications of the model to Bergmann's rule, predicting the mouse-to-elephant curve, climatedisease-toxicant interactions, animal 'design' via genetic engineering and energetic constraints on community structure.As a first test of the model we present calculations and metabolic chamber measurements for mammals ranging in size from mice to Holstein calves. We then compare simultaneous measurements on deer mice, Peromyscus maniculatus, of oxygen consumption, doubly labelled water turnover and food consumption with calculations of metabolic rate using body temperature radio-telemetry as input to the endotherm model. The endotherm model derived in the Appendix requires data on allometry (body dimensions, surface area), fur properties, core temperature, air and radiant temperatures and wind speed. The model is useful for calculating energetic expenditure in different microclimates without the need for extensive physiological measurements in the laboratory. Model predictions of metabolic rate at 12°C and at 22°C were well correlated with each of the three empirical estimates. The model shows that the posture an animal assumes can influence measurements of metabolic rate. Model calculations of metabolic rate using postures ranging from a curled-up ball-like geometry to a sprawled-out, cylinder or ellipsoid geometry bracket all three sets of simultaneous empirical data taken on the same animals.Applications of the model show that it can be applied in a wide variety of circumstances to gain insight into physiological and ecological problems.

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