"Why the chances of getting malignant cancer do not sky-rocket with size is a perplexing question. If size really increased cancer risk than baby whales would die of cancer within days of birth," Nature quoted Armand Leroi, an evolutionary developmental biologist at Imperial College London, UK, as saying.
Biologist John Nagy of Arizona State University in Tempe and his colleagues theorized that natural selection would support the most aggressive cells in a tumour that are further nourished by nutrients meant for blood-vessel growth.
The authors added that these tumours grow on parent tumours by creating a hyper-tumour that wipes out the parent by sabotaging its nutrient supply. In case of larger organisms, more time is given to hyper-tumours to evolve.
Nagy and his colleagues employed a computer simulation that started with a single randomly located tumour with a nutrient supply, challenged tumours to alter its growth in realistic ways.
The team created tumour growth and vascularization in six animals from rat-sized American pikas to humans to blue whales. They used the replication 1,000 times for each species.
They found that most for the time, hyper-tumours evolved and hampered the parent tumour's growth. Larger animals had more hyper-tumours, and lower rates of fatal cancers.
In large animals, tumours had to grow quickly and had to be highly vascularized if they had to be deadly.
"The idea of the hyper-tumour is ingenious because it applies Darwinian selection amongst cancer cells as mechanism for explaining what we seem to observe in this size paradox," said Leroi.
"In my opinion, the authors oversimplify the complexity of cancer. They assume an equivalent carcinogenic risk among mammalian cells and this is a false assumption," said Daniel Martineau, a veterinary pathologist at the University of Montreal in Canada.
According to Martineau, it is easier for rodent cells to transform into cancer cells than human cells, since they express telomerase, an enzyme that repairs chromosomes.
And, human cells do not normally express telomerase, may protect against cancer.
"We know our simulation doesn't prove anything, but it does set up testable predictions, the most important of which is that lethal tumours in large animals have to be highly vascularized," said Nagy.