The nature of scaling between the organismal basal metabolic rate B and its body mass M, BpM a , is currently an important focus of debates in theoretical biology (Whitfield, 2001). For decades, it has been widely accepted that a ΒΌ 3/4 for virtually all groups of organisms (Hemmingsen, 1960;Kleiber, 1961). A novel approach developed by West, Brown & Enquist (1997) (WBE) explains the value of a ΒΌ 3/4 (instead of 2/3 that is to be expected from basic dimensional considerations) by noting the fractal-like space-filling structure of networks that transport materials within living bodies.
However, the agreement about the ubiquity of a ΒΌ 3/4 in the living world has recently been seriously challenged by extensive analyses of data unavailable at the time of adopting the ''3/4 rule''. Dodds et al. (2001) showed that a ΒΌ 0.67 for 357 mammalian species with mass less than 10 kg and a ΒΌ 0.71 for the total of 391 species studied by Heusner (1991), while for birds a ΒΌ 0.66 for the 398 species studied by Bennett & Harvey (1987). For unicellular organisms, a re-analysis of Hemmingsen's (1960) data for 17 species by Prothero (1986) showed that a varies from 0.60 to 0.75 depending on which taxonomic groups are considered. Similarly, based on 554 observations for 108 species of Protozoa, Vladimirova & Zotin (1985) reported values of a from 0.66 to 0.86 for different taxonomic groups, with no obvious clustering around any common value. The evidence from plants consistently in favor of the ''3/4 rule'' was also reported by WBE and their collaborator (