The metabolism of an animal is estimated by determining rates of carbon dioxide production (VCO) and oxygen consumption (VO) of individual animals, either in a closed or an open-circuit respirometry system.  Two measures are typically obtained: standard (SMR) or basal metabolic rate (BMR) and maximal rate (VO2max).  SMR is measured while the animal is at rest (but not asleep) under specific laboratory (temperature, hydration) and subject-specific conditions (e.g., size or allometry), age, reproduction status, post-absorptive to avoid thermic effect of food). VOmax is typically determined during aerobic exercise at or near physiological limits. In contrast, field metabolic rate (FMR) refers to the metabolic rate of an unrestrained, active animal in nature.  Whole-animal metabolic rates refer to these measures without correction for body mass.  If SMR or BMR values are divided by the body mass value for the animal, then the rate is termed mass-specific.  It is this mass-specific value that one typically hears in comparisons among species.