Short-duration respirometry underestimates metabolic rate for discontinuous breathers

Metabolic rate is commonly estimated from rates of gas exchange. An underappreciated factor that can influence estimates is patterns of pulmonary respiration. Amphibians display discontinuous respiratory patterns, often including long apnoeas, in addition to cutaneous gas exchange. The contribution of cutaneous exchange increases at low temperatures when metabolic rate is low. Because of the relatively low permeability of skin, measurements that disproportionately capture cutaneous exchange can produce underestimates of metabolic rate. The permeability of amphibian skin to CO₂ is greater than that to O₂; therefore, calculating the ratio of whole-animal CO₂ emission to O₂ uptake (the respiratory exchange ratio, RER) can be used to avoid underestimates of metabolic rate by ensuring that observed values of RER fall within the normal physiological range (∼0.7 to 1). Using data for cane toads, Rhinella marina, we show that short-duration measurements lead to underestimates of metabolic rate and overestimates of RER. At low temperatures, this problem is exacerbated, requiring over 12 h for RER to fall within the normal physiological range. Many published values of metabolic rate in animals that utilise cutaneous exchange may be underestimates.