Adjustments to the WBE model that retain assumptions of network shape predict "larger" scaling exponents, worsening the discrepancy with observed data. But one can retain a similar theory by relaxing WBE's assumption of a nutrient transport network that is both fractal and circulatory. (WBE argued that fractal circulatory networks would necessarily evolve to minimize energy used for transport, but other researchers argue that their derivation contains subtle errors.) Different networks are less efficient, in that they exhibit a lower scaling exponent, but a metabolic rate determined by nutrient transport will always exhibit scaling between and . 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.