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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 reproduction. Although some mammals present molecular mechanisms that confer strong resistance to developing tumors, cancer is widely distributed throughout the tree of life1,2. In fact, ecological systems can experience counterintuitive increases in population size, a phenomenon known as the hydra effect, or stability, when a subpopulation exhibits higher mortality rates3–5. 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 and caring habits have lower cancer risk. We demonstrate by mathematical modeling that higher mortality rates 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 mammal species in which older individuals show supportive habits, population size increases as mortality rates of older individuals decrease