This news article has the answer to cancer in elephants. The first thing that struck me was the lines "every cell can become cancerous so the more of them you have, the more likely you are to get cancer". It goes on to say how an elephant should be 100x more suseptible to cancer than a human although have 5 times less cancer deaths. I think this is a very interesting fact, every cell has to be created and therefore a mutation could be too. This mutation could cause cancer. The reason why elephants don't suffer from cancer as much is a gene called TP53, a tumour supressing gene which humans have too. When the cell cycle checks stop working due to cyclin not functioning the gene will cancel all growth. This gene of course can be mutated and in doing so puts the mammal at a much larger risk to cancer. Elephants have 20 TP53 genes in their body (humans have 1) which means if growth does occur it can stop it but also if one were to mutate it would not matter as there are multiple which could combat it. It is interesting that they say the elephant would be extinct by now if for this amount of TP53. But why in that case do animals of a similar size like whales not die out from cancer?
http://www.bbc.co.uk/news/health-34466220
http://www.bbc.co.uk/news/health-34466220