A. Relation between mammalian mass, lifespan and size of the active stem-cell pool, and the risk of HSC disorders. (A) Comparison between known values for the lifespan of several terrestrial mammals²⁴  and the 2 scaling exponents considered in this work: the empirical scaling L_E = 11.8M^0.20 (- - -) and the ¼ scaling L = 8.6 M^¼ (—). While both scaling relations account for the qualitative trend of the data, the conclusions drawn from the panel below do not depend on the specific scaling relation used to describe the lifespan of a given mammal. (B) The probability of diagnosis during the lifespan is plotted as a function of the mass of the mammal, for different values of the relative selective advantage r of CSC. For most usual tumors one expects r ≥ 1.7, which moves the maximum of probability (illustrated by the yellow circles) outside of the plotted (and terrestrial mammals) range. Consequently, for such tumors it is an advantage to be as smaller a mammal as possible. The curves plotted predict that, for r = 1.05, only mammals with M > 18 kg are more protected against mutations with this value of r than mice; for r = 1.10 the threshold is already 125 kg; for r = 1.15 we obtain 870 kg, whereas for r = 1.20 the threshold becomes 5800 kg. These values correspond to the ages at which each line crosses the dashed horizontal line.