The expression of telomerase, and therefore risk of cancer, has apparently been tailored by evolution to scale by body mass (meaning total number of cells): "Until now, scientists believed that our relatively long lifespans controlled the expression of telomerase - an enzyme that can lengthen the lives of cells, but can also increase the rate of cancer. ... Mice express telomerase in all their cells, which helps them heal dramatically fast ... but the flip side of it is runaway cell reproduction - cancer ... evolution has found that the length of time an organism is alive has little effect on how likely some of its cells might mutate into cancer. Instead, simply having more cells in your body does raise the specter of cancer - and does so enough that the benefits of telomerase expression, such as fast healing, weren't worth the cancer risk. ... What, then, does this mean for animals that are far larger than humans? If a 160-pound human must give up telomerase to thwart cancer, then what does a 250,000-pound whale have to do to keep its risk of cancer at bay? ... It may be that whales have a cancer suppressant that we've never considered."

Link: http://www.rochester.edu/news/show.php?id=2698