Carabidae_metabolic rate

The origin of the allometric relationship
between the standard metabolic rate (MR) and body mass (M), often described as
MR=aMb, remains puzzling. Much effort has been directed at
determining the mass-scaling exponent, b,
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<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>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.