researchers argue that their derivation contains subtle errors.) If larger metabolic rates are evolutionarily favored, then low-mass organisms will prefer to arrange their networks to scale as , but large-mass organisms will prefer to arrange their networks as , which produces the observed curvature. Modified thermodynamic models An alternative model notes that metabolic rate does not solely serve to generate heat. Metabolic rate contributing solely to useful work should scale with power 1 (linearly), whereas metabolic rate contributing to heat generation should be limited by surface area and scale with power . Basal metabolic rate is then the convex combination of these two effects: if the proportion of useful work is , then the basal metabolic rate should scale as where and are constants of proportionality. in particular describes the surface area ratio of organisms and is approximately ; typical values for are 15-20%. The theoretical maximum value of is 21%, because the efficiency of glucose oxidation is only 42%, and half of the ATP so produced is wasted. Criticism of explanations Kozłowski and Konarzewski have argued that attempts to explain Kleiber's law via any sort of limiting factor is flawed, because metabolic rates vary by factors of 4-5 between rest and activity. Hence any limits that affect the scaling of basal metabolic rate would in fact make elevated metabolism — and hence all animal activity — impossible. WEB conversely argue that animals may well optimize for minimal transport energy dissipation during rest, without abandoning the ability for less efficient function at other times. Other researchers have also noted that Kozłowski and Konarzewski's criticism of the law tends to focus on precise structural details of the WEB circulatory networks, but that the latter are not essential to the model. Experimental support Analyses of variance for a variety of physical variables suggest that although most variation in basal metabolic rate is determined by mass, additional variables with significant effects include body temperature and taxonomic order. A 1932 work by Brody calculated that the scaling was approximately 0.73. A 2004 analysis of field metabolic rates for mammals conclude that they appear to scale with exponent 0.749. Generalizations Kleiber's law only applies to interspecific comparisons; it (usually) does not apply to intraspecific ones. Indeed, Kleiber's law only appears when studying animals as a whole; scaling exponents within taxonomic subgroupings differ substantially. In other kingdoms A 1999 analysis concluded that biomass production in a