Altruism

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Altruism, defined as an action that benefits the receiver but comes at some cost to the performer, is one of the four types of social interactions that can occur between animals of the same species. Figure 1 summarizes these four interactions. Cooperation, where both actor and receiver benefit, and selfishness, where the action benefits the actor at the expense of the receiver, are by far the most common of the four interactions in nature. Spite, where both actor and receiver are harmed, and altruism are very rare.

The prevalence of cooperation and selfishness over altruism and spite is explained by the rules of natural selection. The currency of natural selection is offspring. Any anatomical, physiological, or behavioral trait that enhances an individual's ability to produce more offspring will be favored, and the trait will be selected regardless of the effects on others. For example, seagulls sometimes steal food from nesting neighbors to feed themselves and their chicks. This behavior clearly increases the fitness of the actor whiledecreasing the fitness of the receiver; it is selfish. Imagine an altruistic seagull that willingly provided food for its neighbors. This trait would not last very long in the population because the helpful gull would not be able to feed many of its own offspring.

Despite the odds against altruism evolving, it does exist in nature. Some biologists, however, consider these instances to be examples of pseudoaltruism, and insist that true altruism has yet to be found. Pseudoaltruistic acts appear to be altruistic, but "in the long run" are actually beneficial to the actor. There are two types of pseudoaltruism—reciprocal altruism and kin selection.

This occurs when the actor acts altruistically in expectation of having the same done in return at a later time. Many animals that live in groups will post sentinels to watch for predators while the rest forage for food. The sentinel changes several times daily, so the animal "on duty" is assured of being protected later when it is his turn to forage. Vampire bats provide another example. If, when the group returns from hunting, one individual has not found food, a neighbor will regurgitate a portion of its meal for the hungry one. The next evening, the helpful bat may be the hungry one and need the favor returned.

This other type of pseudoaltruism, kin selection, was proposed by British scientist W. D. Hamilton in 1964. He realized that an individual could not only increase his fitness by having its own offspring, but it could also help a close relative raise its offspring, since they share genes. The combination of individual fitness and fitness through kin selection is inclusive fitness. Hamilton argued that if the benefits the actor receives by helping its relatives outweighs the cost of the action, then altruism can evolve. This concept can be expressed mathematically through Hamilton's Rule: br c, where b is the benefit to the actor, r is the relatedness of the actor to the receiver, and c is the cost to the actor. Relatedness is measured by the proportion of genes that are identical between two individuals. Because of Mendelian inheritance, half of a diploid individual's genes are shared with each of its parents, siblings, and children. Diploid grandparents share one-quarter of their genes with their grandchildren, and cousins share one-eighth of their genes with each other. An individual who helps two of its siblings, four of its grandchildren, or eight of its cousins is just as fit as the individual who helps only itself.

Kenyan bee-eaters of the bird genus Merops, have evolved behaviors by kin selection. Male bee-eaters will typically forgo reproducing when they are young, instead opting to help more mature birds raise their young. These young males help relatives more often than nonrelatives, thus raising their inclusive fitness. Young males that attempt to have their own offspring actually fare worse than helpers because their territories are too poor to raise more than one chick.

The classic example of altruism occurs in the eusocial bees. Honeybee workers rarely reproduce, letting the queen provide all the offspring. An unusual chromosome condition, called haplodiploidy, produces unusual relatednessamong the bees in a hive. Workers are actually more related to their sisters (eggs laid by the queen) than their own offspring! Although honeybees are considered the classic example of altruism, they really practice a form of kin selection. True altruism has not yet been found in nature, and some scientists believe that true altruism can be found only in human populations.

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