Coevolution of cooperative lifestyles and low cancer incidence in mammals

The development of cancer has been traditionally seen as a ‘side-effect’ of other processes under selection or conceived as selectively neutral since cancer tends to occur after reproduction1. Although some mammals present molecular mechanisms that confer resistance to developing tumors, cancer is widely distributed throughout the tree of life2–4. In fact, ecological systems can experience counterintuitive increases in population size, a phenomenon known as hydra effect, when a subpopulation exhibits higher mortality rates5–7. Here, we work under the hypothesis that cancer risk across the tree of life might have been fine-tuned by evolution. A high incidence of cancer could have a neutral, negative or even positive adaptive value, depending on life history and lifestyle traits associated with the eco-evolutionary context of each species. By using public databases on cancer-related mortality of mammals, we show that species with cooperative habits have lower cancer risk. We demonstrate by mathematical modeling that higher cancer incidence in older less or post-reproductive individuals can lead to an increase in the population size (hydra effect) in a context of intraspecific competition. On the contrary, in species in which older individuals show supportive habits, population size increases as cancer mortality rates of older individuals decrease.