The relationship between body mass and field metabolic rate among individual birds and mammals

Abstract

1. The power-law dependence of metabolic rate on body mass has major implications at every level of ecological organization. However, the overwhelming majority of studies examining this relationship have used basal or resting metabolic rates, and/or have used data consisting of species-averaged masses and metabolic rates. Field metabolic rates are more ecologically relevant and are probably more directly subject to natural selection than basal rates. Individual rates might be more important than species-average rates in determining the outcome of ecological interactions, and hence selection. 2. We here provide the first comprehensive database of published field metabolic rates and body masses of individual birds and mammals, containing measurements of 1498 animals of 133 species in 28 orders. We used linear mixed-effects models to answer questions about the body mass scaling of metabolic rate and its taxonomic universality/heterogeneity that have become classic areas of controversy. Our statistical approach allows mean scaling exponents and taxonomic heterogeneity in scaling to be analysed in a unified way while simultaneously accounting for nonindependence in the data due to shared evolutionary history of related species. 3. The mean power-law scaling exponents of metabolic rate vs. body mass relationships were 0.71 [95% confidence intervals (CI) 0.625-0.795] for birds and 0.64 (95% CI 0.564-0.716) for mammals. However, these central tendencies obscured meaningful taxonomic heterogeneity in scaling exponents. The primary taxonomic level at which heterogeneity occurred was the order level. Substantial heterogeneity also occurred at the species level, a fact that cannot be revealed by species-averaged data sets used in prior work. Variability in scaling exponents at both order and species levels was comparable to or exceeded the differences 3/4-2/3 = 1/12 and 0.71-0.64. 4. Results are interpreted in the light of a variety of existing theories. In particular, results are consistent with the heat dissipation theory of Speakman & Król (2010) and provided some support for the metabolic levels boundary hypothesis of Glazier (2010). 5. Our analysis provides the first comprehensive empirical analysis of the scaling relationship between field metabolic rate and body mass in individual birds and mammals. Our data set is a valuable contribution to those interested in theories of the allometry of metabolic rates.

Keywords: allometry; birds; body mass; body size; daily energy expenditure; doubly labelled water; energetics; field metabolic rate; mammals; metabolic scaling.

Figure 1

Field metabolic rates (FMR) against M for (a) birds and (b) mammals. Each point is for an individual animal; some points are the average of more than one measurement.

Figure 2

Estimates of slope by order for (a) birds and (b) mammals. Filled circles and horizontal lines mark the best model’s random-effects estimates together with their 95% confidence intervals, offset by the best model’s fixed-effects estimates. Vertical lines mark the model-averaged fixed-effects estimate. Crosses mark model-averaged values per order, computed by summing model-averaged fixed-effect slopes and model-averaged conditional means of the random effect of order on slope. Models without a random effect of order on slope were treated as having a conditional mean of zero. As far as we are aware, it is not possible to compute model-averaged confidence intervals on predictions that include random effects, so the crosses are not accompanied by confidence intervals.