Altruistic behavior in nature is the unselfish behavior exhibited by an organism for the benefit of another organism or group of organisms (not closely related).
Honey bees stinging attacker, therefore sacrificing their life trying to protect the colony.
Birds giving warning calls, attracting attention (& danger) to themselves to warn the rest.
Cleaning symbiosis of several fish species.
de Waal, F. (2008). Putting altruism back into altruism: The evolution of empathy. Annual Review of Phycology. 59, 279-300.
Trivers, R. (1971). The evolution of reciprocal altruism. The Quarterly Review of Biology. 46(1), 35-57.