Carabidae_metabolic rate

The origin of the allometric relationship between the standard metabolic rate (MR) and body mass (M), often described as MR=<i>a</i>M<i>^b</i>, remains puzzling. Much effort has been directed at determining the mass-scaling exponent, <i>b</i>, however, the conclusions depend on the methodological approach. We investigated the mass scaling of MRs within and between species of Carabidae beetles. We used ordinary least squares (OLS) regression, phylogenetically generalized least squares (PGLS) regression and standardized major axis (SMA) regression to explore the effects of different model-fitting methods and data clustering caused by phylogenetic clades (shift grade) and gas exchange patterns (discontinuous (DGE), cyclic and continuous). At the interspecific level, the relationship between MR and M was either negatively allometric (b&lt;1) or isometric (b=1), depending on the fitting method. At the intraspecific level, the relationship either did not exist or it was isometric or positively allometric (b&gt;1) and it was significantly improved after the analysed dataset was split to gas exchange patterns. The studied species originated from two distinct phylogenetic clades that had different intercepts but a common scaling exponent (OLS, 0.61) that was much shallower than the scaling exponent for the combined dataset for all species (OLS, 0.71). The best scaling exponent estimates were obtained by applying OLS while accounting for grade shifts or applying PGLS. Overall, our findings warn that conclusions about a relationship between MR and M in insects can depend heavily on the model fitting method, the structure of phylogenetic non-independence and ecological factors that elicit different modes of gas exchange.