Comparative analyses of basal rate of metabolism in mammals: data selection does matter

Genoud, Michel; Isler, Karin; Martin, Robert D. (2018). Comparative analyses of basal rate of metabolism in mammals: data selection does matter. Biological reviews, 93(1), pp. 404-438. Cambridge University Press 10.1111/brv.12350

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Basal rate of metabolism (BMR) is a physiological parameter that should be measured under strictly defined experimentalconditions. In comparative analyses among mammals BMR is widely used as an index of the intensity of the metabolicmachinery or as a proxy for energy expenditure. Many databases with BMR values for mammals are available, butthe criteria used to select metabolic data as BMR estimates have often varied and the potential effect of this variabilityhas rarely been questioned. We provide a new, expanded BMR database reflecting compliance with standard criteria(resting, postabsorptive state; thermal neutrality; adult, non-reproductive status for females) and examine potentialeffects of differential selectivity on the results of comparative analyses. The database includes 1739 different entriesfor 817 species of mammals, compiled from the original sources. It provides information permitting assessment of thevalidity of each estimate and presents the value closest to a proper BMR for each entry. Using different selectioncriteria, several alternative data sets were extracted and used in comparative analyses of (i) the scaling of BMR tobody mass and (ii) the relationship between brain mass and BMR. It was expected that results would be especiallydependent on selection criteria with small sample sizes and with relatively weak relationships. Phylogenetically informedregression (phylogenetic generalized least squares, PGLS) was applied to the alternative data sets for several differentclades (Mammalia, Eutheria, Metatheria, or individual orders). For Mammalia, a ‘subsampling procedure’ was alsoapplied, in which random subsamples of different sample sizes were taken from each original data set and successivelyanalysed. In each case, two data sets with identical sample size and species, but comprising BMR data with differentdegrees of reliability, were compared. Selection criteria had minor effects on scaling equations computed for largeclades (Mammalia, Eutheria, Metatheria), although less-reliable estimates of BMR were generally about 12 – 20% largerthan more-reliable ones. Larger effects were found with more-limited clades, such as sciuromorph rodents. For therelationship between BMR and brain mass the results of comparative analyses were found to depend strongly on thedata set used, especially with more-limited, order-level clades. In fact, with small sample sizes (e.g.<100) results oftenappeared erratic. Subsampling revealed that sample size has a non-linear effect on the probability of a zero slope fora given relationship. Depending on the species included, results could differ dramatically, especially with small samplesizes. Overall, our findings indicate a need for due diligence when selecting BMR estimates and caution regardingresults (even if seemingly significant) with small sample sizes.

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