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

Abstract

Basal rate of metabolism (BMR) is a physiological parameter that should be measured under strictly defined experimental conditions. In comparative analyses among mammals BMR is widely used as an index of the intensity of the metabolic machinery or as a proxy for energy expenditure. Many databases with BMR values for mammals are available, but the criteria used to select metabolic data as BMR estimates have often varied and the potential effect of this variability has 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 potential effects of differential selectivity on the results of comparative analyses. The database includes 1739 different entries for 817 species of mammals, compiled from the original sources. It provides information permitting assessment of the validity of each estimate and presents the value closest to a proper BMR for each entry. Using different selection criteria, several alternative data sets were extracted and used in comparative analyses of (i) the scaling of BMR to body mass and (ii) the relationship between brain mass and BMR. It was expected that results would be especially dependent on selection criteria with small sample sizes and with relatively weak relationships. Phylogenetically informed regression (phylogenetic generalized least squares, PGLS) was applied to the alternative data sets for several different clades (Mammalia, Eutheria, Metatheria, or individual orders). For Mammalia, a 'subsampling procedure' was also applied, in which random subsamples of different sample sizes were taken from each original data set and successively analysed. In each case, two data sets with identical sample size and species, but comprising BMR data with different degrees of reliability, were compared. Selection criteria had minor effects on scaling equations computed for large clades (Mammalia, Eutheria, Metatheria), although less-reliable estimates of BMR were generally about 12-20% larger than more-reliable ones. Larger effects were found with more-limited clades, such as sciuromorph rodents. For the relationship between BMR and brain mass the results of comparative analyses were found to depend strongly on the data set used, especially with more-limited, order-level clades. In fact, with small sample sizes (e.g. <100) results often appeared erratic. Subsampling revealed that sample size has a non-linear effect on the probability of a zero slope for a given relationship. Depending on the species included, results could differ dramatically, especially with small sample sizes. Overall, our findings indicate a need for due diligence when selecting BMR estimates and caution regarding results (even if seemingly significant) with small sample sizes.

Coevolution of body size and metabolic rate in vertebrates: a life‐history perspective

“…Therefore, a relationship between the relative brain size and MinMR may be sensitive to the biological characteristics of studied taxa. An effect of relative brain size on the MinMR was found by Dworak et al (2010) in 51 placental mammals, by Isler & van Schaik (2006) in 347 mammals, by Weisbecker & Goswami (2010) and Genoud, Isler, & Martin (2018) in placental but not marsupial mammals, and by Sobrero et al (2011) in rodents. In carnivores, the relationship was found by Genoud et al (2018) but not by Finarelli (2010).…”

Copulas and their potential for ecology

“…The hypothesis is that asymmetric tail association occurs in evolutionary changes in bivariate characters, and gives rise to asymmtric tail association between the two character values across extant species. We simulated bivariate character evolution on an estimate of the phylogeny, taken from Genoud et al (2018), of 817 mammal species. The root character state and the change across each branch were randomly chosen from matrices of one million independent draws from bivariate distributions showing one of five distinct types of copula structure: 1) extreme or 2) moderate left-tail dependence, 3) symmetric tail dependence, or 4) moderate or 5) extreme right-tail dependence (Appendix S8).…”

Energetic constraints on mammalian home‐range size

“…Home range (HR; ha), basal metabolic rate (BMR; ml O 2 /hr; measured on animals in a post‐absorptive state, resting at thermoneutral conditions) and maximum metabolic rate (VO 2max ; ml O 2 /hr; measured during graded running exercise until rate of oxygen consumption no longer increases with increased work; included one record on swimming exercise for Castor has no marked effect on the final results) data were collected from reviews (Genoud et al, ; Gillooly, Gomez, & Mavrodiev, ; Kelt & Van Vuren, ; Tamburello et al, ; Tucker et al, ). Only adult individuals were included to reduce body mass variation among records for the same species.…”

“…The energetic view of ecology posits that fitness‐related traits are determined by metabolic rates and that body mass is an indirect correlate to fitness through metabolism (Brown et al, ; Glazier, ). Comparative phylogenetic regression and path analyses are conducted here to test these predictions on recently revised home‐range size and metabolic rates data for mammals (Genoud, Isler, & Martin, ; Kelt & Van Vuren, ).…”