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

Abstract

Summary1. 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 ( 86 Rb k b ) with DLW depletion and the rate of CO 2 production ( _ V co2 ) measured by flowthrough respirometry (FTR) for two dunnart species (Marsupialia: Dasyuridae), Sminthopsis macroura (17 g) and Sminthopsis ooldea (10 g). 2. The rate of metabolism as assessed by _ V co2 (FTR) and 86 Rb k b was significantly correlated for both species (S. macroura, r 2 = 0Á81, P = 1Á19 9 10 À5; S. ooldea, r 2 = 0Á63, P = 3Á84 9 10 À4 ), as was _ V co2 from FTR and DLW for S. macroura (r 2 = 0Á43, P = 0Á039), but not for S. ooldea (r 2 = 0Á29, P = 0Á168). There was no relationship between _ V co2 from DLW and 86Rb k b for either species (S. macroura r 2 = 0Á22, P = 0Á169;S. ooldea r 2 = 0Á21, P = 0Á253). We conclude that Rb 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).

Special K: testing the potassium link between radioactive rubidium (86Rb) turnover and metabolic rate

“…As V · CO2 was not different between the two groups, presumably the effect was related to K + elimination, independent of the role of K + in what we measured as the metabolic rate (V · CO2 ). (Peters et al, 1995;Peters, 1996;Tomlinson et al, 2013). The results for the control jam diet were not congruent with a previous metaanalysis of 86 Rb k b for ectotherms (Tomlinson et al, 2013) (Fig.…”

“…(Peters et al, 1995;Peters, 1996;Tomlinson et al, 2013). The results for the control jam diet were not congruent with a previous metaanalysis of 86 Rb k b for ectotherms (Tomlinson et al, 2013) (Fig. 3), registering at a lower elevation, with lower…”

“…Although the DLW method has broad support and application for measuring FMR, the technique has theoretical and practical limits, where some species and habitats are not suitable for the technique (for review, see Tomlinson et al, 2013). The biological turnover of radioactive 86 Rb has been proposed as an alternative that avoids some of the limitations of DLW, and may be applicable to taxa that are unsuitable for DLW (Peters et al, 1995;Peters, 1996;Tomlinson et al, 2013). A problem with using 86 Rb k b as a measure of an animal's metabolism is a lack of understanding of the mechanistic link between metabolism and the turnover of 86 Rb (Bradshaw and Bradshaw, 2007;Tomlinson et al, 2013 86 Rb k b and DEE to make it ecologically relevant as a measure of FMR.…”

“…It has now been fairly well established that the turnover of rubidium-86 ( 86 Rb), a gamma and beta radio-emitter, correlates well with metabolic rate in vertebrates (Mishima and Odum, 1963;Fairbanks and Burch, 1968;Baker and Dunaway, 1975;Williamson, 1975;Peters et al, 1995;Peters, 1996;Tomlinson et al, 2013), and that there are some general patterns to this relationship that span all of the taxa studied so far (Tomlinson et al, 2013). Radio-isotope enrichment, particularly of gamma emitters, can be measured by whole-body counting, obviating the need for sub-sampling of the body pool .…”

“…Furthermore, radio-isotope enrichment provides a simpler avenue for measuring FMR, and one that extends the recapture period far beyond that of DLW in small animals because the biological halflife of the isotopes is longer Tomlinson et al, 2013). The method may be particularly applicable to measuring FMR in insects (Tomlinson et al, 2013 Rb k b and metabolic rate remained even after accounting for food intake. The other possibility is that 86 Rb k b is measuring metabolic rate via some mechanism that is related to K + , such as ATP utilisation by the ubiquitous Na ].…”