The measurement of 86 Rb turnover recently has been suggested as a useful method for measuring field metabolic rate in small animals. We investigated a proposed mechanism of Rb in the beetles on the K + and the Rb + diets was higher than that for beetles on the jam diet (F 2,311 =32.4; P=1.58×10 −13 ). We also exposed the beetles to different ambient temperatures to induce differences in metabolic rate (V · CO 2 ) while feeding them the jam and K + diets. V · CO 2 was higher at higher ambient temperature (T a ) for both jam (F 1,11 INTRODUCTIONThe metabolic rate of an animal is a measure of that animal's energy expenditure and reflects the cost of living (Hulbert and Else, 2004). The basal metabolic rate (BMR) is generally used to compare the metabolic rate of endotherms to each other, being the metabolic rate measured under defined conditions when an animal is inactive, postabsorptive and non-reproductive within the thermoneutral zone (Ricklefs et al., 1996;McNab, 1997). The standard metabolic rate (SMR) is measured under less stringent conditions when an animal is at rest, and is broadly considered to represent the minimal cost of living (Hulbert and Else, 2004). In insects there is no definition for BMR, and the SMR is usually used for comparative purposes (Quinlan and Gibbs, 2006). Metabolic rate can be readily measured in the laboratory by direct calorimetry, which involves measuring the heat produced by an animal (Kleiber, 1961;McLean and Tobin, 1987). More commonly used is indirect calorimetry, including flowthrough respirometry, which measures the gas metabolism of an animal (either oxygen consumption, V · O2 , or carbon dioxide production, V · CO2 , or both) and is recognised as one of the easiest and most accurate approaches to the measurement of metabolism (Frappell, 2006). These are often used as proxies for true calorimetric representations of energy expenditure by virtue of relatively straightforward conversions to calorimetry, providing the respiratory exchange ratio (RER) or respiratory quotient (RQ) is known. METHODS & TECHNIQUESWhile laboratory measures of metabolic rate are informative in a comparative context (Brody and Procter, 1932;Kleiber, 1932;Elgar and Harvey, 1987;McNab, 1997), it is often difficult to extrapolate these measurements under controlled conditions to the cost of living in dynamic ecological scenarios. The utility of extrapolating metabolic rate data from captive animals to free-living animals has been questioned, especially when the latter experience different conditions such as food supply, quality of life and weather compared with laboratory conditions (Nagy, 1987). In contrast, the field metabolic rate (FMR) is measured in animals that are free-ranging in their natural environment (Ricklefs et al., 1996) and reflects the summary cost of living for that animal in its natural environment. The FMR includes the costs of BMR, SMR, thermoregulation, locomotion, feeding, predation avoidance, alertness, posture and digestion (Nagy, 1987). As FMR is measured over se...