For all its pervasiveness and the efforts to study it, cancer is still somewhat of a mystery. Why do some animals get it at a higher rate than others? This is the question at the heart of Peto's paradox, the observation that large animals, by virtue of their number of cells, are statistically more likely than smaller animals to develop and accumulate genetic mutations that lead to cancer, yet they don't. In fact, some large animals, including whales and elephants, get much less cancer than expected for an animal of its body size and number of cells.