© 2014. Published by the Company of Biologists Ltd. The measurement of 86Rb turnover recently has been suggested as a useful method for measuring field metabolic rate in small animals. We investigated a proposed mechanism of 86Rb turnover, its analogy to K+, by comparing the turnover of 86Rb in a model insect, the rhinoceros beetle Xylotrupes gideon, fed a diet of plum jam or plum jam enriched with K+ or Rb+. The turnover of 86Rb in the beetles on the K+ and the Rb+ diets was higher than that for beetles on the jam diet (F2,311=32.4; P=1.58×10-13). We also exposed the beetles to different ambient temperatures to induce differences in metabolic rate (VCO2) while feeding them the jam and K+ diets. VCO2 was higher at higher ambient temperature (Ta) for both jam (F1,11=14.56; P=0.003) and K+ (F1,8=15.39; P=0.004) dietary groups, and the turnover of 86Rb was higher at higher Tafor both jam (F1,11=10.80; P=0.007) and K+ (F1,8=12.34; P=0.008) dietary groups. There was a significant relationship between 86Rb turnover and VCO2 for both the jam (F1,11=35.00; P=1.0×10-3) and the K+ (F1,8=64.33; P=4.3×10-5) diets, but the relationship differed between the diets (F1,19=14.07; P=0.001), with a higher 86Rb turnover in beetles on the K+-enriched than on the jam diet at all Ta. We conclude that 86Rb turnover is related to K+ metabolism, and that this is the mechanism of the relationship between 86Rb turnover and VCO2. Studies relating 86Rb turnover to VCO2 should maintain dietary [K+] as close as possible to that of natural diets for the most accurate calibrations for free-ranging animals.