  From a Darwinian viewpoint, the existence of altruism in nature is at first sight puzzling, as Darwin himself realized. Natural selection leads us to expect animals to behave in ways that increase their own chances of survival and reproduction, not those of others. But by behaving altruistically an animal reduces its own fitness, so should be at a selective disadvantage vis-à-vis one which behaves selfishly. To see this, imagine that some members of a group of Vervet monkeys give alarm calls when they see predators, but others do not. Other things being equal, the latter will have an advantage. By selfishly refusing to give an alarm call, a monkey can reduce the chance that it will itself be attacked, while at the same time benefiting from the alarm calls of others. So we should expect natural selection to favour those monkeys that do not give alarm calls over those that do. But this raises an immediate puzzle. How did the alarm-calling behaviour evolve in the first place, and why has it not been eliminated by natural selection? How can the existence of altruism be reconciled with basic Darwinian principles?