Fig. 1.

An illustration of Peto’s Paradox. Cancer is a disease of uncontrolled cell growth and division, and the risk of developing cancer increases with the number of cell divisions during the lifetime of an organism. Thus, the expected cancer rate for large and/or long-lived species is higher than for smaller short-lived ones. The solid red line indicates a linear relationship between cancer rate and (body mass)*(lifespan) and the dashed red line represents an approximation of the expected cancer rate assuming a model describing the probability of an individual developing colorectal cancer after a given number of cell divisions [4]. The solid blue line represents the observation that there is no relationship between cancer risk and (body mass)*(lifespan) [5]. For instance, cancer risk, which is 11–25% in the human population, is not vastly different between mice and humans. In contrast, cancer risk was estimated to be 5% in elephants [5]. Metastatic cancer was found in a duck-billed dinosaur [26], suggesting cancer was common enough in that lineage to be preserved in the fossil record, but not in other species of large dinosaurs. While adult body mass is approximately the same for the dinosaur and the elephant, duck-billed dinosaurs are thought to have had a shorter lifespan [28, 31]. This suggests that the trade-offs between reproduction and growth and cancer defense mechanisms [22] left these dinosaurs more susceptible to cancer than elephants