Altruism refers to behavior by an individual organism that benefits other members of its species at some sacrifice to the individual. Animals of the same species sharing the same habitat are in competition with each other for food, nest sites, mates, and protection from predators. It might be expected that in order to survive, animals would strive to take the best food and other resources for themselves--and to put other individuals between themselves and a lurking predator. Yet many animals are observed to act in ways that help one another: a ground squirrel, spotting a hawk, gives a shrill call alerting others at the risk of drawing the hawk's attention; a lioness nurses cubs that are not her own; a honeybee defends its hive by stinging a trespasser, an act that proves fatal to the bee. Why would an animal put its own survival below that of other members of its species?
The biological success of a species depends upon its ability to reproduce and pass its genes to offspring. Biologists explain altruism as a way for animals to ensure the continuation of their types of genes (units that carry hereditary information). The alarm-sounding ground squirrel, for example, may be risking attack by the hawk, which would end its reproductive life. However, nearby squirrels, scattering for cover at the sound of the alarm, are spared and can continue to reproduce. By helping its relatives, with whom it typically shares many genes, the alarm-sounding ground squirrel is ensuring that its kinds of genes will be passed on to more offspring than it, alone, could produce. This process is known as kin selection.
The altruistic behavior of honeybees, lionesses, and ground squirrels living in close proximity is understandable, as they are surrounded by genetic relatives whose welfare is of direct interest to a potential helper. More difficult to explain is altruism that occurs between individuals who are not related. A pair of unrelated male olive baboons in Africa team up to steal a sexually receptive female from a higher-ranking rival male. As one interloper harasses the dominant male, the other mates with the female. The next time, the two allies may switch roles, so that each benefits by its association with the other. Helping relationships based on favors being returned have been identified as reciprocal altruism by sociobiologist Robert L. Trivers (1943-). Reciprocal altruism does not require that the actor and the recipient be genetic relatives, but it occurs with the expectation that aid given by one will be reciprocated (or the favor will be returned) by the other at a later time. Individuals who renege or cheat in these relationships are likely to be abandoned or even punished by the partner they let down.
Many acts of altruism performed by human beings--particularly those carried out anonymously--are especially challenging to explain. Newspapers and other media relate numerous acts of selfless behavior: a bystander pulling a stranger from a burning car; individuals donating food, money, and clothing to refugees in distant lands; someone anonymously funding a person's medical care. All require some sacrifice on the part of the doer with no immediate personal benefit. Sociobiologists (scientists who study social behavior as shaped by evolution) argue that these selfless deeds may be performed with the expectation that help will be returned to them in some way at some future time. Further, individuals who develop a reputation for helping others are likely to be highly desirable as partners in reciprocal altruism.