  Understanding the cross-species behavior of cancer is important for
uncovering fundamental mechanisms of carcinogenesis, and for translating
results of model systems between species. One of the most famous interspecific
considerations of cancer is Peto's paradox, which asserts that organisms with
vastly different body mass are expected to have a vastly different degree of
cancer risk, a pattern that is not observed empirically. Here we show that this
observation is not a paradox at all but follows naturally from the
interspecific scaling of metabolic rates across mammalian body size. We connect
metabolic allometry to evolutionary models of cancer development in tissues to
show that waiting time to cancer scales with body mass in a similar way as
normal organism lifespan does. Thus, the expectation across mammals is that
lifetime cancer risk is invariant with body mass. Peto's observation is
therefore not a paradox, but the natural expectation from interspecific scaling
of metabolism and physiology. These allometric patterns have theoretical
implications for understanding life span evolution, and practical implications
for using smaller animals as model systems for human cancer.
