The sterile worker castes found in the colonies of social insects are often cited as the archetypal examples of altruism in nature. The challenge to biologists has been to explain how losing the ability to mate has evolved as a superior strategy for transmitting genes into future generations. Recent theory and evidence have demonstrated that monogamy provides a unifying explanation for the evolution of altruism in family groups. However, sterile helpers have not evolved in all monogamous cooperative species. Why? Although monogamy is necessary, it is not sufficient for the evolution of sterility. Crucially, breeders also need to live longer than helpers, as this allows helpers to invest in raising full siblings for the duration of their lives. This eliminates any potential conflict over reproduction. When these two conditions, monogamy and completely overlapping generations, are met we consistently observe the evolution of sterile helpers. We explain the theory and evidence behind these ideas, and discuss the importance of ecology in predicting whether sterility will evolve using examples from social birds, mammals and insects. In doing so, we offer an explanation for the extraordinary lifespans of some cooperative species which hint at ways in which we can unlock the secrets of long life.
