From doubly labelled water to half-life; validating radio-isotopic rubidium turnover to measure metabolism in small vertebrates

Abstract

The doubly labelled water method (DLW ) is widely used to measure field metabolic rate (FMR ), but it has some limitations. Here, we validate an innovative technique for measuring FMR by comparing the turnover of isotopic rubidium (86R b kb) with DLW depletion and the rate of CO 2 production (urn:x-wiley:2041210X:media:mee312056:mee312056-math-0001) measured by flow‐through respirometry (FTR) for two dunnart species (M arsupialia: D asyuridae), S minthopsis macroura (17 g) and S minthopsis ooldea (10 g). The rate of metabolism as assessed by urn:x-wiley:2041210X:media:mee312056:mee312056-math-0002 (FTR) and 86R b kb was significantly correlated for both species (S . macroura , r 2 = 0·81, P = 1·19 × 10−5; S . ooldea , r 2 = 0·63, P = 3·84 × 10−4), as was urn:x-wiley:2041210X:media:mee312056:mee312056-math-0003 from FTR and DLW for S . macroura (r 2 = 0·43, P = 0·039), but not for S . ooldea (r 2 = 0·29, P = 0·168). There was no relationship between urn:x-wiley:2041210X:media:mee312056:mee312056-math-0004 from DLW and 86R b kb for either species (S . macroura r 2 = 0·22, P = 0·169; S . ooldea r 2 = 0·21, P = 0·253). We conclude that 86R b kb provided useful estimates of metabolic rate for dunnarts. Meta‐analysis provided different linear relationships between urn:x-wiley:2041210X:media:mee312056:mee312056-math-0005 and 86R b kb for endotherms and ectotherms, suggesting different proportionalities between metabolic rate and 86R b kb for different taxa. Understanding the mechanistic basis for this correlation might provide useful insights into the cause of these taxonomic differences in the proportionality. At present, it is essential that the relationship between metabolic rate and 86R b kb be validated for each taxon of interest. The advantages of the 86R b technique over DLW include lower equipment requirements and technical expertise, and the longer time span over which measurements can be made. The 86R b method might be particularly useful for estimating FMR of groups for which the assumptions of the DLW technique are compromised (e.g. amphibians, diving species and fossorial species), and groups that are practically challenging for DLW studies (e.g. insects).