Power scaling relationships between body mass and organismal traits are fundamental to biology. Compilations of mammalian masses and basal metabolic rates date back over a century and are used both to support and to assail the universal quarter-power scaling invoked by the metabolic theory of ecology. However, the slope of this interspecific allometry is typically estimated without accounting for intraspecific variation in body mass or phylogenetic constraints on metabolism. We returned to the original literature and culled nearly all unique measurements of body mass and basal metabolism for 695 mammal species and (1) phylogenetically corrected the data using the fullest available phylogeny, (2) applied several different regression analyses, (3) resampled regressions by drawing randomly selected species from each of the polytomies in the phylogenetic hypothesis at each iteration, and (4) ran these same analyses independently on separate clades. Overall, 95% confidence intervals of slope estimates frequently did not include 0.75, and clade-specific slopes varied from 0.5 to 0.85, depending on the clade and regression model. Our approach reveals that the choice of analytical model has a systematic influence on the estimated allometry, but irrespective of the model applied, we find little support for a universal metabolic rate-body mass scaling relationship.

The recently proposed metabolic theory of ecology (MTE) claims to provide a mechanistic explanation for long known allometric relationships between mass and metabolic rate. The MTE postulates that these patterns of allometry are driven by the primary selective constraint of transport of energy and materials. However, recent evidence along several different lines has called into question both the adequacy and the universality of this mechanism. We review the accumulating body of literature on this subject, adding our own concerns and criticisms. In addition to other difficulties, we argue that MTE fails as a mechanistic explanation of mass versus metabolic rate allometries because: 1) circulatory cost minimization is not a tenable criterion for evolutionary optimization, 2) the Boltzmann type relationships on which MTE depends are inadequate descriptors of complex metabolic pathways, and 3) most of the hypotheses advanced by the MTE do not, in fact, depend on the proposed mechanism and therefore cannot be used to test the theory. We conclude that the MTE should be abandoned as a monolithic explanation for allometric patterns, and that a more realistic path toward a better understanding of allometry would be to consider multiple explanatory mechanisms for physiological allometries.Biologists have been intrigued by allometric scaling relationships between organismal form, function, and ecology for more than a century. Such scaling relationships are taxonomically pervasive and have inspired a literature that, by 20 years ago, resulted in three nearly simultaneous, but remarkably different, book-length reviews (Peters 1983, Calder 1984, Schmidt-Nielsen 1984. Nonetheless, there is currently no widely accepted general mechanism for these patterns. Hypothesized explanations include structural and functional factors (e.g. surface area/volume effects on exchanges of heat and metabolites; reviewed by Calder 1984, Dodds et al. 2001, biomechanical requirements for support and fracture resistance (McMahon 1973, Biewener 2005, and correlated natural selection on body size and life history (Kozlowski and Weiner 1997).Recently, West et al. and others have advanced a metabolic theory of ecology (MTE Á West et al. 1997, Brown et al. 2004a which:1. conjectures that a quasi-fractal structure of distributional networks like circulatory systems requires an allometry (e.g. metabolism 0/a mass b , where a is a ''normalization constant'' and b is a scaling exponent) in order to minimize transport costs of energy and materials 2. argues that minimizing these transport costs requires a scaling exponent of b 0/3/4, which in

The recently proposed metabolic theory of ecology (MTE) claims to provide a mechanistic explanation for long known allometric relationships between mass and metabolic rate. The MTE postulates that these patterns of allometry are driven by the primary selective constraint of transport of energy and materials. However, recent evidence along several different lines has called into question both the adequacy and the universality of this mechanism. We review the accumulating body of literature on this subject, adding our own concerns and criticisms. In addition to other difficulties, we argue that MTE fails as a mechanistic explanation of mass versus metabolic rate allometries because: 1) circulatory cost minimization is not a tenable criterion for evolutionary optimization, 2) the Boltzmann type relationships on which MTE depends are inadequate descriptors of complex metabolic pathways, and 3) most of the hypotheses advanced by the MTE do not, in fact, depend on the proposed mechanism and therefore cannot be used to test the theory. We conclude that the MTE should be abandoned as a monolithic explanation for allometric patterns, and that a more realistic path toward a better understanding of allometry would be to consider multiple explanatory mechanisms for physiological allometries.Biologists have been intrigued by allometric scaling relationships between organismal form, function, and ecology for more than a century. Such scaling relationships are taxonomically pervasive and have inspired a literature that, by 20 years ago, resulted in three nearly simultaneous, but remarkably different, book-length reviews (Peters 1983, Calder 1984, Schmidt-Nielsen 1984. Nonetheless, there is currently no widely accepted general mechanism for these patterns. Hypothesized explanations include structural and functional factors (e.g. surface area/volume effects on exchanges of heat and metabolites; reviewed by Calder 1984, Dodds et al. 2001, biomechanical requirements for support and fracture resistance (McMahon 1973, Biewener 2005, and correlated natural selection on body size and life history (Kozlowski and Weiner 1997).Recently, West et al. and others have advanced a metabolic theory of ecology (MTE Á West et al. 1997, 2002, Brown et al. 2004a which:1. conjectures that a quasi-fractal structure of distributional networks like circulatory systems requires an allometry (e.g. metabolism 0/a mass b , where a is a ''normalization constant'' and b is a scaling exponent) in order to minimize transport costs of energy and materials 2. argues that minimizing these transport costs requires a scaling exponent of b 0/3/4, which in

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Physiological and ecological allometries often pose linear regression problems characterized by (1) noncausal, phylogenetically autocorrelated independent (x) and dependent (y) variables (characters); (2) random variation in both variables; and (3) a focus on regression slopes (allometric exponents). Remedies for the phylogenetic autocorrelation of species values (phylogenetically independent contrasts) and variance structure of the data (reduced major axis [RMA] regression) have been developed, but most functional allometries are reported as ordinary least squares (OLS) regression without use of phylogenetically independent contrasts. We simulated Brownian diffusive evolution of functionally related characters and examined the importance of regression methodologies and phylogenetic contrasts in estimating regression slopes for phylogenetically constrained data. Simulations showed that both OLS and RMA regressions exhibit serious bias in estimated regression slopes under different circumstances but that a modified orthogonal (least squares variance-oriented residual [LSVOR]) regression was less biased than either OLS or RMA regressions. For strongly phylogenetically structured data, failure to use phylogenetic contrasts as regression data resulted in overestimation of the strength of the regression relationship and a significant increase in the variance of the slope estimate. Censoring of data sets by simulated extinction of taxa did not affect the importance of appropriate regression models or the use of phylogenetic contrasts.