Respiration rate (R) as a function of body mass (W) is usually expressed as
R = alpha W-b. Empirically, the b value is often close to 3/4 when organis
ms covering a large span in body mass are compared. But recent years resear
ch on the energetic cost of growth demonstrate that young and fast growing
stages show higher weight specific respiration rates than older and adult s
tages, and this implies that the b values tend to be higher: b similar to 1
in small (young) organisms falling to b = 0.6-0.7 in larger (older) stages
. Thus, respiration and growth are integrated through the energetic costs o
f growth. This explains why the b value is not a "natural constant" and why
a "3/4 power scaling law" cannot be deduced from the interplay between pur
e physical and geometric constraints of the transport of oxygen.