On the Relation Between Basal and Field Metabolic Rates in Birds and Mammals

Basal metabolic rate (BMR) is widely assumed to be an index of energy expenditure of free-living animals. However, few attempts have been made to check if the assumption is valid. In this study field metabolic rates (FMR) of 58 species of birds and mammals have been compared with their BMRs. FMR tends to increase with increasing body mass slower than BMR, but the difference was significant only in marsupials. A correlation between mass-independent FMR and BMR (residuals from regressions) is very high in eutherians (mainly rodents), but it is weak in birds and approaches zero in marsupials. The assumption of a proportionality between BMR and energy expenditure of animals does not have a sound experi-