How are animal cancer rates established in the first place? It seems like you'd have to go through the tissues of a large number of creatures to do this. Plus you have the issue that the carcasses you get might be selected unevenly, eg the cancer rate among zoo elephants might be different to the ones in the wild?
You don't necesssarily need to know the rates. "If blue whales got 1,000 times more cancer than humans, they would likely die before they were able to reproduce and the species would quickly go extinct" yet the species does exist. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3060950/
