The importance of metabolism for the selection of mass explains allometric transitions across the tree of life
The often observed Kleiber scaling in taxa of multicellular animals (with 1/4 and 1/6 exponents) is a special case where the final allometries resemble mass-rescaling. This is because the majority of the body mass variation evolves from the handling of different resources across a variety of niches, with only minor variation in the pre-mass component of metabolism.
Allometries on other scales include metabolic-rescaling. With an apparent 5/6 exponent for mass specific metabolism (DeLong et al., 2010), prokaryotes follow the prediction for the smallest single celled self-replicators. They are predicted to have a metabolic-rescaling exponent of unity (ββ•=1) and body mass variation that follows from evolutionary differences in mass specific metabolism.
Having a b-exponent that declines from about 0.61 to -0.20 with an increasing mass (Witting, 2017), protozoa follow the prediction for larger single celled self-replicators with interactive competition and a metabolic-rescaling exponent that declines from one to zero.
And mass specific metabolism is invariant of mass on the macro evolutionary scale from prokaryotes to mammals (Makarieva et al., 2005, 2008; Kiorboe and Hirst, 2014). This follows from evolution around an upper metabolic bound, where the metabolic decline from mass-rescaling is balanced by a metabolic increase from primary selection on the net energy of the organism.
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