Abstract
ABSTRACTDespite many decades of research, the allometric scaling of metabolic rates (MRs) remains poorly understood. Here, we argue that scaling exponents of these allometries do not themselves mirror one universal law of nature but instead statistically approximate the non‐linearity of the relationship between MR and body mass. This ‘statistical’ view must be replaced with the life‐history perspective that ‘allows’ organisms to evolve myriad different life strategies with distinct physiological features. We posit that the hypoallometric allometry of MRs (mass scaling with an exponent smaller than 1) is an indirect outcome of the selective pressure of ecological mortality on allocation ‘decisions’ that divide resources among growth, reproduction, and the basic metabolic costs of repair and maintenance reflected in the standard or basal metabolic rate (SMR or BMR), which are customarily subjected to allometric analyses. Those ‘decisions’ form a wealth of life‐history variation that can be defined based on the axis dictated by ecological mortality and the axis governed by the efficiency of energy use. We link this variation as well as hypoallometric scaling to the mechanistic determinants of MR, such as metabolically inert component proportions, internal organ relative size and activity, cell size and cell membrane composition, and muscle contributions to dramatic metabolic shifts between the resting and active states. The multitude of mechanisms determining MR leads us to conclude that the quest for a single‐cause explanation of the mass scaling of MRs is futile. We argue that an explanation based on the theory of life‐history evolution is the best way forward.
Highlights
Despite m any decades of research, the allom etric scaling of m etabolic rates (MRs) rem ains poorly understood
T his diversification is accom panied by a slower than linear increase of metabolic rates (MRs; see T able 1 for glossary) with body mass, which has fascinated biologists since R ubner (1908), who proposed that the surface-to-volume ratio dictates that M R increases with body mass at a rate of 2 /3
The 3 /4 scal ing proposed by K leiber (1932, 1947) becam e popular; in this scaling, the exponent was initially not considered a m an ifestation of biological laws but as an approxim ation of empirical data rounded to 3 /4, which m ay facilitate utilitar ian calculations with a slide rule (Hulbert, 2014)
Summary
E norm ous diversity o f body mass is observed w ithin orders of anim als and even narrow er clades. T his quest has been futile, as illustrated by a recent sequence of papers on basal metabolic rate (BMR) scaling in mammals: W hite & Seym our (2003) argued for a slope of 2 /3 , Savage etal. U nderstanding the mechanisms of multivariate selection shaping body mass and M R requires a life-history approach that considers a rel evant fitness measure. W ithout such an approach, efforts to explain the hypoallometric scaling of M Rs with various m echanistic m odels m ay be fruitless because body mass is routinely treated as an independent variable, and only M R is perceived as the direct targ et o f selection (see online S u p p o rting inform ation, A ppendix S1 for term inology an d the form of scaling equations). Suggested future directions and conclu sions are presented in Sections V II and VIII
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