Altruism refers to animal behavior that benefits other animals of the same species. Living in the company of other animals presents numerous drawbacks, including increased competition for food, nest sites, and mates, and increased visibility to predators, to name just a few. We might expect animals to strive to outdo the competition whenever possible, to take the best food and other resources for themselves, and to put other individuals between themselves and the lurking predator. Yet many animals are observed to act in ways that help one another: a ground squirrel, spotting a hunting hawk, stands tall and gives a shrill alarm call, potentially drawing the hawk's attention to itself; a lioness allows cubs that are not her own to suckle alongside her cubs; a honeybee comes to the defense of its hive by stinging an encroacher, an act which proves fatal to the bee. Such self-sacrificing acts of altruism require an explanation, because they seem to contradict what we would expect in a world shaped by natural selection. If competition is the name of the game, why should animals sometimes place the interests of another creature before their own, even to the point of suicide?
Biologists recognize altruism when an animal, like an alarm-calling ground squirrel, sustains some cost to its present or future reproduction by aiding another animal, whose reproductive success is thereby given a boost. The alarm-calling squirrel presumably risks attack by calling attention to itself; in addition, if the squirrel is watching out for predators, it will not be able to forage or perform other activities well. Squirrels nearby can profit, however, as they scatter for cover from the predator. Evolutionary biologists have puzzled over how altruistic behavior of all kinds could evolve, since evolution requires maximizing reproductive success, not sacrificing it. An explanation was provided by William D. Hamilton in 1964, who showed that if a helper directs aid to a genetic relative, it may be more than compensated, in reproductive terms, by the increased reproduction of that relative—with whom it typically shares many genes. The key to genetic representation in future generations—and evolutionary success—can thus be brought about either by having offspring, or by helping relatives to do so, in a phenomenon known as kin selection. Biologists have postulated the existence of so-called "green-beard" genes, which result in traits, such as a "green beard," a patch of color, or a certain odor, that enable an animal to recognize kin and non-kin. Recently, a gene of this type was identified in red fire ants.
The altruistic behavior of honeybees, lionesses, and ground squirrels is almost certainly facilitated by the social context in which these animals live: 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. 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 solicits and mates with the female. The next time, the two allies may switch roles, so that each benefits by the association. Helping relationships based on such reciprocity have been identified as reciprocal altruism by sociobiologist Robert L. Trivers. Reciprocal altruism does not require that the actor and the recipient be genetic relatives, but the actor has the expectation that the aid will be returned in kind at a later time. Individuals who defect or cheat in these relationships are likely to be abandoned or even punished by the defrauded partner.
There also are many examples among cooperative animal societies of indirect reciprocity, wherein altruistic actions benefit the social group as a whole, and thereby indirectly benefit the perpetrator of the altruistic act. In these instances, it is unclear whether natural selection is acting on the genes of the individual or of the group as a whole, a model known as "group selection." The altruistic behaviors of Arabian babbler birds clearly benefit the group; however, these birds actually compete among themselves by altruistic acts. The most altruistic babblers are at the top of the social hierarchy and therefore are the most likely to reproduce and pass on their genes. Thus, altruism may signal the biological fitness of an individual to a potential mate.
Sociobiologists, who apply many of the biological principles observed in insect societies to societies of higher animals, including human beings, have been challenged to explain human altruism. Newspapers and other media are full of accounts of acts of heroism: people donate blood and other organs, dive in a raging river to save a floundering stranger, or leave the waitress a tip in a restaurant even though the tipper will never return. Sociobiologists argue that while the benefits of such actions are delayed in time, they may be cumulative and profound. In a supremely social species like humans, reputation may be everything; individuals who establish themselves as reliable partners in social exchange are likely to be highly desirable as associates in reciprocity. With this in mind, it may not be surprising to discover that many "anonymous" acts of altruism are not anonymous at all: for example, although blood donors never learn who receives their blood, and never reap a reward directly from the recipient, they often wear a sticker proclaiming, "Be nice to me—I gave blood today!" The reward may come from other individuals who become favorably inclined toward the donor, and the donor further buttresses his or her reputation as a desirable partner in reciprocal altruism.
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