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1. INTRODUCTION

In this context, the term "altruism" refers in general to behavior that benefits another individual, usually of the same species, at the expense of the agent. The phenomenon is widespread among various species, and has been interpreted by some as apparently at odds with Darwinian theory. Theories of altruism in biology are often concerned with "cost-benefit" analysis as dictated by the logic of natural selection.

The term "Hamilton's rule" refers to the prediction that genetically determined behavior that benefits another organism, but at some cost to the agent responsible, will spread by natural selection when the relation (rb-c} > 0 is satisfied, where (r) is the degree of relatedness between agent and recipient, (b) is the improvement of individual fitness of the recipient caused by the behavior, and (c) is the cost of the agent's individual fitness as a result of the behavior. The rule was first proposed by William D. Hamilton (1936-2000), and Hamilton's theory is often referred to as "kin selection". As an example: A mutation that affected the behavior of a sterile worker bee so that she fed her fertile queen but starved herself would increase the inclusive fitness of that worker because, while her own fitness decreased, her actions increased the fitness of a close relative.

ON NATURAL SELECTION, KIN SELECTION, AND ALTRUISM

"Natural selection working on groups of close genetic relatives is called kin selection. In species in which individuals sometimes meet one another, such as in social groups, individuals may be able to influence each other's reproduction. Biologists call a behavior pattern altruistic if it increases the number of offspring produced by the recipient and decreases that of the altruist. (Notice that the term in biology, unlike in human action, implies nothing about the altruist's intentions: it is a motive-free account of reproductive consequences.) Can natural selection ever favor altruistic actions that decrease the reproduction of the actor? If we take a strictly organismic view of natural selection, it would seem to be impossible. Yet, as a growing list of natural observations records, animals behave in an apparently altruistic manner. The altruism of the sterile 'workers' in such insects as ants and bees is one undoubted example. In such cases, the altruism is extreme, as the workers do not reproduce in some species.

"Altruistic behavior often takes place between genetic relatives, where it is most likely explained by the theory of kin selection. Let us suppose for simplicity that we have two types of organism, altruistic and selfish. A hypothetical example might be that, when someone is drowning, an altruist would jump in and try and save him or her, whereas the selfish individual would not. The altruistic act decreases the altruist's chance of survival by some amount which we call c (for cost), because the altruist runs some risk of drowning. The action increases the chance of survival of the recipient by an amount b (for benefit). If the altruists dispensed their aid indiscriminately to other individuals, benefits will be received by other altruists and by selfish individuals in the same proportion as they exist in the population. Natural selection will then favor the selfish types, because they receive the benefits but do not pay the costs.

"For altruism to evolve, it must be directed preferentially to other altruists. Suppose that acts of altruism were initially given only to other altruists. In such a case, what would be the condition for natural selection to favor altruism? The answer is that the altruism must take place only in circumstances in which the benefit to the recipient exceeds the cost to the altruist. This relation will hold true if the altruist is a better swimmer than the recipient, but it does not logically have to be true (if, for instance, the altruist were a poor swimmer and the recipients were capable of looking after themselves, the net result of the altruist's heroic plunge into the water might merely be that the altruist would drown). If the recipient's benefit exceeds the altruist's cost, then a net increase occurs in the average fitness of the altruistic types as a whole. This condition has only theoretical interest. In practice, it is usually (maybe always) impossible for altruism to be directed only to other altruists, because they cannot be recognized with certainty. It may be possible, however, for altruism to be directed at a class of individuals that contains a disproportionate number of altruists relative to their frequency in the population. For example, altruism may be directed toward genetic relatives. In this case, if a gene for altruism appears in an individual, it is also likely to be in its relatives."

Mark Ridley: Evolution. 2nd Edition. Blackwell Science 1996, p.321.

ON ALTRUISM OF INDIVIDUALS IN INSECT SOCIETIES

Altruism is self-destructive behavior performed for the benefit of others. The use of the word altruism in biology has been faulted by Williams and Williams (1957), who suggest that the alternative expression "social donorism" is preferable because it has less gratuitous emotional flavor. Even so, altruism has been used as a term in connection with evolutionary argumentation by Haldane (1932) and rigorous genetic theory by Hamilton (1964), and it has the great advantage of being instantly familiar. The self-destruction can range in intensity all the way from total bodily sacrifice to a slight diminishment of reproductive powers. Altruistic behavior is of course commonplace in the responses of parents toward their young. It is far less frequent, and for our purposes much more interesting, when displayed by young toward their parents or by individuals toward siblings or other, more distantly related members of the same species. Altruism is a subject of importance in evolution theory because it implies the existence of group selection, and its extreme development in the social insects is therefore of more than ordinary interest. The great scope and variety of the phenomenon in the social insects is best indicated by citing a few concrete examples:

1) The soldier caste of most species of termites and ants is virtually limited in function to colony defense. Soldiers are often slow to respond to stimuli that arouse the rest of the colony, but, when they do, they normally place themselves in the position of maximum danger. When nest walls of higher termites such as Nasutitermes are broken open, for example, the white, defenseless nymphs and workers rush inward toward the concealed depths of the nest, while the soldiers press outward and mill aggressively on the outside of the nest. Nutting (personal communication) witnessed soldiers of Amitermes emersoni in Arizona emerge from the nest well in advance of the nuptial flights, wander widely around the nest vicinity, and effectively tie up in combat all foraging ants that could have endangered the emerging winged reproductives.

2) I have observed that injured workers of the fire ant Solenopsis saevissima leave the nest more readily and are more aggressive on the average than their uninjured sisters. Dying workers of the harvesting ant Pogonomyrmex badius tend to leave the nest altogether. Both effects may be no more than meaningless epiphenomena, but it is also likely that the responses are altruistic. To be specific, injured workers are useless for most functions other than defense, while dying workers pose a sanitary problem.

3) Alarm communication, which is employed in one form or other throughout the higher social groups, has the effect of drawing workers toward sources of danger while protecting the queens, the brood, and the unmated sexual forms.

4) Honeybee workers possess barbed stings that tend to remain embedded when the insects pull away from their victims, causing part of their viscera to be torn out and the bees to be fatally injured. A similar defensive maneuver occurs in many polybiine wasps, including Synoeca surinama and at least some species of Polybia and Stelopolybia and the ant Pogonomyrmex badius. The fearsome reputation of social bees and wasps in comparison with other insects is due to their general readiness to throw their lives away upon slight provocation.

5) When fed exclusively on sugar water, honeybee workers can still raise larvae -- but only by metabolizing and donating their own tissue proteins. That this donation to their sisters actually shortens their own lives is indicated by the finding of de Groot (1953) that longevity in workers is a function of protein intake.

6) Female workers of most social insects curtail their own egg laying in the presence of a queen, either through submissive behavior or through biochemical inhibition. The workers of many ant and stingless bee species lay special trophic eggs that are fed principally to the larvae and queen.

7) The "communal stomach", or distensible crop, together with a specially modified proventriculus, forms a complex storage and pumping system that functions in the exchange of liquid food among members of the same colony in the higher ants. In both honeybees and ants, newly fed workers often press offerings of ingluvial food on nestmates without being begged, and they may go so far as to expend their supply to a level below the colony average.

These diverse physiological and behavioral responses are difficult to interpret in any way except as altruistic adaptations that have evolved through the agency of natural selection operating at the colony level. The list by no means exhausts the phenomena that could be placed in the same category.

Adapted from: Edward O. Wilson: The Insect Societies. Harvard University Press 1971, p.321.

ON ALTRUISM AND LOVE

"Even the most powerful human sentiment does not escape an explanation calling on natural selection. Maternal love, mate bonding, altruism have their equivalent in numerous animal species. It has been pointed out that such behaviors favor the protection and multiplication of genomes and may, for this reason, have been selected in a purely passive fashion.

"This explanation is plausible for the instinct that drives a female to go so far as to sacrifice her life in defense of her young. From the point of view of evolutionary success, several young lives are worth more than a single old one. Even a single young life is likely to produce more progeny than an older one, provided, of course, it has acquired a sufficient degree of independence. The solidity of the bond between sexual partners, although it varies considerably from one animal species to another, generally parallels, as predicted by the theory, the importance of the joint roles of the two sexes in reproductive success. Birds are a characteristic example. Finally, even altruistic behavior -- the sacrifice of one member of a group for the group's benefit -- is explainable by natural selection. This has been shown by the late British biologist William Hamilton in his theory of kin selection, which evaluates the evolutionary benefit of the sacrifice as a function of the degree of kinship between the 'altruist' and the other members of the group. There is benefit if the loss of the altruist s genes allows a greater number of the same genes to be saved in the group.

"All this, we are told, is explained by the fact that individuals genetically disposed to protect their young at the risk of their lives, to remain united in a manner that favors the welfare of their offspring, or to sacrifice themselves under circumstances such that a danger threatening their kin is lessened have a greater chance of propagating their genes, and thus their behaviors, than those devoid of those genetic characteristics. Those are seen as mathematical truths. Whether feelings are associated with such behaviors may be guessed from the attitudes of the animals, but in a purely anthropomorphic framework; we don't know what goes on in their minds.

"The naturalistic explanation of altruism is convincing. But thence to reduce to the mere play of the evolutionary lottery the flame that burns in the hearts of lovers, the tenderness of a mother for her child, the complicity between two old people contemplating, hand in hand, a life-time spent together; to bring down to a purely utilitarian function the sentiment that has inspired so many poets, writers, musicians, and artists, motivated so many heroic acts, and engendered so many bitter rivalries and conflicts, even between nations; there cannot be many prepared to take this step. Love transports, transfigures, gives a feeling of participating in a sort of cosmic rapture, to the point of sometimes blinding the senses and reason. As with other mental manifestations, one has the impression that the development of the human brain has drawn love out of its primitive shell and allowed it to blossom in a more subtle sphere, a sort of 'love for love's sake', so to speak.

Christian de Duve: Life Evolving: Molecules, Mind, and Meaning. Oxford University Press 2002, p.200.

ON ALTRUISM AND MORALITY

The importance of social interactions in developing behavioral and communication skills can be seen throughout primate groups in the panoply of calls, grimaces, gestures, and activities they use to indicate social positions (e.g., dominance, subordination, group affiliation), needs (e.g., food, sex, reassurance), and changes in any of these areas (e.g., new social positions, alliances, sexual states, or dietary interests). Such behaviors range from transmitting only information on themselves as individuals to actions that may immediately affect the survival of other group members.

Information affecting the survival of other group members is most obvious, for example, when a monkey encounters a leopard and reacts with a loud scream, signaling nearby listeners to take refuge. Thus, although this warning signal may call the predator's attention to the screamer and diminish its own chances for survival, the effect can nevertheless help preserve its relatives or compatriots.

Population geneticists beginning with Haldane and Wright have suggested that there were genetic advantages in such altruistic behavior in which individuals may even go so far as to endanger their own genetic future for those who carry closely related genotypes. In 1964, Hamilton popularized this cooperative process under the name "kin selection", and provided formulas by which some of its benefits could be evaluated. As Maynard Smith has pointed out, "the main reason for thinking that kin selection has been an important mechanism in the evolution of cooperation is that most animal societies are in fact composed of relatives."

Some years after Hamilton's proposals, Trivers introduced a concept of altruism that seemed to have special applicability to human social behavior. Trivers's theory of "reciprocal altruism" suggested that altruism can become established in a group where the frequency of interaction between individuals is high and the life span sufficiently long to enable recipients of altruistic acts to return favors to the altruists. The benefits to individuals who partake in such reciprocal altruism can far outweigh the costs, since even slight expenditures of altruistic energy (such as throwing a life preserver to a drowning individual) may have significant benefits to the altruist when it is reciprocated by the previous beneficiary or other group members. Frequent interaction and exchange of roles ("sometimes an altruist, sometimes a beneficiary") is necessary in order to recognize "cheaters" early on, who would otherwise continually try to act as beneficiaries and exploit the altruists.

By refusing cheaters the benefit of future aid, through either punishment or exile, moral sentiments are developed and enhanced in such cooperative groups, and emphasis is put on precise accounting and balancing of exchanges among individuals. As pointed out by Trivers, the maintenance of such systems is therefore supported by introducing or reinforcing a variety of emotional traits: friendship, moral indignation and resentment, gratitude, sympathy, guilt and repentance.

Adapted from: Monroe W. Strickberger: Evolution. 2nd Edition. Jones and Bartlett 1996, p.475.

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ALTRUISM

K. Sigmund and C. Hauert (University of Vienna, AT) discuss altruism, the authors making the following points:

1) Altruistic actions are generally seen as "noble". Yet some "lowly" organisms are apt to match the most heroic human acts of devotion and self-sacrifice. To use a widely-quoted example, consider Dicrocoelium dendriticum, also known as brainworm. These parasites spend some of their stages in the innards of cows, exit in the feces and, in the form of cercaria, are eaten by ants a few stages later. Once ingested, a gang of cercaria will break through the ant's stomach wall. One of them makes it to the brain of the ant, and causes it to climb on the tips of grass blades, thus exposing itself to be taken up by the grazing cattle. The other cercaria form cysts in the ant's body, ready to pursue their life-cycle within the cow that swallows them. But the one who made it to the brain -- the "brainworm" -- dies without leaving offspring. It has effectively sacrificed itself for the survival of its gang. In humans, comparable feats would be the stuff of epic poetry.

2) Small wonder that evolutionary biologists feel challenged by such behavior and see it as a high priority aim "to take the altruism out of altruism". To begin this task, they define the term "altruism" in purely Darwinian terms, devoid of any moralistic undertones. An action performed by individual A and affecting individual B is said to be altruistic if it increases the fitness -- the reproductive success -- of the recipient B, and decreases the fitness of A. In this context, one may as well give names to the other possible scenarios: if the action increases the fitness of both A and B, one speaks of cooperation; of spite, if it decreases both fitnesses; and of selfishness if A's fitness is enhanced and B's fitness diminished. Both altruistic and spiteful traits lower the reproductive success of their bearers and seem at first inconsistent with the action of natural selection. Yet they abound.

3) The suicidal behavior of the brainworm is a spectacular example of reproductive altruism. The other cercaria could not even reach their egg-laying stage otherwise. Further forms of reproductive altruism occur in the brood care of social insects. The worker castes consist of sterile individuals representing a dead end for the germ line. Nevertheless, it is their task to take care of the brood and even to commit suicide for the community's defense. In a wide variety of tropical bird species, unmated individuals mostly males help a breeding pair raise offspring. Similar alloparenting behavior occurs among some fish and mammals, for instance cichlids, naked mole rats, or jackals.

Current Biology 2002 12:R270

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